Focusing the Search for Biosignatures on Mars: Facies Prediction with an Example from Acidalia Planitia
Published:January 01, 2012
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Dorothy Z. Oehler, Carlton C. Allen, 2012. "Focusing the Search for Biosignatures on Mars: Facies Prediction with an Example from Acidalia Planitia", Sedimentary Geology of Mars, John P. Grotzinger, Ralph E. Milliken
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The search for martian biosignatures can be enhanced by focusing exploration on locations most likely to contain organic-rich shales. Such shales both concentrate and preserve organic matter and are major repositories of organic geochemical biomarkers in sediments of all ages on Earth. Moreover, it has been suggested that for Mars, accumulations of organic matter may be the most easily detected and least ambiguous of possible biosignatures (Summons et al. 2010). Since current surface conditions on Mars are unfavorable for preservation of organic matter, focusing exploration on locations predicted to contain ancient organic-rich shales would offer one of the best chances of detecting evidence of life—if it ever evolved on the planet.
Orbital data can be used to evaluate regional sediment sources and sinks on Mars, and, based on that, facies can be predicted and locations identified that are most likely to contain organic-rich sediments. An example is presented from Acidalia Planitia, in the martian lowlands, where this approach led to the conclusion that facies in southern Acidalia were likely to be dominated by fine-grained, muddy sediments. That conclusion added weight to the hypothesis that mounds in Acidalia are martian versions of mud volcanoes as well as the suggestion that organic materials, if present, would have been deposited in the same area as the mounds. This allowed speculation that potential mud volcano clasts in Acidalia could include preserved, organic biosignatures and, thus, that the mounds in Acidalia constitute an untested class of exploration target for Mars.
Facies prediction using orbital data is particularly applicable to planetary exploration where ground truth is most often lacking but orbital data sets are increasingly available. This approach is well suited to the search for potential geochemical biomarkers in organic-rich shales. The approach additionally could be applied to exploration for other categories of biosignatures (such as stromatolites or morphologically preserved microfossils) and to more general planetary objectives, such as the search for hydrothermal sediments, carbonates, or any particular type of geologic deposit.
One of the primary goals of the Mars exploration program is to determine if living systems ever arose on that planet by seeking evidence of past or present life. Water is a critical requirement for life, but today Mars is extremely dry. Nevertheless, martian river valleys attest to a wetter past, with surface accumulations of liquid water likely in the early part of martian history (e.g., Squyres and Kasting 1994, Craddock and Howard 2002). Accordingly, the first phase of martian exploration was focused on a “follow the water” strategy (Briggs 2000).
An adjunct to the “follow the water” strategy has been a focus on areas with clay minerals (phyllosilicates), since clays form in the presence of water and therefore are an indicator of past water. Phyllosilicates on Mars have been located with near-infrared spectra acquired from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the National Aeronautics and Space Administration’s (NASA’s) Mars Reconnaissance Orbiter (MRO) (Bishop et al. 2008, Ehlmann et al. 2008, Mustard et al. 2008) and the Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activite’ (OMEGA) spectrometer on the European Space Agency’s Mars Express Orbiter (Bibring et al. 2006). Using OMEGA data, Bibring et al. (2006) reported that global distributions of martian phyllosilicates correspond to the oldest terrains, indicating that earliest Mars was most likely to have had the most habitable (i.e., the wettest) conditions. That, plus the fact that organic compounds can be adsorbed within smectite clays (Kennedy et al. 2002), has stimulated interest in searching for clays as potential repositories of organic biosignatures from the early—most habitable—time on Mars (Ehlmann et al. 2008).
A complexity with this approach is that the presence of phyllosilicates does not necessarily imply associated organics. Clays can form in settings lacking organic matter. They can also accumulate and persist in environments where organics—though initially present—may have been destroyed by postdepositional processes such as oxidation, radiolysis, or photocatalytic decomposition. This is a concern on Mars, where surface oxidation appears to be significant over much of the planet’s surface, and oxidants such as perchlorates and possibly peroxides have been postulated (e.g., Encrenaz et al. 2004, 2008; Ten Kate et al. 2006; Hurowitz et al. 2007; Hecht et al. 2009). While adsorption of organic matter within smectite interlayers plays a role in organic preservation on Earth (Kennedy et al. 2002), it is not certain that comparable preservation would occur on Mars, since peroxides are stronger oxidants than terrestrial oxygen. In addition, Shkrob et al. (2010) have suggested that potential organics on Mars may be destroyed by photocatalytic decomposition as a result of the reaction with particulate iron (III) oxides that are abundant in the martian surface. They conclude that there may be “no safe haven” for organics from this process since it involves small, mobile, and highly reactive radicals. Finally, preservation of organic materials on Earth is enhanced by burial, which isolates sediments from surface oxidants and reduces permeability and porosity, thereby minimizing interaction with surficial fluids and gases. So burial on Mars might be part of the equation for preservation of organics.
Others have focused on different attributes of living systems, and there have been suggestions that Mars exploration should “follow the nitrogen” (Capone et al. 2006), “follow the energy” (Hoehler et al. 2007), “follow the carbon” (National Research Council 2007), or “follow the chemistry” (Hecht 2009). Each of these strategies captures important aspects of habitability or life, and each has different advantages and challenges. However, all of the above can be incorporated within a strategy focused on predicting settings in which martian life not only may have thrived but also where it is likely to have been both concentrated and preserved within sediments.
Potential for biosignatures on mars
Present-day Mars has a surface that is relatively hostile to life—as it is very cold, very dry, and lacks an atmosphere that could shield organisms from ultraviolet and cosmic rays. There are no indications that the planet is teeming with life, though it is conceivable that microbial life may exist in the subsurface. In contrast, early Mars may have had surface conditions more conducive to life, with a wetter, possibly warmer climate and a denser CO2-rich atmosphere that could have provided protection from ultraviolet and cosmic radiation and contributed to a warming greenhouse effect (Molina-Cuberos et al. 2001, Lammer et al. 2002, Bibring et al. 2006, Fairén et al. 2009, Tian et al. 2009, Morris et al. 2010, Warner et al. 2010).
Of the three Martian eras (Noachian 4.65 to 3.7 Ga, Hesperian 3.7 to 3.0 Ga, and Amazonian ∼3.0 Ga to present) (Hartmann and Neukum 2001, Neukum et al. 2001, Neukum 2008), the most habitable time is considered to be in the Noachian and earlier parts of the Hesperian. It has been suggested further that the Late Noachian–Early Hesperian (∼3.5 Ga) may have been the most favorable time for development of life, as by that time, major impact bombardment would have ceased but Mars may have still possessed an atmosphere that contributed to warmer, wetter conditions and afforded protection from radiation (Grady and Wright 2006).
On Earth, the paleontological and geochemical records indicate that life was thriving by at least ∼3 Ga and perhaps by ∼3.5 Ga. Evidence derives from (1) stromatolites (Walter et al. 1980; Hofmann et al. 1999; Allwood et al. 2006, 2009), (2) organically preserved microfossils (Rasmussen 2000; Altermann and Kazmierczak 2003; Schopf 2006; Schopf and Walter 2007; Schopf et al. 2007; Sugitani et al. 2007; Oehler et al. 2009, 2010; Javaux et al. 2010), and (3) organic chemical biomarkers in cherts and shales (Duck et al. 2007, Marshall et al. 2007, Van Zuilen et al. 2007, Derenne et al. 2008, De Gregorio et al. 2009, Oehler et al. 2009). This indicates that life developed and diversified relatively early in the history of the Earth (Oehler et al. 2010) and may have done so on a similar timescale on Mars—during its most habitable period.
Biosignatures include both inorganic and organic remains of living systems. Inorganic biosignatures consist of biologically influenced minerals as well as inorganic morphological fossils or bio-sedimentary structures like stromatolites. Organic biosignatures include morphologically preserved remains of microbial life and geochemical remnants (organic biomarkers) of biological materials. For Mars, it has been suggested (Summons et al. 2010) that accumulations of sedimentary organic matter (including potential organic geochemical biomarkers) may be the most easily detected and the least ambiguous of potential biosignatures. On Earth, organic-rich shales and cherts go back in the geologic record to nearly 3.5 Ga and are commonly present even when morphologically recognizable microfossils are not preserved.
In fact, organic-rich shales are major repositories for terrestrial organic matter of all ages. These organic-rich sedimentary rocks represent environments in which organic matter has been both concentrated and preserved. These are the sorts of environments that should be sought in martian exploration, as they would enhance the chances of finding organic biosignatures—if life were ever present on Mars.
Organic-rich shales are common source rocks for oil and gas, and petroleum companies have developed methods for predicting their occurrence in areas with limited ground truth. Using these methods, regional structure is typically assessed first from satellite-derived topography and bathymetry using radar, gravity, and magnetics coupled with paleogeographic reconstructions. From that assessment, basin configurations are identified along with likely sediment sources and sinks; sedimentary facies are predicted from analysis of sediment input and basin geometry. Since much is known about the settings in which organic-rich sediments occur, source rock accumulations can be predicted from the mapped facies (see Allen and Allen 1990, Klemme and Ulmishek 1991, Gautier 2005, Tommeras and Mann 2008). Examples of these types of predictive studies can be found in the works of Christ et al. (2003) and Dickson et al. (2003, 2005, in press).
In general, organic-rich shales are found in both marine and lacustrine, basinal settings within distal, quiet-water facies (e.g., Macquaker and Curtis 1989, Tyson 1996, Gautier 2005, Jenkyns 2007, Davies-Vollum and Smith 2008). These are environments in which in situ living systems may have proliferated, in which organic material is concentrated by transport processes, and in which burial with associated muds and sometimes evaporites enhances preservation of organic matter by isolating it from destructive surface processes. The sedimentary processes resulting in accumulation and preservation of organic-rich shales are applicable to sediments of any age and can be applied beyond oil exploration to the general search for organics as biosignatures of early life.
Similar predictive approaches can be applied to the search for organic biosignatures on Mars. Orbital topographic data can be used for first-pass designations of basin architecture, sediment sources, and sediment sinks, since Mars has had a comparatively stable structural framework since ∼3.7 Ga (Golombek 2005, Golombek and Phillips 2009). Based on that assessment, facies can be predicted where organic-rich shales would be expected. The regional evaluation can be fine-tuned with geomorphic and hyperspectral data from the higher resolution imaging spectrometers now orbiting Mars. This approach would allow prediction of facies and expected sediment composition for many of the areas on Mars that are lacking ground truth. It is rationally a first step that should be taken in identifying exploration targets.
Example from acidalia planitia
An example from Acidalia Planitia, in the martian lowlands (Fig. 1), is provided for which facies have been predicted from orbital data (Oehler and Allen 2010). Results were applied to assessment of the mounds in Acidalia, supporting an interpretation that the mounds represent a form of mud volcanism. Multiple alternatives were considered, but the geologic context added considerable weight to the interpretation of mud volcanism. The analysis further allowed prediction of locations for accumulation of potential organics as well as the suggestion that the mounds in Acidalia represent an untested class of exploration target for Mars.
The Acidalia example is presented here in abbreviated form, as it is not the intention of this article to make the argument for mud volcanism but rather to illustrate the methods used for determining geologic context and the value of this approach in interpreting planetary features. The Acidalia example provides a case study for which facies have been predicted that could contain organic-rich shales, including potential biosignatures. A comprehensive study of the mounds in Acidalia is reported in Oehler and Allen (2010), and the reader is referred to that article for detailed treatment of the mapping methods, geomorphology and spectral signatures of the mounds, facies prediction, alternative hypotheses, timing of events, potential relationships to methane, triggers for sediment mobilization, differences between the Acidalia mounds and those elsewhere in the lowlands, and expected differences between terrestrial and martian mud volcanism.
Acidalia Planitia is part of the northern lowlands of Mars (Fig. 1); it extends nearly 3000 km in an east–west direction and lies between Chryse Planitia to the south, the Tharsis volcanic province to the southwest, Arabia Terra to the southeast, and the North Polar province to the north (Fig. 1). Recent work (Andrews-Hanna et al. 2008, Marinova et al. 2008) indicates that the martian lowlands were formed very early in the history of the planet, perhaps >4 Ga, by a giant impact that formed the huge northern Borealis Basin. Frey (2006, 2008, 2010a, 2010b) proposes that the lowlands were subsequently affected by several very large impacts, perhaps during the martian equivalent of a late heavy bombardment ∼3.9 Ga, and that remnants of those partially buried impact basins can be mapped as quasi-circular depressions (QCDs). Chryse and Acidalia are both proposed to be large impact basins of this type.
Analysis of the tectonic history on Mars (Golombek 2005, Golombek and Phillips 2009) indicates that development of the enormous Tharsis volcanic edifice prior to ∼3.7 Ga may have contributed to a warm and wet climate of early Mars. This early emplacement of Tharsis would have altered the elliptical shape of the Borealis Basin (Andrews-Hanna et al. 2008), leaving the Chryse– Acidalia region as a relatively restricted embayment (Figs. 1, 2; Oehler and Allen 2010). The tectonic analysis further indicates that the lithospheric structure of Mars has been relatively unchanged since ∼3.7 Ga, and this, plus the observation that ancient river valleys flow down the present topographic gradient, combine to argue that major, regional topographic trends on Mars have not varied substantially since ∼3.7 Ga.
There is considerable discussion as to whether the northern lowlands were the site of a past ocean on Mars (proposed by Parker et al. [1987, 1989, 1993] and summarized by Dohm et al. ). The evidence that the northern plains have remained a planet-wide topographic low since the late Noachian indicates that, at a minimum, the lowlands were a focal point for fluid accumulation during much of martian history. Moreover, there is evidence that massive floods occurred in the Hesperian and that the floodwaters debouched into the Chryse Basin (Fig. 2) from its southern and western perimeters (Golombek et al. 1995a, 1995b; Rice and Edgett 1997). These floods appear to have been unique on the planet and would have introduced significant sediment and water into Chryse and southern Acidalia. Some even consider that as a result of the Hesperian floods, Chryse, Acidalia, and perhaps the entire northern lowlands were under water (Parker et al. 1989, 1993; Baker et al. 1991; Scott et al. 1991, 1995; Rice and Edgett 1997; Head et al. 1999; Boyce et al. 2005; Dohm et al. 2009).
There are multiple data sets available for Mars from orbiting satellites. Those used in analysis of the mounds in Acidalia (Oehler and Allen 2010) included topography from the Mars Orbiter Laser Altimeter (MOLA) on NASA’s Mars Global Surveyor orbiter (MGS), nighttime and daytime infrared data from the Thermal Emission Imaging Spectrometer on NASA’s Mars Odyssey orbiter, and mosaics for these data sets provided by the US Geological Survey in their MarsGIS DVD v. 1.4. For detailed geomorphology, image data were used from the Mars Orbiter Camera (MOC) on MGS and from the Context (CTX) and High Resolution Imaging Science Experiment (HiRISE) cameras on MRO. For reference geology, the geologic map of the Northern Plains (Tanaka et al. 2005) was used. CRISM data from MRO were incorporated to provide information on mineralogy.
In this article, the term facies is used to describe distinctive rock types that broadly correspond to certain environments or modes of origin. This concept is sometimes referred to as “sedimentary facies” or “lithofacies.”
For the Chryse–Acidalia Embayment, analysis of topographic slopes and mapped river valleys predict a catchment area that covers a large portion of the southern highlands (Fig. 3). A depositional sink in Chryse is clear from the river and outflow channels. Moreover, streamlined islands in northeastern Chryse (Fig. 4) indicate that water and sediments from the outflow floods spilled from Chryse into the deeper Acidalia Basin (Scott et al. 1991, Rotto and Tanaka 1995, Rice and Edgett 1997). Within this framework, proximal channel facies (with boulders, pebbles, gravels, and sands) would be expected in Chryse, and distal, fine-grained facies (with mainly muds) would be expected in Acidalia (Oehler and Allen 2010).
Tens of thousands of high-albedo mounds (Fig. 5) occur in Acidalia. These were first noted in Viking imagery (Allen 1979, Frey et al. 1979, Frey and Jarosewich 1982) and have been interpreted as pseudocraters, cinder cones, tuff cones, pingos, ice disintegration features, mud volcanoes, or a combination of mud volcanoes and evaporites (Tanaka 1997; Tanaka and Banerdt 2000; Tanaka et al. 2003, 2005; Farrand et al. 2005; Rodríguez et al. 2007; McGowan 2009; Skinner and Mazzini 2009).
Oehler and Allen (2010) mapped more than 18,000 of the mounds and estimated that more than 40,000 occur in southern Acidalia (Fig. 2). The mounds generally overlie early Amazonian-aged units mapped by Tanaka et al. (2005); mound diameters average ∼800 m and analysis of one HiRISE stereo pair yielded a height of tens of meters. Many alternative hypotheses were considered to explain their origin (see Oehler and Allen  for discussion of all hypotheses), but the data were most consistent with an analog of mud volcanism (e.g., Fig. 6). This interpretation was strongly supported by the geologic context developed from regional analysis.
Requirements for mud volcanism are (1) subaqueous deposition of thick sequences of fine-grained sediments, (2) development of overpressure (Kopf 2002, Deville et al. 2003, Van Rensbergen et al. 2003, Deville 2009), and (3) subsequent triggering events to initiate sediment mobilization. These requirements are likely to have been met in southern Acidalia. The embayment-like geometry of the Chryse– Acidalia region coupled with its large catchment would have fostered accumulation of sediments in that region from as early as the late Noachian, with the finer-grained muds accumulating in the distal facies in Acidalia. Overpressure is likely to have developed as a result of rapid and massive Hesperian outflow sedimentation, and triggering events could have included such things as hydrothermal or compressive pulses from Tharsis, sublimation of ice and resulting loss of overburden, or destabilization of clathrates, with consequent release of gas (Oehler and Allen 2010).
The geologic setting of Acidalia provides advantages for potential biosignature concentration, preservation, and access to rovers. For example, the proposed catchment (Fig. 3) could have drained a substantial portion of the highlands (Fig. 3), carrying organic microfossils or organic geochemical biomarkers derived from any life in that large area. Since organic materials are of relatively low density, they tend to stay in suspension during fluvial transport and accumulate with the muddy fraction of sediments. Accordingly, organic remnants of possible life in the Chryse–Acidalia watershed are most likely to have been deposited with muds in Acidalia.
There is the additional possibility that the subsurface in Acidalia could have supported in situ life forms—in microhabitats within fluid-filled porosity (Fig. 7C). On Earth, microbial assemblages occur in the subsurface of mud volcano systems of the Gulf of Mexico, supported by upwelling fluids containing dissolved organic matter (Joye et al. 2009). Similarly, mud volcanism in Acidalia could have promoted upward migration of fluids from depth. Such fluids could have contained dissolved organics from Hesperian or Noachian strata or from any subsurface geological or biological sources of methane. These fluids could have supplied a continuing source of nutrients for potential microbial life (e.g., Niederberger et al. 2010, Pohlman et al. 2011) and also may have provided a source of warmth on an increasingly cold planet. Andrews-Hanna et al. (2007) have suggested that Acidalia may have been a region of enhanced upwelling from regional hydrological considerations. If this were the case, then these upwelling waters could have been channeled to the surface through conduits created by mud volcanism (Fig. 8) (e.g., Van Rensbergen et al. 2005a, 2005b; Van Rooij et al. 2005), as analogous conduits on Earth provide long-lived pathways (up to kilometers deep) for fluid migration (Etiope et al. 2010).
Finally, burial of transported or in situ biosignatures with outflow muds could have protected entrained organics from destructive processes on the surface of Mars, and such preserved materials could be contained in rock clasts brought to the surface by mud volcanism. Terrestrial mud volcanoes extrude clasts of lithified rock that are ripped from the walls of subsurface strata during mud eruption. Many of these clasts contain well-preserved organic biomarkers and nannofossils (Fig. 7) (Robertson and Kopf 1998, Kopf 2002, Stewart and Davies 2006). If burial in Acidalia were adequate for lithification of Noachian and early Hesperian strata, rock clasts containing potential biosigna-tures might be present on the surface in this region. Since no Mars mission has analyzed structures interpreted as mud volcanoes, the mounds of Acidalia can be considered to represent a new class of exploration target.
Mars exploration has been centered on the goal of determining if life was ever present on that planet, with the primary emphasis being the search for organic geochemical biosignatures. However, the surface of Mars currently appears to be unfavorable for preservation of organic materials as a result of potential oxidation from peroxides and perchlorates and decomposition from photocatalytic processes. One way to improve the chances of finding organic biosignatures (if they are present) would be to seek facies in which ancient, organic-rich sediments would be expected. Additionally, and importantly for Mars, if sites could be located where predicted organic-rich sediments are likely to have been compacted and lithified into shales, then the preservation potential of any organic constituents would be enhanced.
Orbital data can be used to evaluate regional sediment sources and sinks, and from this, predictions can be made of locations likely to have basinal, quiet-water, distal facies. These represent settings in which organic-rich sediments typically occur. An example is presented in which this approach was used in an evaluation of mounds in Acidalia Planitia, in the martian lowlands. In that study (Oehler and Allen 2010), regional assessment and facies prediction provided geologic context that supported interpretation of the mounds as martian equivalents of mud volcanoes. The facies model further predicted that organic materials would have been deposited coincidently with the muds. This allowed speculation that the mud volcano clasts erupted to the surface from depth in Acidalia might include preserved biosignatures.
This approach could also be used to search for other types of potential martian biosignatures, such as morphologically preserved, organic microfossils or stromatolites—the macroscopic, mineralized remnants of microbial mats. Each of these provides significant information about the Archean biosphere on Earth and could represent signatures of early life on Mars. A search for microfossils could be focused on cherts, shales, or evaporite deposits, all of which preserve organic microfossils on Earth (Schopf 2006; Schopf and Walter 2007; Schopf et al. 2007, 2010; Javaux et al. 2010). Similarly, exploration for martian stromatolites could be directed to locations predicted to have facies analogous to the carbonate-, silica-, or evaporite-precipitating settings in which terrestrial stromatolites occur (e.g., Walter et al. 1980, Jones et al. 2005, Allwood et al. 2006, Allwood and Kanik 2010).
Geologic assessment from orbital data is a rational first step in the exploration process. It is commonly used in pursuit of petroleum and mineral deposits on Earth in remote areas, incorporating satellite-derived topography/bathymetry, gravity, and magnetics for assessment of tectonic structure, basin architecture, and facies. The same approach should be particularly useful for planetary exploration where ground truth is commonly lacking but where orbital data sets are increasingly available. The approach is well suited to the search for extraterrestrial biosignatures and could be applied to more general planetary objectives, such as exploration for hydrothermal sediments, carbonates, or any particular type of geologic deposit.
This article grew out of a presentation made at the 1st International Conference on Mars Sedimentology and Stratigraphy in April 2010. We are grateful to Drs. D. Beaty (Jet Propulsion Laboratory) and J. Grotzinger (California Institute of Technology) for encouraging development of these ideas and to Drs. G. Etiope (Istituto Nazionale di Geofisica e Vulcanologia), A. Feyzullayev (Azerbaijan National Academy of Sciences), and K. Tanaka (Yamaguchi University) for providing mud volcano samples and insights into the process of mud volcanism. We are also appreciative of valuable discussions with Dr. P. Van Rensbergen (Shell Oil) regarding subsurface sediment mobilization. Dr. D. Van Rooij (Ghent University) provided the seismic images for Fig. 8. We thank the reviewers for many helpful suggestions and the Astromaterials Research and Exploration Science Directorate at Johnson Space Center for providing facilities and support.
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Sedimentary Geology of Mars
Often thought of as a volcanically dominated planet, the last several decades of Mars exploration have revealed with increasing clarity the role of sedimentary processes on the Red Planet. Data from recent orbiters have highlighted the role of sedimentary processes throughout the geologic evolution of Mars by providing evidence that such processes are preserved in a rock record that spans a period of over four billion years. Rover observations have provided complementary outcrop-scale evidence for ancient eolian and fluvial transport and deposition, as well as surprisingly Earth-like patterns of diagenesis that involve recrystallization and the formation of concretions. In addition, the detection of clay minerals and sulfate salts on Mars, coupled with large-scale morphologic features indicative of fluvial activity, indicate that water-rock interactions were once common on the martian surface.