3. NOBLE GAS (AND OTHER) STUDIES ON METEORITES AND OTHER SAMPLES FROM FAR AWAY

In the early 1980s, Uli Herpers and Rolf Sarafin of the University of Cologne, together with Rolf Michel who just had moved from Cologne to the University of Hannover, presented us with their programme to study the production systematics of cosmogenic nuclides in meteorites. They proposed what eventually led to a broad collaboration between the groups in Hannover, Cologne, Mainz, and Zürich. These studies as well as our extensive further work on cosmogenic noble gases and radionuclides in meteorites will cover the first part of this section. Toward the end of the 1980s, I realised that our gas release technique by in vacuo etching (CSSE) might also be a powerful tool for studying the enigmatic primordial noble gases in meteorites known as “phase Q” (for Quintessence). This and our other work on trapped noble gases in extraterrestrial samples form the second part of this section, which ends with a few – hopefully interesting – examples of projects somewhat off our main lines of research.

Cosmogenic nuclides are formed when primary or secondary particles of galactic cosmic rays (GCR), or in some cases solar cosmic rays (SCR, energetic particles emitted by the Sun), interact with atomic nuclei in extraterrestrial or terrestrial matter. The overwhelming part of primary cosmic ray particles are protons and – to a lesser extent – alpha particles, i.e. ⁴He nuclei. When interacting with matter, these primary particles produce not only secondary protons, but also secondary neutrons, which in turn again produce cosmogenic nuclides. Cosmogenic nuclides are observable mainly in noble gas isotopes and radioactive nuclides, whose abundances in the target materials are otherwise extremely low. In solid matter, the cosmic ray flux has a mean attenuation length of roughly 50 cm. Therefore, cosmogenic nuclides in meteorites are mainly used to determine their exposure age, i.e. the time they spent as metre-sized or smaller bodies in interplanetary space before falling to Earth, or in some cases their residence time in the uppermost metres of their larger parent body.

Research into cosmogenic nuclides started after World War II with the suggestion by Willard Libby that secondary cosmic ray neutrons should produce ¹⁴C in the upper atmosphere of the Earth by interacting with ¹⁴N. This idea led to radiocarbon dating. A few years later, Raymond Davis and Oliver Schaeffer suggested that cosmogenic nuclides produced in rocks at the Earth’s surface could be used to study geological problems (Davis and Schaeffer, 1955). However, production rates of cosmogenic nuclides at the Earth’s surface are several orders of magnitude below those outside the Earth’s atmosphere and magnetic field, which act as an efficient shield for cosmic rays. Therefore, cosmogenic nuclides produced in rocks were routinely studied first in meteorites and lunar samples. The first successful application of cosmogenic isotopes to a meteorite was the determination of the cosmic ray exposure age of Norton County by Begemann et al. (1957). The most likely range reported for the tritium-helium-3 (³H-³He) age of 240–280 Ma is at least twice the modern value for Norton County (which is still one of the two highest exposure ages ever recorded for a stony meteorite; Miura et al., 2007). Nevertheless, this pioneering study showed that meteorites have been broken off from a much larger parent object relatively late in the history of the solar system. Since then the question of how meteorites find their way to Earth has remained a major topic in cosmogenic nuclide research. Answering this question requires constant interplay between nuclide analyses and modelling efforts to improve our understanding of cosmogenic nuclide production mechanisms, production rates, and orbital dynamics (e.g., Arnold et al., 1961; Leya et al., 2021; Gladman et al., 1997). A more detailed introduction to cosmogenic nuclides is given by Wieler (2021).

Peter Signer was a pioneer in cosmogenic noble gas research. While working in Minneapolis, he developed what has become known as the Signer-Nier model, which describes the production systematics of cosmogenic noble gases in iron meteorites, based on analyses of samples taken from cross sectional slabs of the two large meteorites Grant and Carbo. In Zürich, Peter continued to systematically study cosmogenic noble gases in meteorites together with Larry Nyquist, Jack Huneke, Herbert Funk, and Ludolf Schultz (see also next section). He therefore immediately accepted the proposal by Michel and Herpers to participate in their joint programme to study cosmogenic nuclide production. In Cologne, Uli Herpers (with Peter Englert and Wilfried Herr) analysed cosmogenic radionuclides (mainly ²⁶Al and ⁵³Mn) in meteorites by low level counting (for ⁵³Mn combined with neutron activation analysis) and planned to extend these studies by collaborating with the Accelerator Mass Spectrometry (AMS) team at ETH in Zürich (see below). Rolf Michel and his group in Hannover were modelling cosmogenic nuclide production in meteorites and were interested in experimental data as input to, and tests of, the models. Our contribution therefore consisted of measuring cosmogenic noble gases in meteorites, for which the Cologne group provided radionuclide data, and of noble gases in high purity artificial samples irradiated with high energy protons, as described in the next paragraph.

Over the years, Rolf Michel’s group has isotropically irradiated five roughly spherical targets of different sizes (between 5 and 25 cm radius) with high energy protons of 600 MeV and 1600 MeV, respectively, comparable to the energy range of GCR particles (e.g., Michel et al.,1989; Leya et al., 2004a). Figure 3.1 shows a sketch of the r = 15 cm “stony meteorite” irradiated with 600 MeV protons at CERN in Geneva and the r = 10cm “iron meteorite” irradiated with 1.6 GeV protons in Saclay near Paris. The idea behind these “thick target” irradiations was to get a better handle on the development of the secondary particle cascade within a meteorite than is possible based only on measured or calculated “thin target” cross sections for nuclide production. Thin target irradiations measure the probability that a particular nuclide will be produced in a particular target element irradiated by a proton (or more often a neutron) with a particular energy. Thick target experiments are especially important to study the nuclide production by secondary neutrons. They often dominate the total production, and also only few thin target data for neutrons are available. Four of the meteorite mockups were made of granodiorite or gabbro rocks very low in water to simulate stone meteorites, while the r = 10 cm sphere consisted of iron. Tubes inserted in each meteorite dummy were filled with a large variety of targets relevant for cosmogenic nuclide production, such as high purity Mg, Al, Si foils, or, for the elements O, Na, S, K, and Ca, compounds such as FeS₂ or Na₂MoO₄ (e.g., to irradiate elemental Na would have been too dangerous). In addition, the bores were filled with degassed samples from two meteorites. Irradiations were performed at CERN and at Laboratoire National Saturne in Saclay near Paris. Isotropic irradiation of the “meteorites” was achieved by a superposition of two rotational and two translational motions. In addition to the thick target experiments, foils were irradiated upstream of the dummy meteorites to determine thin target cross sections for many nuclides and target elements. Short lived and long lived radionuclides were analysed by gamma spectroscopy and AMS, respectively (e.g., Englert et al., 1984; Leya et al., 2000a). Noble gases (He, Ne, and Ar) in the pure target elements were measured in Zürich, and the degassed meteorite samples and compounds were analysed in Mainz by Hartwig Weber and Friedrich Begemann. This entire huge data set (involving more people than I can list here) served as input to the nuclide production models developed in Hannover. I will come back to these modelling efforts below, where I will address the work of Ingo Leya, who came to Zürich after completing his doctoral thesis in Hannover with Rolf Michel.

The collaboration between the Hannover-Cologne team, our group in Zürich, and (as we soon will see), the Accelerator Mass Spectrometry group at ETH Zürich, led by Willy Wölfli, Martin Suter, and Georges Bonani was key to my education in cosmogenic nuclides in extraterrestrial samples. Two crucial events happened in 1984. The first was the “Third International Symposium on Accelerator Mass Spectrometry” in Zürich, organised by the AMS team at ETH, and the second the “Workshop on Cosmogenic Nuclides” in Los Alamos, organised by Robert (Bob) Reedy and Peter Englert. Only a few years earlier had it been recognised that tandem Van de Graaff accelerators, while becoming outdated in high energy physics research, could be used as very efficient detectors of radionuclides in Earth and environmental sciences. AMS systems count atoms rather than their decays, so much smaller samples are needed for nuclides with relatively long half-lives, such as ¹⁴C, ¹⁰Be, ²⁶Al, and others. In Switzerland, Hans Oeschger, the pioneer of climate science at the University of Bern, had motivated the group of Willy Wölfli to develop ¹⁴C analyses by AMS. Figure 3.2 shows the 6 MV Tandem Accelerator used for these analyses and, a few years later, also for cosmogenic ¹⁰Be, ²⁶Al, and other rare nuclides. The figure also shows one of the many later instruments developed by the Zürich AMS team. Most ¹⁴C analyses today are done with much smaller accelerators that require considerably lower acceleration voltages to separate the rare nuclide of interest from a huge isobaric background. At the 1984 Zürich meeting, the Cologne-Zürich collaboration presented their first ¹⁰Be analyses in meteorites by AMS (Sarafin et al. 1984). It was also at this meeting that I first met Kunihiko (Kuni) Nishiizumi, who became a very good friend and collaborator. Kuni was, and still is, keen to always using the latest technological developments, so he introduced us to the then new world of e-mail. We immediately decided that we too needed this wonderful tool, but we also were convinced that one single e-mail address would do for the entire Signer group. Well, it didn’t stay that way for long.

A few months later, the “Workshop on Cosmogenic Nuclides” became even more important to me. Arriving after midnight, I realised that Los Alamos is a somewhat special place. I had been a bit nervous about how I would find my hotel so late at night, but no problem, a US government limousine was waiting at the airport, offering a ride to everybody. The next day, Rolf Sarafin and I enjoyed the Los Alamos summer sun when we decided we desperately needed a cold beer. No bar or restaurant was open in mid-afternoon, but we found the solution in form of an army veterans club. We had to become club members, which was recorded in a very thick book, but helped us to finally get our much needed beer.

At the Los Alamos workshop I met for the first time many colleagues who since became good friends. Let me mention Jim Arnold, Marc Caffee, Jitendra Goswami, Gregory Herzog, Charles Hohenberg, Tim Jull, Kurt Marti, Kuni Nishiizumi, Bob Pepin, Bob Reedy, and Tim Swindle, among others. This two-day topical meeting helped me find my own place in the cosmogenic nuclide family. I guess my first presentation of noble gas and ¹⁰Be data on the Knyahinya meteorite discussed next was well received.

In 1983, Peter Signer visited Gero Kurat at the Natural History Museum in Vienna, which hosts one of the world’s most important meteorite collections. Peter was much impressed by Gero’s office, which was perhaps ten times the size of his own modest office in Zürich and was overlooked by a large portrait painting of Emperor Franz Josef I. But Peter became even more thrilled when Gero showed him the largest stone meteorite in the Vienna collection. With more than 500 kg recovered mass, Knyahinya had been the largest known stone meteorite worldwide when it fell in 1866 in what is now Ukraine, but at that time belonged to the empire of Austria-Hungary.

Knyahinya is exhibited in a hall which was aptly described in a newspaper feature: “The meteorite hall of the Naturhistorische Museum in Vienna looks as if it had fallen out of the sky itself. It has been allowed to preserve its aura of timelessness. It has remained unchanged since 1889”. Figure 3.4 shows a mural of the fall of Knyahinya and its two largest pieces. Most importantly, the main mass actually consists of three pieces that broke on impact and fit almost perfectly together. The two largest pieces expose a nearly perfectly planar cross section, which, as it turned out, includes the meteorite’s pre-atmospheric centre! This reminded Peter of the cosmogenic noble gas profile that Ludolf Schultz and he had obtained from samples along a drill core through the St. Severin chondrite taken by Paul Pellas and co-workers at the Muséum National d’Histoire Naturelle in Paris (Schultz and Signer, 1976). With Knyahinya, it was now possible to sample not only a linear profile, but a complete, two dimensional cross section through a very large chondrite without having to drill a core. Peter convinced Gero Kurat to allow us to sample across the cross section on one of the two large pieces. So it was that I went to Vienna several times to take gramsized samples with a small core drill. In all, I drilled about forty holes, each perhaps a centimetre deep, prompting Gero to remark that only a Swiss could turn a meteorite into something resembling Emmental cheese (Wieler, 1997). After sampling, the two large pieces were arranged in such a way that museum visitors can barely see the holes.

Knyahinya has probably become the most systematically studied meteorite for cosmogenic nuclides, and hence the Knyahinya data serve as benchmark for tests of cosmogenic nuclide production models. Our first data set was presented at the 1984 Los Alamos workshop, where I was able to show a correlation between the ¹⁰Be activity in Knyahinya and the ²²Ne/²¹Ne ratio. This ratio is a well known “shielding parameter”, i.e. a somewhat loosely defined measure of the depth of a meteorite sample from the pre-atmospheric surface of a “meteoroid” (the pre-atmospheric meteorite before it gets ablated upon its passage through the atmosphere) and also of the size of a meteoroid. Figure 3.5 shows the production rate of ²¹Ne in H chondrites of different radii as a function of depth from the (pre-atmospheric) surface, calculated using a model of cosmogenic nuclide production by Ingo Leya and co-workers that will be discussed in detail in Section 3.1.5. Note that the production rate of most cosmogenic nuclides is not highest at the immediate surface of a meteoroid, where the flux and energy of the primary protons of the GCR are highest, but somewhat below this pre-atmospheric surface. This is because secondary cosmic ray protons and neutrons produce a large fraction of many nuclides, and the flux of secondaries increases from the surface down to a certain depth. Because the production of ²¹Ne by secondary neutrons is higher than that of ²²Ne (the main process is ²⁴Mg(n,α)²¹Ne), samples from deeper within a meteoroid have lower ²²Ne/²¹Ne ratios than samples from near the surface, and samples from large meteoroids tend to have lower ²²Ne/²¹Ne ratios than samples from small meteoroids, as shown in the next paragraph. The correlation of the ²²Ne/²¹Ne shielding parameter with the ¹⁰Be activity in Knyahinya implies that the production rate of ¹⁰Be has a shielding dependency similar to those of Ne and other cosmogenic nuclides.

In other words, the production of ¹⁰Be (mostly from oxygen) by secondary cosmic ray particles increases from the surface up to a certain depth. The ¹⁰Be data of Knyhinya that I showed in Los Alamos and the ¹⁰Be measurements on a series of chondrites presented by Rolf Sarafin at the same meeting were the first data we obtained in our long and fruitful collaboration with the Zürich AMS group, later also on terrestrial cosmogenic nuclides, as discussed in Section 4.

Our work on Knyahinya received a new impetus in 1985 with the arrival of Thomas Graf as a doctoral student of Peter Signer. In the early 1960s, Peter and Al Nier had developed in Minneapolis a semi-empirical model that described the production of cosmogenic noble gases in iron meteorites. The Signer-Nier model, as it became known (Signer and Nier, 1962) was based on work by K. Ebert and H. Wänke in Mainz. Cosmogenic noble gas data from a cross section of the large iron meteorite Grant (see also Section 3.1.11) were used to fit two free parameters A and B in a semi-empirical production equation. Parameters A and B describe production by primary and secondary cosmic ray particles, respectively. This model was first extended to chondrites by Larry Nyquist. Thomas Graf now showed that the model also successfully reproduced measured concentrations of He, Ne, Ar, ¹⁰Be, ²⁶Al, and ⁵³Mn as well as the ²²Ne/²¹Ne ratio in Knyahinya (Graf et al., 1990a;b). Figure 3.6 shows measured ²²Ne/²¹Ne ratios in the Knyahinya cross section and – based on these data – modelled ²²Ne/²¹Ne ratios as a function of shielding. This semi-empirical model allows the calculation of production rates in chondrites of variable sizes. Thomas also showed that an analysis of Ne and ¹⁰Be in a single sample of a chondrite allows the derivation of a shielding-corrected exposure age.

Knyahinya turned out to be a remarkable stroke of luck. First, the centre of the meteoroid is preserved in the recovered meteorite, very close to the edge of the sampled cross section (Fig. 3.6). Second, from one edge of the meteorite only a few cm were lost by ablation upon atmospheric entry, as became evident by the ²²Ne/²¹Ne shielding parameter. Third, Knyahinya was found to have experienced a one stage exposure history, avoiding complications in data interpretation in the event the meteoroid had broken apart during its journey to Earth. Such “complex” exposure histories will be discussed in the next section.

All these fortunate circumstances – together with Thomas Graf’s excellent work – contributed to Knyahinya being studied for cosmogenic nuclides by many groups. For example, Bernard Lavielle and co-workers in Bordeaux and Tim Jull and co-workers at the University of Arizona analysed Kr, Xe, and ¹⁴C, respectively, in cross section samples (Jull et al., 1994; Lavielle et al., 1997) and Ingo Leya used Knyahinya data to calibrate the GCR particle flux in the inner solar system, which he needed for his nuclide production model (Leya et al., 2000b). Many other workers compared their meteorite data with those of Knyahinya. After his doctoral exam, Thomas Graf spent many years in Kurt Marti’s group in La Jolla at the University of California, San Diego. Best known is his work on exposure age statistics of various chondrite classes (e.g., Graf and Marti, 1995).

Knyahinya was the first meteorite on which I was involved in a detailed cosmogenic nuclide study, but many others followed. Most of these studies were carried out in collaboration with one or more other teams, combining noble gas measurements with analyses of cosmogenic radionuclides and often also with modelling of nuclide production. The first such study was on the iron meteorite Grant (Section 3.1.3), using samples from the cross section already studied by Peter Signer in Minneapolis. In collaboration with Stephan Vogt in Cologne and the AMS group in Zürich we showed that the ¹⁰Be/²¹Ne ratio is independent of a sample’s shielding and hence can be used to determine shielding-corrected ²¹Ne cosmic ray exposure ages of iron meteorites (Graf et al., 1987). Grant and Carbo, another large iron meteorite, were later used extensively to study very small but potentially important modifications of the isotopic composition of elements important to cosmochemistry by cosmic rays (Section 3.1.11).

Several studies were done on large pre-atmospheric meteorites with up to hundreds of recovered fragments, such as Bur Gheluai (Vogt et al., 1993), Gold Basin (Welten et al., 2003), or Almahata Sitta (Welten et al., 2010; Riebe et al., 2017c). Other large studies were done on groups of selected meteorites, for example from Mars (Wieler et al., 2016) or the Frontier Mountain region in Antarctica (Welten et al., 1999). The latter project was also a result of my wish to study some of the meteorites I had found myself. Some projects involved collaborations that went beyond the study of cosmogenic nuclides, such as the consortium formed by Derek Sears on the Fayetteville regolith breccia (Wieler et al., 1989a,b), the work on Almahata Sitta (Riebe et al., 2017c), a meteorite that had fallen in Sudan less than a day after the discovery of its immediate parent asteroid of a few metres size, and the work on meteorites containing cluster chondrite clasts (Müsing et al., 2021). These clasts are characterised by deformed and indented, closely spaced chondrules thought to have accreted within hours or days after they had formed (Metzler, 2012). A special place in this list is reserved for the long standing controversy between my good friends Charles Hohenberg and Marc Caffee and myself over whether or not some samples in meteorites with very substantial excesses of cosmogenic noble gases testify to a very active early Sun (Section 3.1.7). Also the collaboration with Birger Schmitz and colleagues on fossil meteorites (Section 3.1.9) and the work with Philipp Heck and colleagues on presolar grains (Sections 3.1.7, 3.1.9 and 3.1.10) should be mentioned here.

Many of the projects mentioned in the previous paragraph address the broad topic of “complex exposure histories”. An exposure history of a meteorite is said to be “complex” if not all of its cosmogenic nuclides were produced during a single stage of irradiation of the pre-atmospheric meteorite. For example, a meteorite may have broken up by a collision on its way to Earth, or part of its nuclide complement may have already been acquired near the surface of its parent body. Complex exposure histories may also reflect differential irradiation of certain components of a meteorite prior to its final compaction. This may happen, for example, in a parent body regolith (Section 3.1.6), or some grains may even “remember” early irradiation in the solar nebula or in presolar environments. If the last change of the irradiation geometry occurred earlier than a few half-lives of a radionuclide (e.g., prior to some 3 Ma for ²⁶Al), essentially all the now detectable atoms of this nuclide will have been produced during the last irradiation stage. However, the same meteorite may inherit (stable) cosmogenic noble gas isotopes from (one or more) earlier irradiation stages. Perhaps some of the longer-lived cosmogenic radionuclides from an earlier stage also survived. To recognise complex exposure histories, it is therefore necessary to analyse several cosmogenic nuclides in the same sample or multiple samples from the same meteorite, or both (e.g., Vogt et al., 1993). Combinations of measurements of stable noble gases with suites of cosmogenic radionuclides with different half-lives and different depth dependencies of their production rates are ideal. In addition, determinations of cosmic ray track densities are also desirable, but unfortunately have rarely been done in recent years. Such comprehensive studies require a large effort, which means that it remains a challenge to constrain the fraction of meteorites with a complex exposure history. Yet, it is important to obtain good estimates of the fraction of meteorites that suffered complex exposures in order to better understand the history of meteorites as they travel from the asteroid belt to Earth.

Perhaps the most important general finding of the above work – and many similar studies by others – is that it is difficult to unequivocally confirm or deny that a meteorite suffered a complex exposure history. However, it seems likely that complex histories are much more common than the few confirmed cases would suggest (e.g., Herzog et al., 1997). Wetherill (1980) had argued that fragmentation of meteorite-sized bodies on their way to Earth should be common. For a long time, this seemed to be in contrast to the small numbers of meteorites with well documented complex exposure histories, apart from those which clearly represent material from the regoliths of their parent-body asteroids. Among the well studied meteorites mentioned above, Bur Gheluai, Gold Basin, and Almahata Sitta likely suffered a complex history. George Wetherill’s prediction may thus turn out to be correct.

One excellent result of working with others in the field of cosmogenic nuclides has been that it has allowed me to make many of my best and most lasting friendships within the scientific community. One of these good old friends is Kuni Nishiizumi (who showed me what e-mail is, see above). In the 1980s, Kuni invented the “Cosmogenic Dinner”. He is a gourmet, and his main motivation probably was that he did not like the “chilli-cookoff and barbecue dinner” on Wednesday nights of the annual LPSC in Houston. The community of cosmogenic nuclide aficionados was (and is) quite small (usually we were crammed into the smallest available conference room at NASA’s Gilruth Center). Kuni managed to convince many of us to skip the chilli-cookoff and gather at a good restaurant instead. Over the years Cosmogenic Dinners became a tradition and were later extended to the annual meetings of the Meteoritical Society. With the integration of colleagues from sample return missions such as Stardust and Genesis, the event was eventuelly renamed “Cosmogenic and Sample Return Dinner”. Kuni always asked the older ones among us to bring their younger colleagues. For many early career scientists, this was a great opportunity to integrate into the community. There are too many good friends I have enjoyed Cosmogenic Dinners or worked closely with over the years to name them all. Marc Caffee, Goswami, Gregory Herzog, Charles Hohenberg, Tim Jull, Candace Kohl, Kurt Marti, Jozef Masarik, Kuni Nishiizumi, Larry Nyquist, Uli Ott, Paul Pellas, Bob Reedy, Ludolf Schultz, Tim Swindle, and Kees Welten are just a few of them. Particularly entertaining were good storytellers like Don Brownlee and Don Burnett, the Principal Investigators of the Stardust and Genesis missions, as well as Jim Arnold, one of the pioneers in cosmogenic nuclide research.

As a highly important outcome of the Hannover-Zürich collaboration, two persons that were to become among my most important collaborators and best friends found their way to Zürich from Rolf Michel’s group in Hannover. Henner Busemann in 1994 became the very first doctoral student for whom I acted as the main supervisor. Two years later, Ingo Leya was to become my first postdoc (and later research associate) after completing his doctoral thesis in Michel’s group. My long lasting collaborations with Henner – which continue to this day – are described below. Here is a good place to summarise Ingo’s work in Zürich and later at the University of Bern.

Ingo’s training in nuclear physics in Hannover enabled him to continue the modelling work on cosmogenic nuclide production in meteorites initiated by his mentor Rolf Michel, mentioned above. Here below a few more details about the physics and the modelling of nuclide production. More details can be found in, e.g., Herzog and Caffee (2014), and David and Leya (2019).

Knowledge of nuclide production rates forms the basis for almost all applications of cosmogenic nuclides. Production rates of radioactive nuclides can to a first order be determined empirically, since their production rates equal their decay rates after some five half-lives of irradiation, and their concentrations therefore reach an equilibrium that will be approximately the same in all meteorites with identical major target element concentrations and a more or less “typical” size and sample depth. If the production rate of a radionuclide is known, its concentration in a meteorite that is not yet in equilibrium can then be used to determine the exposure age of that meteorite and, together with the concentration of a stable noble gas nuclide, also the production rate of that nuclide (Vogt et al., 1990). However, this approach is limited to meteorites of “typical” size and will also fail for meteorites with a complex exposure history. Therefore, more sophisticated methods are needed, not only to obtain more reliable estimates of production rates, but also to better understand the physics of nuclide production and to compare the production systematics of different nuclides. The semi-empirical model used by Signer and Nier (1962) and Graf et al. (1990b) is one such approach. Models based on nuclear physics – in theory and experiment – are another. Such models were pioneered by Jim Arnold, Masatake Honda, Devendra Lal, Truman Kohman, and Bob Reedy (Arnold et al., 1961; Kohman and Bender, 1968; Reedy and Arnold, 1972). On the one hand, these models are based on cross sections for the production of nuclides by protons and neutrons of different energies from relevant target elements. Cross sections are compiled in very large databases and are often values interpolated from rather few measured data points or based entirely on theory. On the other hand, the production of secondary protons and neutrons and their transport within a meteorite must also be modelled. Furthermore, the proton flux of the primary cosmic radiation in the inner solar system is a free parameter that can be fitted to measured radionuclide concentrations in meteorites, e.g., Knyahinya, as explained above.

I mentioned above that Rolf Michel and his colleagues began developing their “Hannover” model of cosmogenic nuclide production in the mid-1980s. The noble gas and radionuclide data from the thick and thin target irradiations at CERN and Paris-Saclay allowed them to determine separately nuclide production in spherical meteorites by secondary protons and neutrons, respectively (e.g., Michel et al., 1989; 1996). Ingo Leya was involved in the latter of these two papers as part of his doctoral dissertation. In Zürich he used the Hannover model to calculate depth dependent production rates of cosmogenic He, Ne, and Ar as well as five radionuclides (¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, ⁵³Mn) from all major target elements in chondrites of various sizes (Leya et al., 2000b). This paper also reported the correlation between the production rate of ²¹Ne and the shielding parameter ²²Ne/²¹Ne (see Fig. 3.9 for an updated version) as well as correlations among, for example, ²⁶Al and ¹⁰Be as a function of meteoroid size and sample depth. This widely used work allowed to calculate in an easy form shielding-corrected exposure ages of meteorites – within certain limits – for samples of different chemical composition. This work was done in friendly competition with others, notably Jozef Masarik and Bob Reedy at Los Alamos (Masarik et al. 2001b). Later, Ingo and Jozef (then at Comenius University in Bratislava) joined their efforts on modelling cosmogenic nuclide production in meteorites in another well known publication (Leya and Masarik, 2009). Figure 3.9 shows the dependence of the production rate of ²¹Ne in H chondrites as a function of the ²²Ne/²¹Ne shielding parameter modelled by these authors. While the model data for meteorites of modest size agree quite well with previously proposed empirical relationships (Nishiizumi et al., 1980; Eugster, 1988), the latter fail for interior samples of large meteorites (r > 65cm) with ²²Ne/²¹Ne ≤ 1.08.

The latest update on Ingo Leya’s work on modelling cosmogenic nuclide production in meteorites can be found in Leya et al. (2021). To summarise, the continuous development of new generations of physical models over the last decades has led to an ever improved understanding of cosmogenic nuclide production, but – as always – further progress is highly desirable and can be expected.

Ingo Leya also extensively models potential modifications of the isotopic composition of elements important for isotope cosmochemistry, such as tungsten. The motivation came when Alex Halliday arrived in Zürich in the late 1990s, and I will describe this work in Section 3.1.11.

Ingo continued to work with us in Zürich after he moved to Bern around 2004. One of the main projects from this time was to use the ³⁶Cl-³⁶Ar method to determine noble gas production rates in chondrites, including a re-evaluation of the ⁸¹Kr-Kr exposure dating method. These studies were done in collaboration with Thomas Graf, Kuni Nishiizumi, Bernard Lavielle, Marc Caffee, Kees Welten, Natalie Dalcher, and others (Leya et al. 2004b, 2015; Dalcher et al., 2013). The ³⁶Cl-³⁶Ar method is based on the fact that in Fe-Ni metal samples, more than 80 % of the cosmogenic ³⁶Ar is produced via its radioactive precursor ³⁶Cl. This means that the production rate of ³⁶Ar in metal samples can be determined directly by an analysis of ³⁶Cl in an aliquot or an adjacent metal sample. The ³⁶Cl-³⁶Ar technique is therefore an elegant way to determine cosmic ray exposure ages of samples without the need to know their pre-atmospheric shielding. The method was first applied to iron and stony iron meteorites (Schaeffer and Heymann, 1965; Begemann et al., 1976; Lavielle et al. 1999). However, Kuni Nishiizumi and his collaborators managed to produce very clean metal separates from chondrites, requiring only minor corrections for trapped ³⁶Ar and cosmogenic ³⁶Ar produced from Ca in the remaining silicate impurities (Graf et al., 2001).

The intrinsic accuracy of the ³⁶Cl-³⁶Ar technique encouraged Thomas Graf, Ingo Leya, and co-workers to further study the details of the most prominent peak in exposure age histograms of meteorites. It has been known for decades that exposure ages of meteorites are not evenly distributed, but show distinct clusters – different for each meteorite class – that indicate large collision events in the asteroid belt or on other parent bodies (Anders, 1964). The largest of these exposure age peaks accounts for about 45 % of all H chondrites, suggesting that the world’s meteorite collections owe about 20 % of all their specimens from documented falls to a single collision in the asteroid belt about 7 Ma ago. However, in 1995, Thomas Graf and Kurt Marti had taken a closer look at the H chondrite histogram and suggested that the 7 Ma peak actually represents not one, but two different collisions (Graf and Marti, 1995; Fig. 3.10). Graf et al. (2001) and Leya et al. (2001) were able to provide further evidence for this double peak, perhaps some 700,000 years apart.

The ⁸¹Kr-Kr technique allows one to directly derive the production rates of stable cosmogenic Kr isotopes in a sample of unknown shielding by a single noble gas analysis (Marti, 1967). The production rate of, e.g., ⁸³Kr is deduced from the concentration of ⁸¹Kr, which has a half-life of 229,000 a. However, to determine the production rate ratio P(⁸³Kr)/P(⁸¹Kr) is less straightforward than for the ratio P(³⁶Cl)/P(³⁶Ar). Also, P(⁸³Kr)/P(⁸¹Kr) likely depends on the concentration ratios of the major target elements for Kr production (Rb, Sr, Y, Zr). Leya et al. (2015) used ³⁶Cl-³⁶Ar ages of 14 chondrites to recalibrate the ⁸¹Kr-Kr system and concluded that ⁸¹Kr-Kr exposure ages calculated according to Marti (1967) and Marti and Lugmair (1971) are too high by up to 30 %. Leya et al. (2015) also showed that the production rate ratio P81/P83 is indeed essentially constant for the entire shielding range covered by their chondrite samples, confirming the fundamentals of the technique. However, it must also be acknowledged that, as elegant as the ⁸¹Kr-Kr method is in principle, it requires very precise analyses of very low ⁸¹Kr amounts and sometimes also critical corrections for trapped Kr, which often limits the precision of ⁸¹Kr-Kr ages.

Cosmogenic nuclides, in particular cosmogenic noble gases, are also excellent for studying the mixing or exhumation history of the top few metres of a meteorite’s parent body, especially regoliths on asteroids or the Moon, and the surface of Mars. Mixing of a planetary regolith by impacts leads to differences in concentrations of cosmogenic nuclides even in samples that ultimately arrive on Earth in the same meteorite. Obviously, such effects are expected in particular in gas-rich meteorites, i. e. meteorites that also contain solar wind implanted noble gases acquired at the surface of their parent body. One of the best known gas-rich meteorites is the H chondritic regolith breccia Fayetteville. We were therefore happy to participate in the Fayetteville consortium set up by Derek Sears at the University of Arkansas in Fayetteville (Schwarz and Sears, 1988). Fayetteville is one of the meteorites with the highest concentrations of solar wind noble gases. It displays a dark-light structure typical for many gas-rich chondrites (Fig. 3.11), with solar noble gases being present only in the dark portions. These consist of compacted dust from the parent body, with many or most of the dust grains having once been at the immediate surface where they collected ions from the solar wind. The light inclusions were cm- or mm-sized pebbles in the regolith. Their interiors are free of solar wind particles which penetrate only a few tens of nanometres into solid matter. We analysed noble gases in samples from both, dark and light lithologies, and Paul Pellas at the Muséum d’Histoire Naturelle in Paris counted solar flare tracks (Wieler et al., 1989a;b; Pedroni, 1989). In the matrix samples, concentrations of cosmogenic ²¹Ne correlate with those of solar wind-implanted Ne (Fig. 3.11). This correlation implies that not all cosmogenic ²¹Ne in the fine grained matrix samples was produced during the Fayetteville meteoroid’s journey to Earth but that a substantial fraction of it was already acquired on the parent body. Also many, if not all, of the solar wind-free light samples must have been irradiated by galactic cosmic rays on the parent body for at least several million years. We concluded that the distance of the Fayetteville parent body to the Sun was about 2-3 times that of the Moon, i.e. between 2 and 3 astronomical units, consistent with a main belt origin. Based on an argument developed by Ed Anders (Anders, 1975), we reached this conclusion by comparing solar and cosmogenic noble gases in gas-rich meteorites and lunar samples, respectively. Anselmo Pedroni, in his doctoral thesis came to a similar conclusion for the parent body of the gas-rich aubrite Kapoeta (Pedroni, 1989). Turning the argument around and assuming a priori that the Fayetteville and Kapoeta parent bodies both orbit in the main belt, the regolith dynamics on these bodies compare well with those on the Moon, in the sense that the ratio of residence time of grains at the top surface of the regolith to that in the top few metres is similar for all three bodies. This reasoning has subsequently been adopted in several other studies (e.g., Obase et al., 2020).

Further studies of asteroidal regoliths in Zürich included work on the Ghubara meteorite and on grains from asteroid Itokawa collected by the Japanese Hayabusa mission. Matthias Meier (Meier et al., 2014b) showed that chromite grains in the Ghubara L5 chondrite were exposed to galactic cosmic rays for up to several 10 Ma, a result that is particularly relevant to studies on fossil meteorites which will be addressed in Section 3.1.9. Meier et al. (2014a) determined a ²¹Ne exposure age of only about 1.5 Ma for one grain from Itokawa, but further grains analysed by Henner Busemann (personal communication, 2023) show exposure ages of up to some 20 Ma, exceeding the upper limit of a few Ma previously obtained by Nagao et al. (2011) for several other grains from Itokawa. These data indicate that the surface of this asteroid appears to be much younger than the lunar regolith. This may be due to either rapid loss of grains to space or rapid regolith turnover rates on Itokawa.

Young stars emit very high fluxes of energetic particles during their formation and early evolution (Feigelson, 2010). Figure 3.12 shows the intense X-ray emission of a five million year old young stellar cluster, which very likely is accompanied by a very intense energetic proton flux. From such observations, Eric Feigelson inferred that also the young Sun emitted a perhaps 10⁵ times higher energetic proton flux than today. The very high activity of the early Sun likely left its mark in the form of several isotopic anomalies in meteorites. In particular, excesses of ¹⁰B in early solar system condensates are thought to be due to the decay of ¹⁰Be produced by spallation reactions induced by energetic particles from the early Sun (McKeegan et al., 2000; Liu et al., 2009). Perhaps some atypically high initial abundances of ⁵³Mn and ²⁶Al in CAIs are also the result of high energetic proton fluxes from the early Sun (Nyquist et al., 2009). It is therefore conceivable that early intense solar radiation also left its mark on the noble gas records of meteorites, perhaps even more conspicuously than on any other element. Can we really observe this?

This question has been the subject of intense debate since the pioneering work of Charles Hohenberg and his colleagues at Washington University in St. Louis in the 1980s. I have been involved in this controversy for many years. Marc Caffee and his co-workers observed that a small fraction of the individual grains of solar gas-rich chondrites and achondrites they studied contained much higher concentrations of cosmogenic ²¹Ne and ³⁸Ar than the bulk meteorite (Caffee et al., 1987). For the carbonaceous chondrites Murchison, Murray, and Cold Bokkeveld, the excess gas would correspond to exposure to galactic cosmic rays for as long as several ten and sometimes more than a hundred million years, if produced by GCR with present day flux. This would be much longer than the accepted exposure ages of the meteorites in space (their “4π age”) of a few Ma (Woolum and Hohenberg, 1993). Prior to the noble gas analysis, groups of grains and later individual grains were first examined by Jitendra Goswami at the Physical Research Laboratory in Ahmedabad, India, for the presence or absence of crystal lattice damages caused by solar energetic particles (solar flare tracks, see Section 2.5). Very remarkably, only the track containing grains showed excesses of cosmogenic noble gases (Caffee et al., 1987; Hohenberg et al., 1990). The St. Louis collaboration therefore clearly favoured the view that excess cosmogenic noble gases (as well as the tracks) were the result of irradiation by the early active Sun.

However, in our work on the gas-rich meteorite Fayetteville we expressed caution about this interpretation (Wieler et al., 1989a). A correlation between solar flare tracks and excess cosmogenic noble gases in grains taken from a regolith should not be viewed as a strong argument that the cosmogenic gases also had been produced by solar – rather than galactic – energetic particles. Take a handful of well mixed soil from the lunar surface. If exposed to the solar and galactic cosmic radiation for a long time, most grains of this “mature” soil will contain solar wind noble gases and thus also solar flare tracks, as almost every grain was once at the immediate surface of the Moon (Wieler et al., 1980). All grains will also contain cosmogenic noble gases, but in a well mixed regolith most of these gases will have been produced by the galactic cosmic radiation with its penetration depth of a few metres. Only a few percent of the cosmogenic gas will have been produced by solar cosmic rays, while the grains were in the top one or two centimetres of the regolith. Now consider another parcel of freshly produced regolith, perhaps from material recently brought to the surface by an impact. Such grains would be mostly free of solar wind noble gases, solar flare tracks, and cosmogenic noble gases. If you now add to this fresh material a few percent of “mature” regolith, you would find an almost one to one correlation between track-rich grains and grains with cosmogenic gases. However, the tracks would be the result of solar energetic particles, while the cosmogenic gases would have been mostly produced by galactic cosmic rays. Probably such a clear cut case does not exist in the lunar sample collections, but in less mature asteroidal regoliths it is easy to imagine that intense mixing of immature with mature material happens. During the meteoroid stage, after ejection from the parent body, the now compacted soil would acquire further cosmogenic gases, as seen in the gas-poor and track-free grains and corresponding to the meteorite’s 4π exposure age to GCR. Hohenberg et al. (1990) and Woolum and Hohenberg (1993) argued that carbonaceous chondrites are very unlikely to have experienced regolith exposures of hundreds of Ma. In contrast, Wieler et al. (2000) argued that the solar flare track densities in the meteorite grains were roughly two orders of magnitude lower than would be expected if the cosmogenic noble gas excesses had been predominantly produced by solar energetic particles.

Hence, in the early 2000s, the situation remained controversial. Many people endorsed the idea that meteoritic noble gases preserve a record of the early active Sun. Work began to focus on individual chondrules rather than individual mineral grains, and more or less clear evidence for cosmogenic noble gas excesses in some chondrules was reported, although by far not to the extent previously seen for track-rich single grains (Eugster et al., 2007; Das and Murty, 2009). However, we remained sceptical that these excesses reflect an intense irradiation by energetic particles from the early Sun.

One major problem had always been that substantial excesses of cosmogenic noble gases were only found in samples from regolith breccias. Irradiation by an early active Sun should, however, also or predominantly have happened in a nebular environment rather than on a parent body surface. Chondrules, for example, should therefore show excesses whether they are now found in a solar noble gas-rich regolith breccia or in a meteorite without evidence of regolith origin. Therefore, in his Master’s thesis at ETH, Antoine Roth (Roth et al., 2011) compared the concentrations of cosmogenic He and Ne in single chondrules from the two carbonaceous chondrites Allende and Murchison, the former being free of solar noble gases and the latter representing a solar windrich regolith breccia. The samples from the latter meteorite were analysed with a high sensitivity mass spectrometer called Tom Dooley described in Section 3.1.8. All Allende chondrules contained very similar concentrations of cosmogenic ³He and ²¹Ne, consistent with the accepted meteoroid exposure age of Allende (Fig. 3.14a). Therefore, no Allende chondrule showed evidence for pre-exposure to either solar or galactic cosmic rays. In contrast, about one in five Murchison chondrules contained sizeable or even very large excesses, comparable to those reported for the individual Murchison grains studied by Hohenberg et al. (1990). The (²²Ne/²¹Ne)_cos ratio in chondrules with excess Ne is also as expected for Ne produced by galactic cosmic rays but at odds with contemporary SCR-Ne.

Thus, while we did not definitively rule out that the excesses in Murchison chondrules were due to an early irradiation by SCR, we felt that Antoine Roth’s data clearly argued for production in a parent body regolith by GCR, at any time in solar system history. This more mundane explanation required further evidence. We therefore decided to carefully discriminate between chondrules taken from the “clastic matrix” of Murchison and from lithic clasts, respectively, the latter being fragments of “primary accretionary rocks” (PAR). Knut Metzler and co-workers at the University of Münster had distinguished these lithologies and concluded that PARs had accreted in the solar nebula (Metzler et al., 1992). Important for our project was that the lithic clasts were single pebbles of a few centimetre size in the regolith and therefore all chondrules from one of these lithic clasts had experienced the same parent body mixing and irradiation history. On the other hand, chondrules from clastic matrix may all have had different mixing and irradiation histories in the regolith prior to the final compaction of Murchison. If all chondrules were exposed to an early active Sun, the lithic clasts (from the PARs) and the clastic matrix would be expected to contain a similar fraction of chondrules with (variable) cosmogenic noble gas excesses. Otherwise, if the chondrules did not record exposure to an early active Sun, all chondrules from a lithic clast would either be free of excess cosmogenic gas (if that clast never was near the parent body surface), or would contain the same concentration of excess gas, if the clast had been near the parent body surface for some time. In contrast, chondrules from the clastic matrix would contain variable excesses, as they would have independent regolith exposure histories.

In collaboration with Knut Metzler of the University of Münster, doctoral student My Riebe and postdoc Liliane Huber analysed Murchison chondrules from clastic matrix and a single lithic clast, respectively, with Tom Dooley (Riebe et al., 2017b). All 26 chondrules from the lithic clast had essentially identical concentrations of cosmogenic He and Ne, corresponding to the 4π exposure age of Murchison (Fig. 3.14b). Also 23 of the 27 chondrules from the clastic matrix had the same concentrations as those from the pebble. The remaining four, however, displayed variable excesses of the same magnitude as previously reported by Antoine Roth (Roth et al., 2011). We therefore concluded that the cosmogenic noble gas record of the Murchison CM chondrite does not provide evidence for irradiation by energetic particles from the early Sun, but can be readily explained by substantial irradiation of a portion of its chondrules by GCR in the parent regolith. The isotopic composition of the cosmogenic Ne in the pre-exposed chondrules – typical of production by GCR – is also readily expected in the regolith scenario but requires an ad hoc assumption in the early Sun scenario, since SCR from the early Sun would have to have had a much harder energy spectrum than present day SCR. While we explicitly proved the parent body scenario for only one meteorite, it seemed likely to us that it could be extended to other regolithic meteorites for which large excesses of cosmogenic noble gases have been reported.

That was the situation in 2017 from our perspective. Perhaps our view was not as spectacular as one might have hoped, given that it is incontrovertible that our early Sun emitted an enormously high flux of solar energetic particles, as young stars do today. After all, as seen above, the presence of ¹⁰Be in early solar system condensates (McKeegan et al., 2000; Liu et al., 2009) is also widely accepted as evidence for a high SCR flux in the early solar system. Had we perhaps just missed the right samples? Yes, I think we did!

Philipp Heck of the Field Museum in Chicago, one of my former doctoral students, told me about the work of postdoctoral associate Levke Kööp at the University of Chicago on so called PLACs (Platy hibonite Crystals). PLACs display large nucleosynthetic isotope anomalies and are thought to have formed particularly early CAIs (Kööp et al., 2016). As mentioned above, they also display ¹⁰B excesses from the decay of early SCR produced ¹⁰Be (Liu et al., 2009), and were therefore primary candidates to search for early SCR derived noble gases. However, these grains are much smaller than typical chondrules and thus more difficult to analyse. The Tom Dooley mass spectrometer with its high sensitivity was therefore clearly the instrument of choice. Levke and her colleague Jennika Greer, also from the University of Chicago, came to Zürich to measure He and Ne in a series of PLACs from Murchison. To possibly distinguish effects from a hypothesised early SCR irradiation from regolith irradiation effects, they also measured spinel-bearing grains. These are also early solar system condensates but are somewhat younger than the PLACs. Spectacularly, Levke and Jennika found large excesses of cosmogenic ³He and ²¹Ne in a majority of the PLACs, while only one out of 15 spinel-bearing samples displayed such an excess (Fig. 3.14c). The latter could easily be explained by GCR irradiation in the Murchison parent regolith, but the contrasting much larger fraction of PLACs with excess gas clearly required another explanation. Obviously, the SCR produced ¹⁰Be in these very early condensates is accompanied by noble gases which also testify to an irradiation by energetic particles from the early Sun (Kööp et al., 2018).

To me this was a revelation. For almost three decades, I had repeatedly warned against interpreting excesses of cosmogenic noble gases in various meteorite samples as unequivocal evidence for irradiation by energetic particles from the “active early Sun”. I had pointed out that in many cases irradiation with “ordinary” GCR particles is not only a simpler explanation but also fits better the available data. I remain convinced that this is true for most of the work referred to above. But now, with the platy hibonites, I became involved in the discovery of what I believe is convincing evidence that noble gases produced by early intense solar irradiation indeed are also present in meteorites. You just have to look at the right samples!

The work on early solar system condensates just described is a good example of how measurements of very small samples benefit from (or were made possible in the first place by) the unique noble gas mass spectrometer “Tom Dooley”. Before presenting more studies based on “Tom”, it is time to present this unique instrument and the person who built it.

Heinrich “Heiri” Baur (Fig. 1.1 and Fig. 3.15) is best known internationally for the Baur-Signer ion source for noble gas mass spectrometers. To this day, this source is used in many laboratories around the world because of its remarkable linearity. Two of the world’s best known noble gas mass spectrometers at the Institut de Physique du Globe in Paris are even partially named after Heiri, along with three other authorities: “ARESIBO I and II” stands for Claude Allègre, John Reynolds, Peter Signer and Heinrich Baur (Moreira, 2013; I suspect that ARESIBO instead of ARESIBA reflects the fact that “Baur” is pronounced more or less like “Booor” in French). Heiri arrived in Peter Signer’s lab as a doctoral student a few years before I joined the group. After Peter’s retirement in 1994, the two of us took over as co-leaders of the group until Heiri’s retirement in 2010. The lab still benefits from his invaluable know how.

“Tom Dooley” is one of the mass spectrometers in our laboratory devised by Heiri. This instrument was primarily designed to measure terrestrial He with a very low ³He/⁴He ratio. This required high abundance sensitivity in the He region, i.e. the very small ³He peak had to be well separated from the much larger ⁴He peak. The flight tube therefore has an “appendix”, hosting a Faraday collector exclusively for ⁴He, whereas the other isotopes are measured by ion counting. To minimise vibrations, Heiri decided to mount the spectrometer on a heavy mass, and doctoral student Anselmo Pedroni organised a beautiful granite slab from a quarry in his home canton of Ticino. However, the ETH building department decided that 600 kg of stone was too heavy for our lab floor to carry. So we instead had to hang the slab with the mass spectrometer on it from the ceiling, which could support more weight than the floor. This inspired Heiri to name the instrument after the unfortunate hero of a song that used to be very popular not only in the US but also in Germany and Switzerland:

Hang down your head Tom Dooley Poor boy you’re bound to die

Wikipedia knows that Tom Dula was hanged in 1868 for alleged murder of the mother of their unborn child, and his legend seems to have survived in North Carolina to this day. I guess we were not aware of the whole story in the 1980s (Wikipedia did not exist yet). The fact is that our Tom Dooley (today mostly just “Tom”) is still alive and providing excellent data.

In its first life, Tom was used primarily to measure He (and tritium) in water samples and cosmogenic and mantle He and Ne in terrestrial rock samples, as will be discussed in the second part of this memoir below.

Later, Heiri Baur upgraded Tom into an essentially new and unique instrument. In a normal gas mass spectrometer, the gas to be analysed fills the entire flight tube plus (part of) the gas extraction and purification system. If the gas could be largely concentrated in the ionisation volume of the ion source, the sensitivity of the instrument could be greatly increased. Obviously, this would be of great benefit to the analysis of very small samples, in which we became increasingly interested, and would be particularly important for the lightest noble gases He and Ne, which have very low ionisation efficiencies due to their large ionisation potentials. One possibility was to concentrate the gas in the ion source region with a turbomolecular pump, placing the source at the high pressure end of the pump (Matsumoto et al., 2010). However, the achievable compression factor would not have been large enough to satify Heiri’s ambitions. He hence designed a molecular pump that pushes the gas very efficiently into the small ionisation volume of the source, as shown in Figure 3.15. To avoid grease, the pump’s rotor, which weighs several kilograms, needs magnetic bearings. This requirement had to be compatible with two other requirements to ensure sufficiently high compression efficiencies: the high thermal speed of He atoms implies a rotation frequency of 80,000 rotations per minute and a very small gap of about a tenth of a mm between rotor and stator. This required sophisticated control electronics to keep the magnetically levitated rotor in position with an accuracy of a few hundredths of a millimetre at 1400 rotations per second! Heiri achieved this in collaboration with a company led by a friend from his student days. Furthermore, to achieve the targeted compression factors, Heiri had to shrink his classical ion source by a factor of two in a linear dimension or a factor of eight in volume. Atoms lost through the source slit are efficiently recycled into the ionisation volume by differential pumping. All of this leads to very stable compression factors of about 200 and 100 for Ne and He, respectively, i.e. the gas pressure in the ion source becomes higher by about two orders of magnitude when the rotor is turned on. This allows sufficiently high count rates for gas amounts too small for precise analysis in conventional mass spectrometers, for example the single grain measurements presented in the previous section or low level tritium analyses in water samples (via the decay product ³He), as described in Section 5. The instrument is useful for He and Ne, for which a sizeable fraction of the atoms are ionised within an analysis time of typically 20 minutes. Krypton and xenon cannot be analysed in this instrument. Their halflives against ionisation would only be on the order of one or two minutes, too short for analysis of even one isotope of these elements, let alone the determination of isotopic ratios, which requires (time consuming) mass-peak jumping.

Like many other instrumental developments in our laboratory, Tom Dooley could not have been accomplished without the excellent technical staff in our research team, mentioned in the acknowledgements section. Since Heiri’s retirement, Colin Maden has been responsible as lab manager for technical developments and smooth functioning. Colin joined us after his dissertation in the AMS group at ETH.

One particularly successful area of research largely based on Tom Dooley has been the work on fossil meteorites and micrometeorites with Birger Schmitz at Lund University in Sweden and colleagues. Birger and I already knew of each other from a book on the accretion of extraterrestrial material that he co-edited with Bernhard Peucker-Ehrenbrink (Peucker-Ehrenbrink and Schmitz, 2001), but I first met him in person at the Meteoritical Society meeting in Rome on September 11, 2001. Nadia Vogel had just delivered her talk on the microdistribution of noble gases in chondrules and their rims when the afternoon session was cut short because of the terrorist attacks in the United States. At first, we had no reliable information about what had happened, so Birger asked Nadia and me to talk about the many fossil meteorites that were being recovered from a quarry in southern Sweden (Fig. 3.16). He imagined that if we were able to measure noble gases in individual chondrules, we might also succeed to detect them in individual chromite grains extracted from otherwise fossilised meteorites. Birger knew that the chromites had survived the fossilisation process. Therefore they might have retained their cosmogenic noble gases during the ~470 Ma since their fall on Earth. As I recall, we were not convinced that this would be possible, but Birger is a person of great enthusiasm and persuasion. So we decided to give it a try, perhaps also just for fun. We agreed that Birger would send us some chromite grains from a number of fossilised meteorites from different sediment strata in the quarry were they were found. After dinner we headed for our hotels and when I turned on the TV to watch the collapse of Manhattan’s Twin Towers, meteorites lost all interest for a while, for me and probably for everyone else at the meeting.

Yet, the first batch of chromite grains arrived in Zürich in 2002. Birger had handpicked them on many evenings, accompanied by his favourite opera recordings. The noble gas analyses of these grains marked the beginning of a long and fruitful collaboration. Besides Birger himself, Philipp Heck and Matthias Meier at our side were the two most important persons in this endeavour.

Both did their dissertations in my group and I have continued to work with both of them for many years after, as we will see below. Philipp now heads the famous meteorite collection at the Field Museum in Chicago, and is also affiliated with the University of Chicago, and Matthias has become director of the Natural History Museum in St. Gallen, Switzerland. Philipp’s results for the first sets of chromite grains extracted from fossil meteorites were spectacular, but before I go into that, I must say a few words about the significance of the fossil meteorites from southern Sweden. The story has been told by Birger more than once (e.g., Schmitz, 2013) and I am giving here only a much abridged and less authoritative version. It all started when Maurits Lindstöm found a fossil meteorite in a slab on a dump of a limestone quarry between Göteborg and Stockholm. Such meteorite-bearing slabs had been thrown away by the quarry workers because they were considered defective. However, once informed of the potential value of the fossil meteorites, the Thorsberg quarry owners and workers were very cooperative and, together with Mario Tassinari, carefully saved them. Mario (Fig. 3.16) was a collector of minerals, fossils, and almost anything else you can imagine, including old radios and corks from Italian and other wine bottles. His dedication to the search and preservation of the fossil meteorites was rewarded with a Doctor Honoris Causa from Lund University. Fossil meteorites were found in different strata within the quarried 3-4 metres, covering between one and two millions years of Mid-Ordovician geology. Birger concluded that the meteorites were not simply different pieces of a large fall, but represented many different falls, although all of them likely belong to the same class, the L chondrites. Large concentrations of extraterrestial chromite grains are also present in sediment strata in southern Sweden coeval to the fossil meteorite-rich beds at Thorsberg (Fig. 3.17; Schmitz et al., 2001). These grains and the fossil meteorites indicate that the meteorite flux in the Mid-Ordovician was between one and two orders of magnitude higher than today. Given the stratigraphic age of the sampled limestone of around 466 Ma, this suggested a connection to the break up of the parent body of the L chondrites, which is reflected in the K-Ar and U-Th-He ages of around 500 Ma of many L chondrites (Bogard, 1995). This link became even clearer when the Heidelberg group determined high quality ⁴⁰Ar-³⁹Ar ages for a series of L chondrites (Korochantseva et al., 2007), which dated the L chondrite parent break up (LCPB) at 470 ± 6 Ma, in agreement with the stratigraphic age of the meteorite-rich sediment layers. Therefore, the fossil meteorites represent a unique testimony of the largest known asteroid collision.

Could it be that noble gases in the chromites survived the meteorite fossilisation? If so, would noble gas analyses provide additional information about meteorite provenance and their journey to Earth? On the one hand, if we could show that all chromites contained cosmogenic noble gases, and in at least approximately similar concentrations for samples from one meteorite or from meteorites in the same stratum, this would indicate that potential gas losses must have been small. On the other hand, this would also be strong additional evidence that the individual meteorites were not just fragments of one very large fall, because, if so, inner pieces should have much lower concentrations of cosmogenic gases than samples from closer to the surface. When we started the project, we did not even dare to think of a third possibility: could it be that the concentrations of cosmogenic noble gases correlate with the age of the layers in which the meteorites were embedded? But all this proved to be true. Heck et al. (2004) showed that all chromite batches (a few grains each) contained cosmogenic ²¹Ne produced in space and that essentially none of it had been lost during the chromite’s almost 500 Ma residence time on Earth, although the rest of each meteorite had been diagenetically altered. This allowed the determination of the cosmic ray exposure ages of the chromite’s parent meteoroids (Fig. 3.17). The ages had considerable uncertainties because no shielding corrections could be applied and the Ne production rates from Cr were not very well constrained. But this was not a major problem. The main surprising observation was that the ages are very low for meteorite standards, ranging from about a hundred thousand to a million years. These unexpectedly low ages allowed us to document a trend with sediment age: meteorites from the lowest level in the quarry showed the shortest exposure ages, those higher up in the stratigraphic column, estimated to have been deposited 1-2 Ma later, had exposure ages about a million years higher than the ones at the bottom. This means that the first meteorites from the LCPB arrived on Earth within 100,000 years or even less, and that the high debris flow from this event lasted for at least a million years but probably longer. Such a short transfer time implies that the first debris from the break up was injected into an orbital resonance in the asteroid belt within a very short time.

Dynamical models of meteorite delivery from the asteroid belt to Earth predict that meteorites will first slowly drift into an orbital resonance and then possibly get ejected into an Earth crossing orbit and collide with our planet within a few million years at most (Gladman et al., 1997). The best known orbital resonances manifest themselves in the “Kirkwood gaps”, regions with low asteroid density. An example is the 3:1 Kirkwood gap at a semi-major axis of about 2.5 astronomical units, where an asteroid or meteorite orbits the Sun exactly three times in one Jovian year. The slow drift is the result of non-gravitational forces generated by asymmetric re-emission of thermal energy from solar illumination (the Yarkovsky effect; Bottke et al., 2006). Once in a resonance, orbital motions of bodies become chaotic, with large swings in eccentricity leading to a rapid transit to the inner solar system (Wisdom, 1987). Overall, the exposure ages of meteorites (usually in the range of a few to tens of millions of years) are thus dominated by their drift phase in the asteroid belt. The very short exposure ages of the fossil meteorites provided evidence supporting Jack Wisdom’s simulations of chaotic motions of particles in resonant orbits. They also showed that the LCPB occurred close enough to a strong resonance that collisional ejecta could directly reach a resonant orbit.

The trend of increasing exposure ages with decreasing statigraphic age must have convinced the sceptics who had doubted that the many fossil meteorites actually represent many different falls. I think that the 2004 paper by Philipp Heck and co-workers in Nature also has helped to establish the fossil meteorites in the minds of the planetary science community.

The LCPB has undoubtedly left its mark worldwide in the form of a global rain of meteorites. But a systematic search for macroscopic fossil meteorites (from the LCPB or other events) elsewhere on the planet will hardly ever succeed. The Thorsberg quarry is a remarkable stroke of luck, not only because it exploits exactly the right sediment layers, but also thanks to its team of exceptionally dedicated workers. However, even though a systematic search for macroscopic fossils is hopeless, sediment sequences on other locations on the globe coeval with those at Thorsberg may contain additional evidence of strong extraterrestrial influx from the LCPB. Extraterrestrial chromite grains dispersed in sediments might be good candidates, since in Sweden many such grains are also found in sediment strata of the same age as at Thorsberg. Meier et al. (2010) measured a suite of such grains from the Thorsberg quarry itself and to our surprise most of them contained solar wind noble gases. This implied that these grains were not remnants of macroscopic L chondrites, which rarely contain solar wind, but parts of micrometeorites that had been irradiated by the solar wind in space. Searching for sediment dispersed chromites from other places, Cronholm and Schmitz (2010) were successful at a location in central China deposited contemporaneously with the meteorite-rich strata in Sweden, and similar coeval chromite-rich sediments also are known near St. Petersburg in Russia. These grains as well have chemical compositions very similar to those of chromites from recent L chondrites. Matthias Meier, together with Carl Alwmark who joined our group as a postdoc after his PhD thesis with Birger in Lund, measured He and Ne in chromite grains from both sites (Alwmark et al., 2012; Meier et al., 2014c). Also these grains mostly contained solar wind noble gases and must, therefore, have arrived on Earth as micrometeorites. The solar noble gases overwhelmed cosmogenic noble gases in most grains, but a few of them allowed us to deduce cosmic ray exposure ages. Not so surprisingly any more, they were about as low as those of the chromites extracted from fossil meteorites (Meier et al., 2014c). This clearly showed that the LCPB brought to Earth not only macroscopic bodies, but also large amounts of cosmic dust, on a global scale. What consequences could this have had for our planet?

Schmitz et al. (2008) proposed that the LCPB accelerated the process known as the Great Ordovician Biodiversification Event (GOBE) by showing that the onset of the main phase of biodiversification coincided with the arrival of LCPB material on Earth around 470 Ma ago. They argued that the LCPB produced not only a high meteorite and micrometeorite flux on Earth, but also frequent impacts of km-sized bodies. The latter would have accelerated biodiversification. This work was challenged by Lindskog et al. (2017), who argued that significant diversification in Baltoscandia began some 2 Ma before the asteroid break up. However, Schmitz et al. (2019) showed that the LCPB coincided with a significant eustatic sea level fall attributed to an Ordovician ice age. In contrast to the 2008 paper, Schmitz and colleagues now proposed that extraordinary amounts of fine dust in the entire inner solar system caused by the LCPB cooled the Earth. This cooling could have been the tipping point that triggered the Ordivician icehouse conditions, which may have led to major faunal turnovers associated with the GOBE.

Birger Schmitz devotes much of his career to fossil meteorites and – more generally – to the effects of extraterrestrial matter accretion on our planet. I am pleased to be part of this endeavour, along with his many other friends and collaborators. Birger’s frequent visits to Zürich are internally known as “Apfelstrudel meetings”, as they usually end at a restaurant whose most important requirement is that it serves Birger’s favourite dessert. I also greatly enjoyed several short research visits in the beautiful city of Lund, including a trip to the Thorsberg quarry, a few hundred km further north.

Tom Dooley has extensively been used to measure He and Ne in individual presolar grains, which are certainly among the most fascinating objects in meteorites (Fig. 3.18). As their name implies, presolar – or circumstellar – grains predate the solar system. They condensed in the outflows of stars in the last stages of their evolution and found their way into the nascent solar system without having been melted or vapourised. Therefore, every presolar grain carries information about the evolution of its parent star, about galactic chemical evolution, and about dust formation in stellar environments (Zinner, 2014; Nittler and Ciesla, 2016). In particular, the very large isotopic anomalies displayed by almost every element studied in presolar grains contributes significantly to our understanding of element synthesis in stars, a line of research aptly named “astrophysics in the laboratory”. Over the past nearly four decades, many different types of presolar grains have been identified. Besides the tiny, nanometre-sized diamonds discovered by Lewis et al. (1987), the most important are micrometre-sized silicon carbide (SiC) and graphite grains (Zinner, 2014). Individual grains are amenable to noble gas analyses, as is addressed next.

It is striking that presolar grains often contain a substantial fraction of the noble gas budget of bulk meteorites. Therefore, noble gases are ideal to trace enrichments or depletions of presolar grains in separated phases of meteorites. Much of the pioneering work on this subject was done at the University of Chicago and Washington University in St. Louis. Carbonaceous presolar phases (diamonds, SiC, and graphite) were identified by a series of acid dissolution steps undertaken to eliminate silicates and other acid-soluble phases (e.g., Lewis et al., 1987; Amari et al., 1990; see review by Anders and Zinner, 1993). This technique, which has become known as “burning down the haystack to find the needle”, was guided by noble gas analyses. In fact, the first ever isotopic anomalies in meteorites were reported for noble gases. These were radiogenic ¹²⁹Xe resulting from the decay of short lived ¹²⁹I in the early solar system (Reynolds, 1960) and primordial Xe-HL, which is enriched in the heaviest (H) as well as the lightest (L) isotopes (Reynolds and Turner, 1964).

However, while noble gases have been instrumental in detecting presolar grains, the minute concentrations of noble gases pose a serious challenge for single grain analyses, much more so than for elements that can be measured by SIMS and other techniques. Even so, we were able to detect nucleosynthetic ⁴He and/or Ne isotopes in a minor fraction (typically 10–20 %) of the presolar graphite grains we studied (Heck et al., 2009a; 2018; Meier et al., 2012). For some of these grains we could assign a stellar source such as so called Asymptotic Giant Branch (AGB) stars or core collapse supernovae.

Perhaps the most rewarding aspect of our work on single presolar grains is that we have been able to detect cosmogenic noble gases in single SiC grains. Cosmogenic noble gases provide an estimate of the presolar ages of presolar grains, a task otherwise not (or not yet?) possible. Conventional radiometric dating is hindered by the very small sample size and the highly anomalous isotopic composition of essentially every element in presolar SiC. The alternative is to determine the length of time the grains were exposed to galactic cosmic rays in the interstellar medium, which is best done with cosmogenic noble gas isotopes (Tang and Anders, 1988; Lewis et al., 1994). The first ages provided by the Chicago group turned out to be unreliable because large fractions of the cosmogenic gases get immediately lost from the tiny grains by recoil (Ott and Begemann, 2000). We have therefore focused on some of the largest known presolar SiC grains where recoil corrections are smaller. Such grains, ranging in size from a few to tens of micrometres, had been separated from the Murchison meteorite by Sachiko Amari and co-workers at the University of Chicago. Most of the grains contained measureable amounts of cosmogenic ³He and/or ²¹Ne, corresponding to highly variable presolar exposure ages of between ~5 and 1000 Ma (Heck et al., 2009b). These ages had large uncertainties due to inaccurate grain volume estimates, recoil corrections, and uncertain production rates of GCR nuclides in the interstellar medium outside the heliosphere. Gyngard et al. (2009) reported mostly considerably higher ages on other large SiC grains from Murchison, based on analyses of lithium isotopes with the Washington University NanoSIMS. So next we attempted a joint analysis of Li and noble gases on the same grains, again large SiC grains from Murchison. Philipp Heck, now at the Field Museum in Chicago, analysed He and Ne in Zürich with “Tom”, along with his colleagues Jennika Greer and Levke Kööp. Reto Trappitsch of Lawrence Livermore National Laboratory calculated new values for interstellar production rates of noble gases, based on an interstellar GCR spectrum derived from data collected by NASA’s Voyager 1 spacecraft at the edge of the heliosphere. As it turned out, these values differ only slightly from the old ones provided by Bob Reedy thirty years earlier (Reedy, 1989). Most grains have presolar exposure ages not exceeding some 300 Ma, with some values being as low as a few Ma. However, a few grains have exposure ages exceeding a billion years, with possible values as high as about 3 billion years (Heck et al., 2020; Fig. 3.18). The uncertainties are substantial (see the inset in Fig. 3.18), but we nevertheless believe that these ages are reliable thanks to improved recoil corrections compared to those for the smaller grains in the pioneering Chicago studies. As already suggested by Heck et al. (2009b), we reiterated in our 2020 paper that the majority of the grains with a presolar age below 300 Ma may have originated from stars born during an episode of enhanced star formation roughly 7 Ga ago proposed by astrophysicists. Philipp Heck issued a press release about this work, in which he also noted that we had dated the so far oldest material ever. This statement immediately made headlines everywhere. It was in the news worldwide for at least a day or two, according to BBC attracting even more attention than the then latest gossip about Prince Harry and his wife Meghan. Even two years later, many internet sites still showed the picture of one of the SiC grains we analysed. Unfortunately, the lithium ages of the grains turned out to be too inaccurate to be useful because of the very large contributions of non-cosmogenic lithium.

Although cosmic ray produced nuclides are important tools in cosmochemistry, sometimes one would prefer to study samples whose isotopic composition has not been altered by cosmic ray interactions. Noble gas researchers have struggled with this problem for decades, since cosmogenic noble gases in meteorites often dominate or even completely overwhelm other noble gas components that may be of primary interest. As I will discuss in the next section on primordial noble gases, it is therefore common practice to separate cosmogenic noble gases from other components to the extent possible, either during sample preparation or during noble gas analysis. Examples include separation of presolar components by chemical means and gas extraction by stepwise heating. For most other elements, changes in isotopic composition caused by cosmic rays are so small that they could be largely ignored for a long time. This changed in recent decades, especially with the advent of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS, even the acronym is a tongue twister), which has enabled ultrahigh precision isotope analyses for a large number of elements of cosmochemical interest (e.g., Halliday et al., 1998; Albarède et al., 2004). Today, cosmogenic effects are considered for several elements of cosmochemical interest, particularly for iron meteorites with their very long cosmic ray exposure ages (Ek et al., 2020) and some lunar samples. In some cases, noble gases play an important role in such corrections, as I will explain next on the example of tungsten, which I became involved with shortly after Alex Halliday joined ETH in 1998.

Alex and Der-Chuen Lee (“call me Albert”) had made pioneering tungsten isotope measurements on lunar and meteorite samples at the University of Michigan in Ann Arbor (Lee and Halliday, 1997). These made it possible to date early metal silicate fractionation, e.g., lunar and asteroidal core formation. However, it was recognised that in some samples the measured ¹⁸²W/¹⁸⁴W ratio reflected not only the decay of radioactive ¹⁸²Hf (T_1/2 = 8.9 Ma), but also a cosmogenic contribution, mainly from the capture of secondary cosmic ray neutrons by ¹⁸¹Ta. This called for a reassessment of the timing of lunar core formation proposed by Lee and Halliday. Therefore, Ingo Leya modelled the production and “burnout” of W isotopes by cosmic ray interactions. Apart from the radiogenic isotope ¹⁸²W, the other tungsten isotopes also had to be considered, as they are used to correct for instrumental mass discrimination in MC-ICP-MS. Leya et al. (2000c) showed that, indeed, in many lunar samples neutron capture on Ta has caused a large part of the observed ¹⁸²W excess, since these samples were often exposed to cosmic rays for hundreds of Ma. However, for some lunar samples with much lower exposure ages (or perhaps much lower neutron fluences if they were mostly close to the surface of the regolith) only a minor or even negligible fraction of the ¹⁸²W excess was deduced to be cosmogenic. These results implied that the formation time of the Moon 50 Ma after CAI formation derived by Lee and Halliday (1997) did not need to be corrected due to cosmic ray effects. Leya et al. (2000c) also concluded that the W isotopic composition of Martian meteorites was not substantially altered by cosmic rays because they have lower Ta/W ratios and much shorter exposure ages than lunar samples.

Ingo Leya also assessed cosmic ray induced variations on isotopic compositions of different extinct nuclide systems, including again ¹⁸²Hf-¹⁸²W (Leya et al., 2003). In this paper he used Gd and Sm isotope shifts as proxies for the secondary cosmogenic neutron fluence in a sample. This approach improves the neutron dose estimate based on the noble gas derived cosmic ray exposure age which incompletely reflects the shielding of a sample during exposure. Although this allowed a better correction for ¹⁸²W, Ingo also showed that cosmic ray effects are negligible for ⁹²Zr and ^98,99Ru. Potentially detectable shifts on ⁵³Cr can be corrected via⁵⁴Cr/⁵²Cr.

Apart from the work with Alex and Ingo, I was also involved in a number of studies on cosmogenic effects in iron meteorites, in collaborations with Ghylaine Quitté, Agnès Markowski, Peter Sprung, Thorsten Kleine, and Thomas Kruijer. Iron meteorites are well suited to date asteroidal core formation with the Hf-W system, as pioneered by Charles Harper (Harper et al., 1991). Hafnium-tungsten ages indicated that many asteroids segregated their iron core very early, within one or a very few million years of CAI formation (Kleine et al., 2005), a finding confirmed with very precise absolute Pb-Pb ages (Blichert-Toft et al., 2010). It is therefore very important to date the core formation of differentiated asteroids very accurately and compare these dates with formation ages of chondritic parent bodies. However, the typically very old exposure ages of hundreds of Ma of iron meteorites require a reliable correction of the tungsten isotopic composition for cosmic ray effects. This is further complicated by the fact that knowing the cosmic ray exposure age of an iron meteorite is often insufficient for an accurate correction. Many iron meteorites have large pre-atmospheric sizes and thus can exhibit large variations in their cosmogenic W concentrations from sample to sample, because cosmic ray neutron fluences first increase steeply with distance from the pre-atmospheric surface before they may decrease again towards the centres of very large meteoroids. The bars cut out of the slabs of the large meteorites Grant and Carbo which Peter Signer had studied in Minneapolis in the 1960s to develop his model of cosmogenic noble gas production in iron meteorites (Section 3.1.3) are still kept at ETH (Fig. 3.20). We used these samples to study to what extent the expected depth dependence of cosmogenic W production can be experimentally verified.

The noble gas concentration data provide a good indication of the location of the pre-atmospheric centres of the two meteorites. Agnès Markowski, doctoral student of Alex Halliday, and her advisor Ghylaine Quitté showed that for both meteorites the ¹⁸²W/¹⁸⁴W ratio indeed correlates well with the inferred distance of the samples from the pre-atmospheric centre (Markowski et al., 2006; Fig. 3.20). Unlike lunar samples, where ¹⁸²W is produced by neutron capture on ¹⁸¹Ta, in iron meteorites neutron capture depletes ¹⁸²W by the reaction ¹⁸²W(n,γ)¹⁸³W. Hence, the ¹⁸²W/¹⁸⁴W ratio near the pre-atmospheric centres of Grant and Carbo is lower than near the surface by ~0.3-0.5 ε -units (1ε= 1 part in 10⁴). Thus this work nicely demonstrated that the W isotopic composition in iron meteorites is affected by cosmic ray interactions and that the magnitude of the required correction varies with the pre-atmospheric shielding of the samples.

Since then, samples from the Grant and Carbo bars have been used to study effects of cosmic ray interactions on several other elements of interest in isotope cosmochemistry (e.g., Qin et al., 2015; Ek et al., 2020). The bars therefore now resemble a mountain panorama (Fig. 3.20).

Unfortunately, in most cases the shielding is not as well constrained as in Grant and Carbo. Historically, ^3,4He/²¹Ne ratios have been used for this purpose, but they are not very reliable proxies for the relevant cosmogenic neutron fluxes. Since ³He is mainly produced by high energy protons, its production is highest near the pre-atmospheric surface, whereas the fluxes of (slow) secondary neutrons increase to a certain depth (leading to the anticorrelation of ³He and burnout of ¹⁸²W indicated in Fig. 3.20). Thomas Kruijer tackled this problem by two different approaches. He had joined ETH in 2009 as doctoral student supervised by Thorsten Kleine and myself. First, Thomas analysed a series of the – unfortunately very rare – iron meteorites with known relatively low cosmic ray exposure ages. They could not only be expected to have (almost) negligible burnouts of ¹⁸²W but also marginal cosmogenic effects on other W isotopes used to correct for instrumental mass fractionation. Thomas showed that the Hf-W ages of those meteorites requiring essentially no corrections all postdated CAI formation, mostly by between 1-2 Ma (Kruijer et al., 2012). This was a very important result because it ruled out earlier reports that some iron meteorites were even older than CAIs. The same conclusion had already been reached by Burkhardt et al. (2012), as will be discussed in Section 3.1.12. However, consistent with the earlier reports, Thomas concluded that the accretion and differentiation of many iron meteorite parent bodies preceded the accretion of most chondritic asteroids.

The second approach developed by Thomas Kruijer was to correct W isotopes by establishing a proxy for the fluences of cosmic ray neutrons in individual samples. Platinum isotopes capture neutrons efficiently, and Pt isotopic abundances are also modified by neutron capture reactions on Ir. As part of his doctoral thesis, Thomas measured Pt and W isotopes in many iron meteorites of various classes in Thorsten Kleine’s laboratory at the University of Münster, after Thorsten moved from ETH to Münster. The variations in Pt isotopic compositions agreed very well with model calculations by Ingo Leya, which also take into account the Ir/Pt ratio of meteorites. Therefore, Pt isotopes are a very reliable tool for quantifying neutron capture effects on W isotopes (Kruijer et al., 2013). This is shown in Figure 3.22, which displays Pt and W data of the two IID iron meteorites Carbo and Rodeo, which have almost identical Ir/Pt ratios. The correlation of Pt and W isotopic compositions defined by the Carbo data (which are affected by cosmogenic effects) extrapolate perfectly to the measured data of the weakly irradiated Rodeo (at ε¹⁹²Pt ~0). The model calculations also agree well with the measured correlation in Figure 3.22. This work was a major advance in dating asteroidal core formation by Hf-W. In principle, platinum (and Ir) data from a single sample of an iron meteorite allow a reliable correction of its W isotopic composition for cosmogenic effects without the need for independent shielding information. Kruijer et al. (2013) definitively showed that there was a time gap of at least ~1 Ma between CAI formation and metal segregation in the parent bodies of the iron meteorite groups they studied (IID, IVA, IVB). The study also confirmed earlier conclusions that the accretion of these differentiated bodies predates that of most chondrite parent bodies. Since then, Thomas Kruijer is contributing further notable studies on the dating of early solar system events.

Numerous discussions and several joint projects with colleagues and friends, mostly in the groups of Alex Halliday, Thorsten Kleine, Bernard Bourdon, and Maria Schönbächler, helped me broaden my horizon in isotope cosmochemistry of non-noble gas elements beyond the topic of correcting for cosmogenic effects.

Thomas Kruijer and colleagues studied nucleosynthetic tungsten isotope anomalies in CAIs. Fine grained inclusions have variable abundances of W isotopes from s- and r-process nucleosynthesis, whereas coarse grained CAIs have hardly any nucleosynthetic W anomalies (Kruijer et al., 2014). Correcting the radiogenic ¹⁸²W abundances for nucleosynthetic isotope anomalies resulted in Hf-W ages of angrites in good agreement with their Al-Mg ages, which, contrary to other claims, argues against a heterogeneous distribution of ²⁶Al in the inner solar system regions where CAIs and angrites formed.

Mathieu Touboul measured tungsten isotopes in metal samples from the Moon that are essentially free of Ta derived cosmogenic W and therefore allow more reliable Hf-W ages than non-metal samples (Touboul et al., 2007). In a second paper, Mathieu measured W in lunar ferroan anorthosites (Touboul et al., 2009). In both papers he concluded that the Moon formed no earlier than some 60 Ma after CAI formation, later than previously thought. This work resolved a discrepancy with ¹⁴⁷Sm-¹⁴³Nd ages that had previously suggested late lunar formation.

Christoph Burkhardt and colleagues studied molybdenum and tungsten isotopes in detail in the Murchison carbonaceous chondrite and many other meteorites. Different leach fractions of Murchison indicated a heterogeneous distribution of W isotopes from s- and r-process nucleosynthesis, implying the presence of at least two different carriers of nucleosynthetic W (Burkhardt et al. 2012). These data allowed Christoph to propose an improved correction of the ¹⁸²W abundance for non-radiogenic anomalies, resulting in a decrease of the ages of iron meteorites by about two Ma, which solved the problem that iron meteorites had suggested that some planetesimals differentiated into core and mantle prior to CAI formation. As noted in the previous section, this puzzle has been further studied by Thomas Kruijer by correcting W isotopes for cosmogenic contributions. Burkhardt et al. (2011) reported Mo isotope data in a wide range of samples ranging from CAIs, chondrites, and differentiated meteorites to samples from Mars and the Earth. These data showed that the Earth must have accreted from material different from any of the meteorites. As Burkhardt (2021) explains, this is also evident from isotope data of other elements such as Zr, Cr, Ti, etc., and indicates that the Earth accreted from material belonging to the so called NC reservoir defined by non-carbonaceous meteorites, but most likely sunwards of the known NC representatives. Paul Warren (Warren, 2011) first pointed out the marked dichotomy between the two reservoirs typified on the one hand by carbonacous chondrites (CC), representing material accreted in the outer solar system, and the NC reservoir formed nearer the Sun on the other.

I very often discussed isotope cosmochemistry with Alex Halliday, although we never had any joint papers beyond those on cosmic ray effects discussed above. I remember particularly well the afternoon when Alex – apparently slightly stressed – told me that he needed my immediate help. That evening he had to submit a manuscript of a paper on the origin of the Moon (Halliday, 2000), but he was uncomfortable with having to repeatedly talk about “the Giant Impactor”. He wanted a more catchy name for that body which would hopefully be embraced by the community but he did not have time to think about a suitable option. So he asked me to propose a name – preferably from mythology – that would evoke things like fire, impacts, thunder and the likes. I considered this an excellent idea and a good challenge for me and so I immediately set to work with the help of the internet which was still quite new at the time. The task turned out to be not that easy, however. Either I, or Alex, or both of us were unhappy with my first suggestions. Hawaiian gods or goddesses like Pele were taboo, since they were reserved for Jupiter’s moon Io, and norse gods like Wotan or Thor did not appeal to Alex. Next I resorted to Greek mythology, but Hephaistos, the Greek god of forge and fire, again appealed neither to Alex nor me. So, already slightly embarrassed, I searched the family tree of Selene, the Moon goddess, and found her mother: Θεία or Theia, one of the twelve titans. I insisted, and Alex finally accepted that name, even though, according to Wikipedia, Theia is not associated with fire but rather with the bright blue sky, precious stones and precious metals. The name indeed stuck. Soon after Alex’s paper was published, “Theia” became synonymous for the Giant Moon forming impactor. At the time we did not delve deeper into Selene’s family tree, so only now, as I write this, do I realise that Theia is the daughter of Gaia. Hence our Moon seems to be both granddaughter and child of the Earth. Whether Alex and I would have been bothered by this if we had noticed it at the time, I cannot tell, but Greek mythology and the naming of planetary bodies is not perfectly plausible anyway. For example, Helios is another son of Theia, so a brother of Selene, which would not be very convincing in a planetary science context.

Once again, I take a leap back in time to my doctoral thesis and the years that followed. The 1970s were a wonderful time for noble gas geochemists, allowing us to explore the world of the many noble gas “components” trapped in meteorites that testify to the history of the early solar system and even presolar system processes. As early as 1964, Xe-HL was discovered by Reynolds and Turner as the first nucleosynthetic component in meteorites ever recognised (Fig. 3.23). A few years later, stepwise heating and physical separation techniques of bulk meteorites revealed a Ne component strongly enriched in ²²Ne compared to solar wind Ne and also compared to the major Ne component in bulk carbonaceous chondrites. This component was termed Ne-E at the time (Black and Pepin, 1969; Eberhardt, 1974). Such work provided clear evidence that presolar solids had survived in meteorites. In search of these, Roy Lewis and co-workers in Ed Anders’ team at the University of Chicago realised that most of the primordial heavy gases Ar-Xe are concentrated in a minor phase that survived the dissolution of bulk meteorites by HF and HCl but lost its noble gases when attacked by oxidising acids such as HNO₃ (Lewis et al., 1975). They named this component “Q” for “Quintessence” (Fig. 3.23) and the carrier Q was identified in 1981 as a carbonaceous phase by Uli Ott in Berkeley (Ott et al., 1981). Xe-HL is a component substantially enriched in the heaviest (“H”) as well the lightest (“L”) isotopes and its carrier was identified as presolar nanodiamonds in Chicago in 1987 (Lewis et al., 1987). The ²²Ne-rich gases eventually turned out to have more than one origin. In one of them, pure ²²Ne was brought to the solar system in presolar graphite grains as the decay product of extremely short lived 22Na (T_1/2 = 2.6 a). In the other, presolar SiC grains carry a nucleosynthetic component with a ²⁰Ne/²²Ne ratio <0.1, i.e. about 100 times lower than atmospheric or solar wind Ne. The study of the multitude of primordial noble gas components in meteorites – of which there are many more than mentioned in this paragraph – is highly interesting for noble gas aficionados. But the field is also a nightmare for the uninitiated (and sometimes even for those of us who consider ourselves to be specialists). For comprehensive reviews on these issues I recommend Ott (2002; 2014), and Wieler et al. (2006). As Ott (2014) puts it succinctly: “The exact origin and the history for most of the trapped components … still present a puzzle”. For example, it is unclear why in Xe-HL the lightest isotopes (¹²⁴Xe, ¹²⁶Xe), which are only produced by the p-process, and the heaviest isotopes (¹³⁴Xe, ¹³⁶Xe), which are only produced by the r-process, are so intimately mixed that a separation between the Xe-H and Xe-L branches has been at best only marginally achieved (Meshik et al., 2001).

In the following, I will focus mainly on the Q component, on which we have concentrated much of our own work, in particular through CSSE. Component Q accounts for the vast majority of the primordial Ar, Kr, and Xe in primitive meteorites and for a minor but important fraction of their primordial He and Ne. This does not, however, mean that the Q component is well understood, as we will see next. To quote Ott (2014) again: “[the puzzle] in particular holds for the Q component, …. although it is often thought to be derived from a gas of originally solar composition”.

As noted at the beginning of Section 3, the exciting discoveries I mentioned in the previous paragraphs only marginally attracted my attention during my doctoral studies. As I recall, I did not take off my blinders until we started thinking about what else our unique CSSE line could be used for besides analysing noble gases from the solar wind. The answer seemed fairly obvious. The groundbreaking discovery of phase Q by Ed Anders’ group in Chicago had relied on the difference between the gas concentrations in an “original” and an oxidised HF/HCl residue of a meteorite. Argon, Kr, and Xe concentrations of the oxidised (e.g., HNO₃-treated) residues were much lower (and had a substantially different isotopic composition) than those of the original HF/HCl residues. In contrast, concentration differences for primordial He and Ne were much smaller than those for Ar, Kr, and Xe, only 10 % or so. While the HF/HCl residues accounted for about one percent of the starting mass of the bulk meteorite sample, oxidation mostly resulted in very little further mass loss (but see also the paragraph after next). Therefore, component Q removed by oxidising acids was thought to represent very little mass with extremely high concentrations of mainly the heavy noble gases. Naturally, it was highly desirable to measure Q’s noble gases directly, rather than deriving their concentrations and isotopic compositions by difference. This is perhaps less urgent for the heavy noble gases but is very important for He and Ne, whose concentrations and compositions in Q had to be calculated as the small difference between two almost equal numbers. We expected that we could characterise the Q component much better by oxidising HF/HCl residues in vacuo with HNO₃. If so, the CSSE line would not only allow stepwise analyses of noble gases implanted near grain surfaces, but would also be an excellent tool to selectively target phases of different susceptibilities to acid dissolution.

I approached Ed Anders and he and Roy Lewis kindly provided us with HF/HCl-resistant residues from the two carbonaceous chondrites Allende and Murchison. We measured several aliquots of both meteorites in the two CSSE lines in use then, one made of HNO₃-resistant glass and the other a more sophisticated line whose acid exposed parts consisted of only gold and platinum (Section 2.5.1). We were immediately successful; the data were of better quality than we could ever have dreamed. We determined a precise value for the isotopic composition of Ne-Q and also He-Q (Wieler et al., 1991). As shown in Figure 3.24, the CSSE data provided a much better separation of Ne-Q from other Ne components than had been obtained by stepwise heating at Caltech (Smith et al., 1977). The CSSE data left no doubt that the Q component contains not only the three heavy noble gases but also some He and Ne. This had been disputed by Sabu and Manuel (1980), who used this putative absence as an argument in favour of their hypothesis that the solar system formed from the debris of a single supernova. Despite my confidence in the quality of our work, I was a little nervous when I first presented the Allende data at the 1989 LPSC. So all the more I was proud of the compliment by Jerry Wasserburg after my presentation, as described in Section 2.3. The next paper on Murchison (Wieler et al., 1992) largely confirmed the Allende data but also showed that etching of HF/HCl residues by HNO₃ also released some Ne-E from presolar graphite and silicon carbide. This showed that the operational definition of phase Q (resistant to HF/HCl but not to HNO₃) is incomplete, although it remains of practical use. Murchison also provided a precise, directly measured isotopic composition of the Q-Xe component, which is slightly different from AVCC-Xe (AVerage Carbonaceous Chondrite), widely used at the time as representing primordial Xe in primitive meteorites. In fact, AVCC-Xe is not a pure component sensu stricto, but a mixture which in addition to Xe-Q also contains Xe-HL plus two additional minor components characterised first by Gary Huss and Roy Lewis (Huss and Lewis, 1994).

Henner Busemann’s doctoral thesis work was the next major contribution of our group to noble gases in the Q component. Henner found his way to Zürich thanks to a flyer I had asked Rolf Michel in Hannover – his diploma thesis supervisor – to post on the bulletin board of his institute. After several postdoctoral and research scientist positions, first in the capital cities of Bern and Washington DC, then in Milton Keynes (narrowly missing the British capital) and Manchester, he found his way back to ETH in 2014, where he now leads the noble gas laboratory in its third generation. During all these years, we continued to work together, as we will see below.

In his dissertation, Henner studied HF/HCl-resistant residues of four carbonaceous and two unequilibrated ordinary chondrites using CSSE with nitric acid (Busemann et al., 2000). Individual runs comprised up to 30 steps with in vacuo etch durations between a few hours and up to almost two weeks per step (Fig. 3.26)! With this huge data set, Henner presented a uniquely detailed characterisation of noble gases in Q. Among other findings, he noted a quite variable Ne isotopic composition of Q gases (²⁰Ne/²²Ne varying between ~10.1–10.6; Allende and Murchison are thus at the upper end of this range, see Fig. 3.24).

In Mainz, Schelhaas et al. (1990) also had observed a relatively low (²⁰Ne/²²Ne)_Q value based on stepwise combustion (heating in the presence of oxygen) of a HF/HCl residue from an unequilibrated ordinary chondrite. Henner further provided evidence that phase Q actually consists of two subphases Q1 and Q2 with affinities to different presolar carrier phases and he showed that variable elemental abundances of Q gases reflect both losses from an originally incorporated carrier by thermal metamorphism and aqueous alteration processes.

Many workers have studied phase Q – both the carrier and its associated noble gases – and many continue to do so. Although there has undoubtedly been much progress since the 1970s, almost 40 years after Lewis et al.’s discovery in 1975 most publications on the subject still refer to Q as “enigmatic” or include statements such as “the precise characterisation of phase Q has eluded decades of investigation”. Unlike for presolar SiC, graphite, or nanodiamonds, there are no electron microscope images that unambiguously show phase Q. Lewis et al. (1975) had chosen the name “Quintessence”, because they realised that a marginal loss of mass, together with an almost complete loss of Ar-Kr-Xe upon oxidation of HF/HCl residues, imply an extremely gas-rich carrier which they could not further characterise at that time. Actually, the Chicago group had borrowed the Quintessence term from Papanastassiou and Wasserburg (1971) who used it to denote a minor glassy phase in an Apollo 12 rock highly enriched in trace elements such as K, Rb, and U, related to KREEP rocks later found at the Apollo 15 site (Hubbard et al., 1971). “Quinta essentia” is latinised for the fifth element postulated by Aristoteles alongside the four traditional elements. He assumed that what he called “ether” was immutable and timeless, unlike the classic four elements which can transform into each other. While ”Quintessence” seems to have fallen out of use in lunar science, the term – or at least the acronym Q – persists in noble gas cosmochemistry, although whether or to what extent the Q noble gases were derived from other noble gas components remains an open question. Today, it is widely accepted that phase Q in chondrites is carbonaceous (e.g., Ott et al., 1981), although sulfides have also been proposed as a second carrier (Marrocchi et al., 2015). The most comprehensive recent characterisation of phase Q has been performed by Sachiko Amari at Washington University in St. Louis, Jun-ichi Matsuda at Osaka University, and colleagues. They studied in great detail the L4 chondrite Saratov, which is particularly well suited for this work because it contains Q noble gases but hardly any primordial gases from presolar SiC and nanodiamonds (e.g., Amari et al., 2013; Matsuda et al., 2016). They reiterated that Q is likely only a minor part of the porous carbon of Saratov, a conclusion already made by Schelhaas et al. (1990) in Mainz. Amari, Matsuda, and colleagues note that Q apparently does not differ in structure and chemistry from the 99 % of other porous carbon phases. Q gases may be released simply by a restructuring, rather than by dissolution of the carbon during oxidation. This would imply that the (usually very minor) mass loss upon oxidation is not a measure of the mass of Q, as already suggested by Ott et al. (1981) and Busemann et al. (2000). Moreover, noble gases with isotopic compositions similar or identical to Q gases in carbonaceous and ordinary chondrites are also ubiquitous in carbon-rich phases in achondrites (ureilites, acapulcoites-lodranites) and even in carbon nodules in iron meteorites (Wieler et al., 2006). Whether the Q gases in these meteorite types are transported in a carrier phase identical or similar to that in chondrites is unknown but doubtful.

The origin of Q gases is as enigmatic as their carrier. A first order observation is that they are highly fractionated in their elemental and isotopic compositions relative to solar noble gases, favouring the heavier elements and isotopes. Thus, there is most likely a connection between the noble gases in the Sun and the Q gases in meteorites. But where and by what process did the fractionation occur? In a presolar environment or later in the solar system? Wieler et al. (2006) review some of the many proposed environments and mechanisms. A presolar origin may more easily explain than a “local” solar system origin the fact that Q gases are so ubiquitous in all primitive and some differentiated meteorite classes. Also the apparent close association of Q gases with presolar dust seems to favour a presolar origin (Huss and Alexander, 1987). An example of a local origin was proposed by Bob Pepin (Pepin, 1991). He explored how the blow off of transient atmospheres of early planetesimals may have fractionated an original solar type noble gas inventory toward Q composition. Also Ozima et al. (1998) suggested that Q gases may have formed in the solar nebula, as a result of some sort of Rayleigh distillation. Numerous laboratory experiments have attempted to simulate gas trapping by phase Q. Based on experiments in which material was deposited on fresh surfaces in the presence of noble gases, Hohenberg et al. (2002) suggested that such “active capture” of noble gases transiently adsorbed on the fresh surfaces might play a role in the trapping process of Q gases. In a series of experiments, Maïa Kuga, Yves Marrocchi, and co-workers at CRPG in Nancy simulated the trapping of noble gases on organic substances in a plasma (e.g., Kuga et al., 2017). The elemental and isotopic fractionation patterns of the trapped gases were consistent with those expected if the Q component is mass fractionated solar gas. However, Kuga and co-workers also note that trapping efficiency of Xe in laboratory experiments is orders of magnitude lower than would be necessary to trap Xe-Q in a canonical solar nebula environment. Jamie Gilmour at the University of Manchester (Gilmour, 2010) explains Q-Xe as mass fractionated solar Xe to which traces of components known to be present in presolar grains were added. It should be noted, however, that the (unknown) isotopic composition of solar Xe might be significantly different from the known composition of Xe in the solar wind (Ott, 2014), such that Q-Xe might actually not be as different from Xe in the Sun as is usually assumed.

Since Lewis et al. (1975) discovered that primordial noble gases in meteorites overwhelmingly reside in HF/HCl-resistant residues that represent only a very small fraction of a meteorite’s total mass, primordial noble gas research had largely focused on such residues. This trend intensified when bona-fide presolar phases such as nanodiamonds, SiC, and graphite were separated from these residues (Lewis et al., 1987; Tang et al., 1988; Amari et al., 1990). We also had used the CSSE technique to study meteoritic HF-HCl residues. However, the well known fact that the primordial noble gas budget is dominated by the acid resistant carriers did not preclude the existence of acid soluble presolar carriers that may yield important information about stellar nucleosynthesis or the history of meteorites and their parent bodies. This became evident when presolar silicates were found by ion imaging of bulk meteorite samples with a NanoSIMS instrument that allowed spatial resolution on a sub-micrometre scale (Nguyen and Zinner, 2004). A comprehensive characterisation of the primordial (and radiogenic) noble gas inventory of meteorites would need to include HF/HCl soluble phases.

Such a study using bulk meteorites would face the difficulty of clearly discerning primordial noble gases against a potentially overwhelming background of cosmogenic gases released from “normal” (primordial noble gas-free) silicates and oxides. We believed that the closed system stepwise etching technique should also be promising in this respect and probably superior to stepwise pyrolysis and combustion. Henner Busemann and colleagues had made pioneering CSSE studies on bulk enstatite chondrites (Busemann et al., 2003; King et al., 2013), so we decided to study next a bulk sample of Ivuna, one of the only five known CI chondrites (Riebe et al., 2017a). My Riebe was the last doctoral student I had the pleasure to supervise, together with Henner. My had written her Master’s thesis in Lund with Birger Schmitz on the possibility of detecting cosmic ray tracks in extraterrestrial chromite grains from terrestrial sediments (Section 3.1.9) and I first met her when Birger invited me for a research stay in Lund. After postdoctoral work at the Carnegie Institution of Washington DC, My is now back at ETH supervising her own first doctoral student.

A few percent of both primordial Ne and Xe were indeed released from HF soluble portions of Ivuna. Both this Ne and Xe have isotopic and elemental ratios that are readily explained as a mixture of the two most abundant primordial noble gas components in Ivuna bulk samples: HL and Q. Four percent of the Xe-Q may be derived from an HF soluble subfraction of phase Q, which again suggests that the operational definition of Q as a phase resistant to HF but attacked by strong oxidising acids is not entirely correct. On the other hand, 3 % of Ne-HL (the primordial Ne component whose bulk fraction is released in parallel with Xe-HL) may actually have resided in a different phase than the presolar nanodiamonds thought to be the carrier of Xe-HL. In another study, My Riebe investigated the effects of aqueous alteration on primordial noble gas carriers (Riebe et al., 2020). She did this by combining CSSE and stepwise pyrolysis analyses on bulk samples of the Tagish Lake carbonaceous chondrite that had undergone varying degrees of aqueous alteration.

My Riebe’s work demonstrated the potential of in vacuo etching to study primordial noble gases in bulk meteorites. Her work has been continued and extended by Daniela Krietsch in her doctoral thesis in Henner Busemann’s group (Krietsch, 2020; Krietsch et al., 2019). Daniela characterised the full noble gas inventory of a very primitive CR chondrite found in Antarctica using CSSE with five different etchants, starting with water, followed by acetic acid, HNO₃, HF, and HCl. Surprisingly, about one quarter to one third of the primordial He and Ne, the latter with a Ne-Q-like isotopic composition, were released in the first (water) step. Almost no Ar was released in this step, but acetic acid dissolved an Ar-rich carrier known to exist and to be lost by moderate parent-body aqueous alteration.

One further example of extending the reach of CSSE is the ³⁹Ar-⁴⁰Ar dating study initiated by Igor Villa at the University of Bern, which was inspired by discussions at one of the meetings at Ringberg Castle in Bavaria (see Box 3.1). I mention this example here, even though this work was done on a terrestrial sample, as it includes the first Ar-Ar analysis by CSSE ever done. Igor suggested that I participate in a consortium study aimed at investigating the mechanisms and pathways by which Ar diffuses through the McClure Mountains (MMhb) hornblende, a well known standard in ³⁹Ar-⁴⁰Ar dating (Villa et al., 1996). Igor performed classical Ar-Ar analyses by stepwise heating on untreated and heated (~850 ºC at 2 kbar) MMhb splits, Simon Kelley at the Open University in Milton Keyes performed IR laser traverses and UV laser depth profiling on single grains, and I analysed samples using CSSE with HF. There were minor but important differences in the Ar-Ar age spectra of the stepwise heating and CSSE runs. For example, the CSSE spectrum of the untreated sample provided evidence of a minor phase of low Ca/K in the first 5 % of Ar released that was not seen in stepwise heating. Gas release by acid attack at room temperature therefore provides information that may be blurred when gas is released by diffusion in the laboratory. Although promising in principle, Ar-Ar dating by CSSE has not become routine, mainly because it is too time consuming.

A similar caveat needs to be made for CSSE analyses in general. Almost every single in vacuo etch run represents a very major experimental effort, in most cases lasting between a few weeks up to several months, in any case much longer than typical stepwise pyrolysis runs (e.g., Fig. 3.26). The technique thus is not expected to ever be used for routine analyses but must be limited to selected tasks.

The CSSE technique has been very efficient in distinguishing carriers of primordial noble gases with different chemical properties, especially with different susceptibilities to acid dissolution. The technique is not well suited, however, to study the location of primordial noble gases with high spatial resolution. This can instead be achieved by in situ extraction of noble gases from petrographically well characterised phases by laser heating, pioneered by Tomoki Nakamura and colleagues in Fukuoka, Japan (e.g., Nakamura et al., 1999; Okazaki et al., 2001). Alternatively, small samples with a well characterised petrographic context and mineralogy can be mechanically extracted, with subsequent noble gas release by laser heating. This second approach was used by Nadia Vogel for her doctoral dissertation in my group (Vogel et al., 2003; 2004a;b). We have already met Nadia in Section 2.5.5 on Genesis and will hear more about her in Section 5.2.2. which is devoted to our investigations of speleothems as palaeoclimate archives.

One major goal of Nadia’s dissertation was to study accretion and alteration histories of meteorite parent bodies based on the microdistribution of primordial noble gases in unequilibrated chondrites. All her studies benefitted significantly from the petrographic expertise of Addi Bischoff at the University of Münster. In her first paper, Nadia showed that in many meteorites noble gas concentrations were higher in fine grained rims around chondrules than in adjacent matrix portions of a meteorite (Vogel et al., 2003). She concluded that this indicates accretion of nebular dust onto chondrules from regions whose noble gas concentrations decreased with time, which is incompatible with the formation of chondrule rims by aqueous alteration on a parent body as proposed by others. Her data imply a heterogeneous dust reservoir in terms of noble gas inventories in the accretion regions of meteorite parent bodies. In her next two publications, Nadia studied noble gases in CAIs and chondrules (Vogel et al., 2004a;b). These high temperature components were known to be poor in, or even devoid of, primordial noble gases, but some workers had reported that in some cases chondrules and even CAIs appeared to have retained at least traces of noble gases carried to the solar nebula by presolar grains. If this were the case, it would have implications for CAI formation and early solar system evolution. Carefully avoiding potential cross contamination with matrix material, Nadia measured He, Ne, and Ar in her CAI study, with the Ne data being particularly revealing. She did not detect even traces of primordial Ne; all her data could be readily explained by cosmogenic Ne derived mainly from minerals rich in Na, Ca, and Cl (Fig. 3.27). In contrast to CAIs, some of the chondrules from the carbonaceous and LL chondrites she studied contain small amounts of primordial Ne and Ar, although the primordial Ar concentrations are much lower than the values reported by Okazaki et al. (2001) for chondrules in the enstatite chondrite Yamato 791790. Nadia concluded that – in contrast to CAIs – heating of chondrules and their precursor materials was not strong enough to quantitatively expel primordial noble gases. The very different primordial Ar concentrations between Nadia’s chondrules on the one hand, and those in Yamato 791790 on the other, rule out for the former a scenario proposed by Okazaki and co-workers for the latter, which is that solar gases were implanted into the chondrule precursor material and later incompletely lost by diffusion. If the E chondrite data are confirmed by other meteorites, this could mean that different chondrule populations might have formed in fundamentally different environments or by different mechanisms.

In a good team, ideas for promising research projects sometimes emerge that do not readily fit into the overall picture laid out in a research grant application. In some cases, such ideas may lead to stand alone projects, and in others to larger collaborations. In this section, I present some examples where such ideas led to what I think became interesting science.

One day Matthias Meier approached me with some papers in which the authors claimed that the nuclear decay constants (λ) of several isotopes show an annual periodicity, being slightly higher in northern winter than in summer (e.g., Jenkins et al., 2012). For ³⁶Cl the reported variability of λ₃₆ is of the order of ±1 %. Since the Earth is nearly 3.5 % closer to the Sun in early January than in early July, these authors and others proposed that the Sun influences nuclear decay rates in an unspecified way. They ruled out all other possible effects as cause for the observed variability in count rates, including environmental factors such as temperature or humidity. After further reading, we realised that there are two camps on this subject that are fiercely opposed to each other. One camp argues that there is increasing evidence for variable decay rates, while the other camp seriously doubts the quality of the experiments conducted by the former. Our gut feeling clearly leaned towards the latter view, but Matthias and I still discussed with an open mind what variable decay “constants” would mean for meteorite research. If decay rates already vary at the percent level for heliocentric distance variations covered by the Earth’s orbit, the difference to the main asteroid belt could be dramatic. If so, cosmic ray exposure ages of meteorites based on radionuclide concentrations might be substantially off. Even more dramatically, radiometric ages of meteorites, e.g., those determined by U-Pb dating, would be substantially in error if the decay constants determined on Earth do not apply for the asteroid belt. Hence, the isotope record in meteorites should provide a highly relevant test for the claim that nuclear decay rates depend on the distance from the Sun.

Matthias and I evaluated a large part of the relevant database on meteoritic, lunar and terrestrial samples, which includes both cosmogenic and radiogenic isotope systems (Meier and Wieler, 2014). The bottom line result of our study was – not at all to our surprise – that geo- and cosmochronological data provide no evidence for heliocentrically variable decay constants, whether for nuclides undergoing alpha-decay, beta-decay, or decay by electron capture. No explicit possible physical explanation for the radial dependency has been offered by the “variable decay” camp, but they compare the alleged variations of λ₃₆ between Earth’s perihelion and aphelion with a 1/R or 1/R² dependence on heliocentric distance R. However, this would lead to larger semi-annual variations of decay rates than reported. Therefore to be compatible with reported data, Matthias and I assumed a proportionality of λ with 1/R^x, x falling between 0.06 – 0.3. If such a dependence extended to the asteroid belt, the production rates and hence the saturation activities of cosmogenic ³⁶Cl would vary much more than is observed for meteorites with different orbital histories. Of particular interest is a comparison between absolute radiometric ages of meteorites of different classes based on the decay of uranium to lead and relative ages using (now extinct) short lived nuclides such as ¹⁸²Hf, ⁵³Mn, or ²⁶Al (Nyquist et al., 2009). We concluded that these ages agree so well with each other that the parent bodies of angrites, CV and H chondrites would have had nearly identical orbits throughout solar system history. Moreover, solar physicists are able to derive an age of the Sun from helioseismology combined with solar modelling. This age of 4.60 Ga (Houdek and Gough, 2011) is independent of meteorite ages based on radioactive nuclides and agrees with the U-Pb ages of the oldest meteorite samples to within its 1 % uncertainty. Last but not least, the U-Pb ages of Earth and meteorites agree much better than would be expected if uranium isotopes decayed much more slowly in meteorite parent bodies. We therefore concluded that none of the nuclides we had considered (including ³⁶Cl, which was extensively studied by Jenkins et al., 2012) provide any hint for an influence of the Sun on their decay rates. We also noted in parentheses that our argument would not be relevant if the alleged seasonal variability were in fact not due to the variable Earth-Sun distance but, however unlikely this may seem, to the motion of the Earth with respect to the interstellar medium.

Matthias and I had a lot of fun when working on this paper, but once it was published we were a little worried whether we had now simply asserted something that no one in our own community would ever have doubted anyway. However, it turned out that our work was favourably received by one of the two camps among the nuclear particle physicists, those, of course, who strongly criticise the experimental expertise of the others with words like “poor metrology and incomplete uncertainty analysis” (Pommé and Pelczar, 2020).

In 2013 I was asked to review a manuscript by Jan Kramers of the University of Johannesburg and his co-workers. They had examined a small piece of a very unusual rock of about 30 g found in the strewnfield of the enigmatic Libyan desert glass. The rock consists of almost pure carbon, is black, shiny and extremely hard. In the manuscript it was dubbed Hypatia, honouring the 4^th century philosopher, mathematician, and astronomer who was the first woman to teach Platon’s philosophy in Alexandria, which led to her execution by a Christian mob (Fig. 3. 28). Kramers and co-workers proposed that the Hypatia stone is a remnant of a comet nucleus that fell in Egypt, possibly the same impact that produced the Libyan desert glass. One argument for this provocative proposal was based on noble gas data. On the one hand, ⁴⁰Ar/³⁶Ar ratios as low as 40 (considerably below the terrestrial atmospheric value of ~300) proved that Hypatia is extraterrestrial. On the other hand, Kramers and co-workers argued that the Xe isotopic composition is consistent, albeit within large uncertainties, with a contribution from the exotic component Xe-G but not the more common Q component. In my opinion, the hypothesis put forward by Jan Kramers and his colleagues was highly interesting, although far from stringent. Therefore, I recommended accepting the manuscript, which was published in the journal Earth and Planetary Science Letters as Kramers et al. (2013). The rationale for my recommendation was that I felt that provocative or speculative ideas deserve to be published as long as they have a fundamentally sound basis that includes interesting experimental data. I point this out because some of my colleagues did not share my opinion and let me know.

Hypatia is not the only sample found on Earth proposed to derive from a comet. Matthieu Gounelle at the Muséum National d’Histoire Naturelle in Paris and colleagues, for example, suggested that the famous Orgueil meteorite derives from a body with cometary affinity (Gounelle and Zolensky, 2014) and Sara Russell and colleagues from the Natural History Museum in London suggested that comets are a likely source of much of the petrologic type 1 meteoritic material (Russell et al., 2022). In any case, if a cometary connection of Hypatia could be confirmed, this would be of utmost importance. I therefore proposed to Jan Kramers that we repeat the noble gas (and nitrogen) measurements by more sophisticated analytical methods than those that had been available for the first study. In collaboration with Jan, this was done by a small consortium at CRPG in Nancy, the Institut de Physique du Globe Paris, and ETH Zürich. In addition, Falko Langenhorst in Jena characterised the sample by transmission electron microscopy. Guillaume Avice in Nancy was the principal investigator of the resulting paper (Avice et al., 2015).

One of my tasks was to ask the Nomenclature Committee of the Meteoritical Society to formally approve Hypatia as a meteorite, as this is mandatory for publication in many journals, such as Geochimica et Cosmochimica Acta or Meteoritics and Planetary Science. However, this did not prove to be as straightforward as I had expected, despite the fact that Kramers et al. (2013) had provided clear evidence that Hypatia is extraterrestrial. The problem was that only about one gram of the original 30 g stone is still available, as the rest had somehow disappeared. This meant that we could not name an approved institution that would curate a type specimen. Therefore, according to the Meteoritical Society, Hypatia (or whatever name would have been accepted) cannot be called a meteorite. Our solution was to publish the follow up study again in Earth and Planetary Science Letters (Avice et al., 2015), which in 2015 had not yet enforced the rule of the Meteoritical Society (they do now). While we did not feel we had done anything illegal we were puzzled by the Nomenclature Committee’s decision. It did seem to be a technicality though, and it is clear that the type specimen rule in general is crucial at a time when commercial interests in meteorites increasingly threaten scientific activity. It also helps ensure the overall integrity of the field by discouraging fraud.

Our data turned out to be broadly consistent with concentrations and isotopic compositions of noble gases and nitrogen in various types of carbon-rich meteoritic materials, including graphite nodules in iron meteorites. Hypatia may be different from all of these, but obviously sampled a similar cosmochemical reservoir. We did not confirm the presence of exotic noble gases (e.g., Xe-G) but clearly detected noble gases with isotopic compositions close to the Q gases ubiquitous in meteorites (Section 3.2.1). We therefore did not confirm that Hypatia is a remnant of a comet nucleus. A relation between Hypatia and the Libyan Desert Glass is not warranted.

In 2012, Matthias Meier showed me a recently published paper entitled “Evidence for the extraterrestrial origin of a natural quasicrystal” by Luca Bindi of the University of Florence and co-authors (Bindi et al., 2012). Thanks to Walter Steurer’s inaugural lecture as Professor of Crystallography at ETH, which I had attended many years earlier, I had a vague idea of what quasicrystals are, although all I really remembered was that they had become an exotic but fascinating topic in modern crystallography. This was enough, however, to catch my attention as well, and in the following paragraph I attempt a rudimentary explanation of what quasicrystals are. Luca Bindi’s paper presented evidence, mainly based on oxygen isotopes, that a mineral grain in which Bindi et al. (2009) previously had observed tiny quasicrystals is a small fragment of a meteorite. Matthias imagined that noble gas analyses of part of this grain might provide further clues as to the origin of quasicrystals formed in nature. He suggested that our Tom Dooley mass spectrometer would be an excellent argument to convince Bindi and co-workers to provide us with a tiny portion of what surely was an extremely precious sample. So our first idea was that such exotic matter in meteorites might also contain exotic noble gases, perhaps as interesting as, or even more so than, noble gases in known presolar grains like nanodiamonds or silicon carbide. A closer reading of Bindi et al. (2012) dampened our enthusiasm somewhat, as it seemed impossible to obtain enough quasicrystal material for a noble gas analysis. Nevertheless, Matthias got in touch with Luca, which was the beginning of our – small but nevertheless quite important – involvement in a truly fascinating story. The full story was presented by Paul Steinhardt in a book (Steinhardt, 2019) that makes as thrilling reading as any good detective story and which I will try to summarise in the following few paragraphs.

Paul Steinhardt of Princeton University is a foremost expert in two seemingly disparate scientific fields, cosmology and crystallography. As theoretical cosmologist he studies the origin and fate of our universe (e.g., Steinhardt and Turok, 2003). In crystallography, he was the first to propose the existence of a new form of matter with a quasiperiodic arrangements of atoms. In the 1980s, Paul began to think about the fact that properties of matter can be dramatically changed simply by reordering the atoms. Perhaps the best example is graphite and diamond. Both are made of carbon atoms only, but graphite is soft enough to be used in pencils, while diamond is among the hardest of all known materials. Ultimately, this led Paul to the idea of quasi-periodic crystals or simply quasicrystals. Simply put, you can cover a floor with regular hexagons without leaving empty space, but you cannot do this with pentagons, at least not in a periodic pattern. But Steinhardt and his student Dov Levine found a way to arrange pentagons in a nearly or quasiperiodic fashion without gaps, inspired by tilings in islamic art. They also demonstrated that the three dimensional equivalent, a quasiperiodic arrangement of atoms with five fold (icosahedral) symmetry, is possible and predicted that this should lead to an X-ray diffraction pattern as shown in Figure 3.29a (Levine and Steinhardt, 1984). Soon after, their hypothesis was verified when Dan Shechtman discovered the first compound with the predicted diffraction pattern in an artificially prepared Al-Mn alloy (Shechtman et al., 1984; Fig. 3.29b), a discovery that earned Shechtman the Nobel Prize for chemistry in 2011. Thereafter many different types of quasicrystals were found, but all were produced under well defined laboratory conditions, usually by quenching metal alloys that often contain Al and Cu.

So, while quasicrystals soon became of interest to application oriented material scientists, Steinhardt’s next burning question was: “can quasicrystals also form in nature, and if so, how”? The answer to the first part of the question seemed to come from Luca Bindi in Florence. Luca was meticulously searching for diffraction patterns with five fold symmetry in his museum’s mineral collection. His Eureka moment came with a rather poorly documented sample a few millimetres in size, apparently collected in 1979 in far eastern Russia (Bindi et al., 2009). Luca discovered a quasicrystal phase with five fold symmetry in this tiny sample, now known as icosahedrite (Fig. 3.29c). A first problem, however, was that most of the sample had been consumed during sample preparation. A second problem was that the sample consisted largely of alloys containing Al and Cu, and all experts consulted insisted that metallic Al does not occur in nature. However, Steinhardt and Bindi did not give up. In 2010, it turned out that the Florence sample was a tiny piece of a meteorite, likely of the type CV3 chondrite such as Allende. This became obvious when the oxygen isotopic composition of silicates was analysed at Caltech in Pasadena by John Eiler and Yunbin Guan (Bindi et al., 2012). Hence, it seemed that Bindi and his co-workers had shown that nature could produce quasicrystals, at least in space. However, to be sure, a watertight proof was required that the quasicrystal-containing Al-Cu alloys were also part of the meteorite, notwithstanding the “impossibility” of the existence of metallic Al in nature. But how was such a proof to be provided, given that almost no material was available for further investigations?

Detective work revealed that the Florence sample came from the Chukotka region in the far east of arctic Russia. Steinhardt was able to contact Valery Kryachko, the man who had found the sample back in 1979. Northeast Siberia was certainly not the easiest place on Earth to search for additional meteoritic material more than 30 years later, but Steinhardt managed to set up an expedition of 13 people and a cat. The story of this expedition alone makes Paul’s book exciting reading. Against all odds they found a few more grains of Khatyrka, as the meteorite was named. The new grains also contained Al-Cu alloys intimately intergrown with silicates, and indeed the quasi-crystalline phase icosahedrite was also found embedded in the Cu-Al alloys. Glenn MacPherson of the Smithsonian Institution in Washington had been among the most outspoken critics of the idea that the Al-Cu alloys in the Florence sample could be natural, but Steinhardt had nevertheless convinced him to join the expedition. In a detailed study of some new grains from the 2011 expedition, MacPherson et al. (2013) concluded that the Cu-Al alloys undoubtedly are natural and extraterrestrial. Khatyrka is described as a complex CV3 (ox) breccia. It appears that many in the meteorite community – including earlier critical voices – have by now largely accepted the natural origin of the Cu-Al alloys, and hence the fact that nature is capable of forming quasicrystals, although some remain sceptical. Alan Rubin and Chi Ma, for example, note that “At present, the origin of the weird assemblages in Khatyrka remains controversial, and Sherlock Holmes is unavailable” (Rubin and Ma, 2021, chapter 13). A leading hypothesis is that the quasicrystals in Khatyrka were formed under ultrahigh pressure, caused by impacts. Further evidence for this was found when Bindi et al. (2021) found a new type of quasicrystal in trinitite, the material formed by the first plutonium bomb test in New Mexico in 1945 (Bindi et al., 2021).

What is our contribution to this story? After Matthias Meier and I had abandoned our original naive hope to possibly detect “superexotic” noble gases in the quasicrystals, we suggested to Luca and Paul that we study noble gases in silicates from Khatyrka. Luca sent us six olivine grains, likely fragments from a Khatyrka chondrule. We organised a small consortium that included Philipp Heck in Chicago (chemical composition and volume of grains) and Nicole Spring in Manchester (transferring the tiny and precious grains between different sample holders with her advanced micro-manipulation skills). Matthias measured He and Ne isotopes in Zürich with Tom Dooley (Meier et al., 2018). All grains contained cosmogenic ³He and ²¹Ne in amounts well above what could reasonably have been produced near the Earth’s surface (see Section 4.2). This provided independent evidence that Khatyrka is indeed a meteorite. Its cosmic ray exposure age is likely between 2 and 4 Ma (Fig. 3.29d). As we saw in Section 3.1, this is rather low for a meteorite, but falls within the range observed for a few other CV chondrites. However, the radiogenic ⁴He concentration from the decay of uranium and thorium yields a U-Th-⁴He gas retention age of only around 600 Ma, shorter than that of any other known CV chondrite. This implies that the Khatyrka parent body experienced a strong shock relatively late in its history, possibly the event that produced the second generation of quasicrystals in Khatyrka identified by Lin et al. (2017). Somewhat unfortunately, Khatyrka’s unique combination of low exposure age and low ⁴He retention age means that no other known CV chondrite lends itself to a detailed follow up search of native Al-Cu alloys or even quasicrystals. For the time being, the few available grains of Khatyrka remain our only known source of extraterrestrial quasicrystals, with the possible exception of a micrometeorite from Antarctica hosting an exotic Al-Cu-Fe assemblage (Suttle et al., 2019). On the other hand, Matthias has identified a possible parent body for Khatyrka. Asteroid 89 Julia has a matching reflectance spectrum, is the parent of a young asteroid family consistent with the ~600 Ma shock event recorded in Khatyrka, and is located near strong orbital resonances in the asteroid belt, which can explain its short cosmic ray exposure age (Meier et al., 2018). This assignment is, of course, tentative. Yet, it inspired Phil Plait, a science fiction blogger calling himself “The Bad Astronomer”, to suggest to the writers of a CBS series on life threatening asteroids a name for the material vital to the fictional engine needed to keep a dangerous asteroid away from Earth: Icosahedrite! Would that not be the ultimate example of how crucial “pure” science is to humanity’s survival? (https://www.syfy.com/syfy-wire/cosmic-sleuthing-an-origin-story-for-a-really-weird-meteorite)

The next example is also from a long term and extensive collaboration, in this case between the Physics Institute of the University of Bern and the Institute for Space Research at the Russian Academy of Sciences. We at ETH were fortunate to become involved in this project at a late stage and yet to be able to provide some critical analyses for its success.

In 1991, Peter Bochsler contacted G. N. Zastenker at a meeting in Vienna with a proposal to capture neutral helium atoms from the interstellar medium in foils exposed on the Russian MIR space station. Zastenker was enthusiastic, and in 1996 the COLLISA experiment on MIR collected interstellar neutral atoms in a BeO layer on the surface of a Cu-Be foil. The helium in this foil was then analysed first at the University of Bern (Salerno et al., 2003) and later in Zürich (Busemann et al., 2006).

Knowledge of the ³He/⁴He ratio in the interstellar medium (ISM) is essential for models of galactic chemical evolution (GCE) and for predicting the overall abundance of ³He (relative to H or ⁴He) in the ISM. In particular, the present day interstellar ³He abundance – compared to protosolar ³He as inferred from Jupiter’s atmosphere or from meteorites – allows us to track the GCE over the last 4.56 Ga. The COLLISA experiment provided a direct way to measure the helium isotopic ratio in the local interstellar cloud (LIC), the region of the local ISM currently traversed by the solar system. Neutral gas of the LIC reaching Earth’s orbit before interacting with the solar EUV (extreme ultraviolet) photons retains its original isotopic abundances. COLLISA was obviously inspired by the Apollo Solar Wind Composition experiment, developed in Bern some three decades earlier, in which ions of the solar wind were trapped in aluminum foils on the lunar surface (Section 2.2). The additional main difficulty of COLLISA was the very low speed of the Earth relative to the neutral gas of the LIC, which never exceeds about 80 km/s during northern spring (Zastenker et al., 2002). This is considerably slower than the speed range of solar wind ions. Therefore, Al foils would not have trapped a sufficient fraction of the interstellar He. The problem was overcome with a CuBe foil covered with a thin BeO layer, resulting in a He trapping efficiency of the order of 10 % and a reasonably well defined He isotopic fractionation upon trapping. Over a period of several months in 1996, interstellar neutral atoms were collected for about 60 hours in ~140 time windows of a few minutes each, carefully avoiding, e.g., neutral atoms from the Earth’s upper atmosphere, while ions of various potential sources were electrostatically rejected (Zastenker et al., 2002). This allowed Emma Salerno in Bern (Salerno et al., 2003) to measure an interstellar ³He/⁴He ratio of (1.7 ± 0.8) × 10^-4. To my knowledge, this was the first laboratory-based direct analysis of interstellar matter. Salerno and co-workers concluded that their value is consistent with protosolar ratios obtained from meteorites and Jupiter’s atmosphere, supporting the hypothesis that the abundance of ³He in the Galaxy has not changed significantly over the past 4.5 Ga. However, the value has a substantial uncertainty, largely due to ³He originating from the radioactive decay of tritium in the CuBe foil.

Henner Busemann was involved in the MIR foil analyses in Bern. A few years later, while he was working at the Carnegie Institution in Washington and occasionally as a guest at ETH, we decided to try to mitigate the ³He blank problem by extracting the gas by CSSE (Section 2.5.1) instead of stepwise heating. Low extraction blanks for noble gas components of interest had previously proven to be a major advantage of gas release by selective acid dissolution of specific carriers in vacuo. For example, solar wind gases implanted at very shallow depths in lunar or asteroidal regolith grains were accompanied by much lower contributions of cosmogenic noble gases from the grain volumes than was possible by stepwise heating. Henner and I therefore proposed to dissolve only the very thin BeO layer of the foil by closed system etching. This would release the interstellar He, but hopefully leave in place much of the tritiogenic ³He that had accumulated in the Cu-Be part of the foil, because no high temperatures would be needed in the gas release. Our plan worked well. Using HF, Henner etched about 50 cm² of a foil piece exposed on the MIR station in several steps and clearly observed interstellar ⁴He as well as ³He (Fig. 3.30; Busemann et al., 2006). Several analyses of control foils showed that the helium blank was much lower than that achieved by Salerno et al. (2003) with stepwise pyrolysis. We obtained (³He/⁴He)_LIC = (1.62 ± 0.29) × 10^-4, the most precise determination of the He isotopic composition of the local interstellar cloud at that time, with an uncertainty almost three times lower than that reached by Salerno and co-workers. Since the ³He/⁴He ratio in the LIC is within uncertainty identical to the protosolar value of (1.66 ± 0.06) × 10^-4 measured in Jupiter’s atmosphere, this reinforces the conclusion of Salerno et al. that during the last 4.6 Ga there has been no significant evolution of the ³He abundance in the Galaxy.

My last example of science apart from research grants also relates to a major topic in planetary sciences: the origin of the Earth’s Moon. As Giant Impact modelling is far from my core competences, it fits best in this section, as I would not have dared to include this project in one of my own research proposals.

A long standing conundrum in the Giant Impact hypothesis of lunar formation was – and perhaps still is – that “canonical” impact models predict that the Moon formed predominantly from material of the impactor Theia rather than the proto-Earth, which apparently contradicts the Moon’s close geochemical similarity to Earth. It was again Matthias Meier who asked me one day what might possibly happen if Theia was modelled not as a body with silicate mantle and iron core, but as an object similar to Ganymed, Jupiter’s largest Moon. Like other large moons in the solar system, Ganymed has a mantle of ice (and partially liquid water) overlying an inner region of silicates and iron. Probably rather naively, Matthias and I wondered whether Theia’s evaporating water mantle might largely leave the system upon the collision with the proto-Earth. If so, this could result in a much larger fraction of the Moon deriving from the proto-Earth than in the case of a pure silicate/iron impactor. Matthias and I knew of no study that modelled such a case, but we did not see why potential impactors in the early solar system should not possibly have been similar to Ganymed or other icy moons. So we approached Willy Benz at the University of Bern, one of the pioneers on Giant Impact modelling, and his graduate student Andreas Reufer.

As part of his dissertation, Andreas modelled impacts by relaxing the then canonical assumption that none or very little of the total mass of proto-Earth plus Theia is lost from the system (Reufer et al., 2012). One consequence was that the collisional angular momentum no longer had to be tightly constrained, as escaping material could also carry angular momentum. “Hit and run” collisions, in which a significant fraction of the impactor and perhaps also the target escapes, were first studied by Asphaug et al. (2006). This approach allowed Andreas to expand the parameter space, permitting higher impact velocities and steeper (less grazing) impact angles. He modelled three types of impactors with different silicate and iron fractions and, in some cases, with a water ice mantle. While none of the model runs could reproduce the constraints of the actual Earth-Moon system, many of his runs resulted in moons that contained a considerably higher fraction of material from the proto-Earth than the canonical cases. In the “best” runs where Theia was a silicate-iron mix, a little more than 50 % of the disk mass from which the Moon would form came from the proto-Earth, and in the case of ice-rich impactors this fraction reached up to about 80 %. However, for ice-rich impactors, the efficiency with which material is brought into orbit is considerably lower than for Theias with higher densities, and the resulting disk mass was therefore not large enough to form a body the size of the actual Moon. Hence, the idea of ice-rich impactors does not seem to be able to easily explain the Giant Impact conundrum. Nevertheless, Andreas Reufer’s work resulted in one of three papers published in 2012 which, I think, gave new impetus to Giant Impact research; the others were Canup (2012) and Ćuk and Stewart (2012). Today, more than ever, the Giant Impact origin of the Moon is a highly active research topic for both the modelling and cosmochemical communities.