Observations of the planets in our solar system cover a wide range of scales and are undertaken using a variety of techniques and platforms, resulting in extremely rich data sets. This review paper provides a basic introduction to the available range of planetary science data sets, and the combination of these data sets over a range of scales, resolutions, and techniques to address geological problems. The wealth of data available and the use of a selected combination of data sets to address geological problems are best illustrated by taking a closer look at the planet Mars. As a result of the increasing precision of spacecraft sensors, we now have data sets that cover the whole planet at spatial resolutions ranging from kilometers down to meters (e.g., Mars Global Surveyor) and multiple wavelengths (e.g., Mars Reconnaissance Orbiter), which have been collected over several years. This global coverage is complemented by surface missions that provide localized data sets down to microscopic resolutions (Mars Exploration Rovers). Thus, it is now possible to study geological features and processes quantitatively over an impressive range of scales. The combination of new data sets from current and future missions to Mars (e.g., Mars Express and Mars Reconnaissance Orbiter) will facilitate attempts to unlock the third dimension of Martian geology. The experience gained at Mars will help us to plan and exploit the 3-D exploration of other planets in the future.
Planetary science provides a basis from which to consider the formation and evolution of Earth in a wider context. It can be used as a prompt to reconsider the bigger picture, and ask fundamental questions such as: Why does plate tectonics occur only on Earth, and not on other planets? In the absence of plate tectonics, how do other planets lose their heat? What are the major mechanisms for planetary resurfacing? What geological processes operate on each planet (and how and when do they operate)?
The data available to us to try to address these questions can be divided into two distinct types: in-situ data and remotely sensed data. In-situ data encompass all the data from the Apollo missions to the moon, and all other data collected by instruments located on the surface of planets, including both stationary landers and rovers. It should be noted that not all in-situ data are obtained by instruments strictly in physical contact with the planetary surface. For example, all imaging and geochemical instruments on board stationary landers and rovers are generally considered to be in-situ instruments, even those that obtain images or analyze samples at distances of up to several meters. Remotely sensed data encompass all the data collected by spacecraft in orbit around a planet, or as a spacecraft flies-by a planet en route to another location. In general, in-situ data provide highly detailed information about a location that is somewhat limited in size, whereas remotely sensed data provide less detailed information about a location that covers a much wider area (often >90% of an entire planetary surface). In-situ data provide “ground truth,” which is then used to expand the interpretation of remotely sensed data sets.
All of the data from NASA planetary missions can be accessed through the NASA Planetary Data System (PDS) (for more information, see http://pds.jpl.nasa.gov/), or by contacting your local NASA Regional Planetary Image Facility (RPIF) (for more information, see http://www.lpi.usra.edu/library/RPIF/). All of the data from ESA planetary missions can be accessed through the ESA Planetary Science Archive (PSA) (for more information, see http://www.rssd.esa.int/index.php?project=PSA). Data from each mission are made available following a proprietary period, which varies in length for each mission (but is typically a number of months following initial acquisition of the data), during which time the data are processed and analyzed by the team of mission scientists. Updates to the available data are made on a regular (e.g., monthly or quarterly) basis, following the proprietary period, to facilitate further analysis and interpretation. The total planetary data archive is vast; for example, the data archive for the Thermal Emission Imaging System (THEMIS) instrument, which is just one of three major instruments on board the Mars Odyssey spacecraft, is expected to grow to approximately 6 TB.
In this review paper, we will use the exploration of the planet Mars to provide examples of the range of planetary data that is available, the range of instruments used in planetary exploration, and the range of scales that these instruments cover. We will discuss the use of planetary data sets to develop our understanding of planetary surfaces and possible ways of accessing the third dimension on a planetary scale.
OVERVIEW OF MARS EXPLORATION
There are three spacecraft currently in orbit around Mars acquiring and returning data to Earth: Mars Odyssey (2001), Mars Express (2003) and Mars Reconnaissance Orbiter (2006), in addition to the two Mars Exploration Rovers (2003), Spirit and Opportunity, that are currently exploring the Martian surface. These current missions continue to acquire data that complement that acquired by previous missions such as Viking (1970s), and the more recent Mars Pathfinder and Mars Global Surveyor Missions (1990s). Each of the spacecraft, landers, and rovers used in these missions carries a wide-ranging suite of scientific instruments, including high-resolution imagers and spectrometers, instruments used to identify the physical properties of the planetary surface such as thermal emission and topography, and instruments used to investigate the geophysical properties of the planet as a whole, such as gravity and magnetic fields. More recently, spacecraft have also begun to include radar sounders to penetrate the planetary surface and attempt to identify the properties of the subsurface. More information about the different types of scientific instrumentation used in Mars Exploration is available through the NASA Mars Exploration Web site: http://mars.jpl.nasa.gov/.
MARS IMAGE AND COMPOSITIONAL DATA
The wide variety of spacecraft, landers, and rovers involved in Mars exploration and their associated instrumentation allow us to investigate the Martian surface over a wide range of scales, covering many orders of magnitude. The rich diversity of the available data can be illustrated by considering the range of resolutions, and the range of wavelengths, of imaging instruments used by a selection of the various missions over the past 30 yr. The Viking orbiters, in the 1970s, provided grayscale images of the Martian surface at resolutions of 150 to 300 m per pixel (e.g., Fig. 1), dependent on the elevation of the spacecraft above the Martian surface (e.g., Snyder and Moroz, 1992). More recently, the Mars Orbital Camera (MOC) instrument, on board the Mars Global Surveyor (MGS) spacecraft, was designed to routinely acquire images at three different, complementary resolutions, using a limited number of color filters: (1) global images, typically at a resolution of 7.5 km per pixel; (2) wide-angle, context images, typically at a resolution of 240 m per pixel, using red and blue filters; and (3) narrow-angle, high-resolution images, typically at a resolution of 1.5 to 12 m per pixel (e.g., Fig. 2) (Albee et al., 1998). Following the huge success of the MOC instrument, the Mars Reconnaissance Orbiter (MRO) spacecraft carries three separate imaging instruments that operate over a range of scales comparable with those covered by MOC and incorporate a wider range of color options (Zurek and Smrekar, 2007). Firstly, the Mars Color Imager (MARCI) takes global images at five visible and two ultraviolet wavelengths, typically at a resolution of 1 to 10 km per pixel (e.g., Fig. 3). Secondly, the Context Camera (CTX) takes grayscale images, typically at resolutions of 6 m per pixel (e.g., Fig. 4), making observations simultaneously with the high-resolution images taken by the High Resolution Imaging Science Experiment (HiRISE). Finally, HiRISE operates at visible and near-infrared wavelengths and takes both panchromatic and color images, typically at resolutions of tens of centimeters per pixel (e.g., Fig. 5) (McEwen et al., 2007). The global coverage provided by orbiting spacecraft described above is complemented by in-situ images obtained by landers and rovers at resolutions ranging from a few centimeters per pixel to tens of microns per pixel. For example, the versatile Panoramic Cameras on the Mars Exploration Rovers, Spirit and Opportunity, can take long-range images typically at resolutions of 2.8 cm per pixel at a range of 100 m through to short-range images at much higher resolutions, while the Microscopic Imagers (MI), also on the Mars Exploration Rovers, take images typically at a resolution of 30 microns per pixel (e.g., Fig. 6) (Bell et al., 2003; Herkenhoff et al., 2004; Squyres et al., 2004a, 2004b). Thus, it is now possible to study geological features and processes on Mars over an impressive range of scales.
The Thermal Emission Spectrometer (TES) on board MGS measured the thermal infrared energy (heat) emitted from Mars, which can be used to constrain both the geology and the atmosphere of Mars. In total, TES collected over 206 million spectra. It made several important discoveries, the most influential of which was the discovery of hematite in Meridiani Planum (Christensen et al., 2001), a key contribution to NASA's decision to send the Opportunity rover to this region (e.g., Fig. 7). The Thermal Emission Imaging System (THEMIS) on board Mars Odyssey is a multi-wavelength camera, operating in five visual bands and ten infrared bands, that combines both thermal emission and imaging techniques. THEMIS is designed to further our understanding of Martian geology by detecting any rocks that have been altered by water, and any “hot spots” that may indicate the presence of subsurface hydrothermal systems. THEMIS has also made several important discoveries, including confirming the presence of hematite in Meridiani Planum and mapping its extent in greater detail (Christensen et al., 2005), thus validating NASA's choice of the region as the landing site for Opportunity.
The landing sites for both Spirit and Opportunity were chosen because they both showed clear but contrasting evidence for the presence of liquid water at their respective locations in the past. Hematite is considered to be a chemical signature for the presence of past water because it is usually produced in an environment including liquid water. In contrast, the landing site for Spirit, Gusev crater, is an enclosed, low-lying area with a channel produced by flowing liquid water running straight into it. Both Spirit and Opportunity have greatly enhanced our understanding of the Martian surface, through detailed analyses of their landing sites, and by enhancing our ability to interpret global data sets (Squyres et al., 2004a; 2004b; 2006; Arvidson et al., 2006). For example, in addition to many other instruments, both Spirit and Opportunity carry a Miniature Thermal Emission Spectrometer (Mini-TES). As the name suggests, Mini-TES is a miniaturized version of TES, and similarly collects high-resolution infrared spectra that help to identify the mineralogy of the Martian surface (e.g., Fig. 8) (e.g., Christensen et al., 2004). Thus, Mini-TES has been used to provide further “ground truth” for TES, THEMIS and other instruments.
The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), on board MRO, is the most recent instrument to arrive at Mars designed to determine the surface mineralogy and hence identify traces of past and present water on the surface of Mars. CRISM spans both visible and infrared wavelengths, specifically from 362 to 3920 nm, and is able to scan through these wavelengths at 6.55 nm per channel, covering the Martian surface at typical spatial resolutions of tens of meters (e.g., Fig. 9). The first results from CRISM are currently being received and analyzed (e.g., Murchie et al., 2006; 2007; Pelkey et al., 2007). Many of these results were presented at the 38th annual Lunar and Planetary Science Conference, including the initial components of a global multispectral survey, targeted observations of regions rich in sulfates and phyllosilicates, the north polar cap, and monitoring of seasonal variations (see http://www.lpi.usra.edu/meetings/lpsc2007/pdf/sess323.pdf).
Compositional data gathered by NASA missions are complemented by those gathered by the Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) investigation, on board ESA's Mars Express mission. OMEGA is designed to map the surface composition of Mars, typically at a resolution of 0.3 to 5 km per pixel, using a series of 352 contiguous spectral channels spanning both visible and near-infrared wavelengths (e.g., Fig. 10)(Bibring et al., 2005). OMEGA data illustrate the diversity and complexity of Martian surface mineralogy at the ∼km scale, developing our understanding of both the mineralogical and aqueous evolution of Mars, for example, through the identification of the alteration of some parts of the most ancient Martian terrains to clays (Bibring et al., 2005; Bibring et al., 2006).
Case Study: Integration of Data over a Range of Scales/Sinus Meridiani
The integration of planetary data from a variety of missions over a range of scales can provide important insights into the formation and evolution of surface features. For example, Edgett (2005) used data from the MGS and Mars Odyssey missions, supplemented with data from Opportunity, Mariner 9, Viking 1, Viking 2, and Phobos 2, to investigate the light-toned, layered rocks of the Sinus Meridiani region (e.g., Figures 1, 2, 5, 6, 7, 8, and 11). Although these rocks had been studied previously (e.g., Edgett and Parker, 1997; Christensen et al., 2000; Edgett and Malin, 2002; Hynek et al., 2002; Arvidson et al., 2003; Newsom et al., 2003; Christensen and Ruff, 2004; Christensen et al., 2004; Hynek, 2004; Ormo et al., 2004), Edgett (2005) used a combination of data from a variety of missions over a range of scales to develop a better overall understanding of these rocks, thus providing a framework for the geology of the Sinus Meridiani region that may be used to more readily understand the results of future field studies of the area in a wider context.
The framework laid out by Edgett (2005) is based on a series of simple, but fundamental, observations. The total stratigraphic section observed in the Sinus Meridiani region from orbit is more than 800 m thick. The Opportunity rover has explored a total of ∼7 m thickness, less than 1% of the total section observed from orbit, all located close to the top of the section. Unsurprisingly, the materials exposed across the region are more diverse than those observed in the ∼7 m of section explored by Opportunity. The light-toned, layered rocks observed in Sinus Meridiani are not unique to this region. Across the region, former valleys and impact craters are interbedded with the exposed rocks, and burial and exhumation of paleosurfaces are observed, confirming the presence of unconformities in the rock record. Differences between the materials deposited inside impact craters and those deposited in the surrounding area indicate that different depositional environments existed in close proximity.
The work of Edgett (2005) illustrates that the light-toned, layered rocks of the Sinus Meridiani region are sedimentary in nature, and are comparable with the sedimentary rocks of the Colorado Plateau in terms of the area covered and the diversity of erosional features, relative albedo, and bedding styles. In addition to the rocks exposed in Sinus Meridiani, similar rocks have been observed in two other regions of Mars (e.g., Figures 4, 9, and 10)—Mawrth Vallis and the plains cut by the Valles Marineris—suggesting that conditions for the deposition, lithification, and diagenesis of sedimentary materials may have been present on Mars in a variety of locations at a variety of times (Edgett, 2005).
MARS 3-D DATA SETS
To gain a deeper understanding of the geology of Mars, more three-dimensional information about Mars is needed. To prove that surface features result from the action of water, to determine how much water was involved in the process, and when the surface features formed, the 3-D shape of the surface feature in question is needed. 3-D information can also be used to identify landing sites for future missions. Firstly, it helps us to identify sites that are inappropriately dangerous places to try to land because of sharp topography or large numbers of boulders. Secondly, it helps us to understand interactions between the surface and the atmosphere, which have a significant impact on the balloons and parachutes that are used to ensure relatively safe landings. Finally, in combination with data from other instruments such as MOC on board MGS, it also helps us to identify places that are most appropriate to continue the search for life on Mars, e.g., where there is recent geologic activity (Malin et al., 2006; Okubo and McEwen, 2007; Crown et al. 2007), or where there may possibly be a frozen sea (Murray et al., 2005; Balme et al., 2007; Jaeger et al., 2007).
We have a high-resolution topography data set for Mars, acquired through the Mars Orbiter Laser Altimeter (MOLA) instrument on board MGS (e.g., Fig. 12) (Smith et al., 2001). The MOLA data have been converted to both gridded and spherical harmonic models for the topography and shape of Mars that have vertical and radial accuracies of ∼1 m with respect to the planet's center of mass. The MOLA global topographic grid has a maximum spatial resolution of 0.0039° × 0.0039° (0.23 × 0.23 km2 at the equator). The absolute horizontal and vertical accuracy of MOLA data is on the order of 1 m and 300 m, respectively (Smith et al., 1999a)
MOLA data are comparable with the Shuttle Radar Topography Mission (SRTM) data set for Earth (Gesch et al., 2006). SRTM data cover ∼80% of Earth's land surface (most of the land surfaces that lay between 60° N lat and 54° S lat), and have been used to produce standard DEMs for these regions of Earth that have a maximum horizontal resolution of 30 m. The absolute horizontal and vertical accuracy of SRTM data is 20 m and 16 m, respectively (Gesch et al., 2006).
The MOLA data set has been used to significantly improve our understanding of Mars. For example, MOLA data have been used to analyze the north polar ice cap on Mars (Fig. 13), which is one of the largest currently identified reservoirs of volatiles on the planet, and has a major impact on geological processes in the local polar region and on the seasonal and climatic evolution of the planet as a whole. Zuber et al. (1998) used MOLA data to calculate the total volume of the north polar ice cap and the seasonal variations in its morphology and extent, which provide important constraints on estimates of the present-day abundance of water on Mars. Zuber et al. (1998) also used the MOLA data to study a variety of geological features in the north polar region, including layered terrain, troughs and chasms, and impact craters, thus providing new insights into their formation and evolution.
Use of the MOLA data set has also resulted in a new understanding of the relative age of the Martian surface, which is critical for models of the formation of the Martian crustal hemispheric dichotomy. Prior to the MGS mission, the northern lowlands of Mars were interpreted to be relatively younger than the southern highlands, because of the apparent lack of significant numbers of impact craters in the northern hemisphere. However, a population of impact craters (Head et al., 2002), and subdued quasi-circular depressions that are interpreted to be ancient impact craters and basins that have been buried by subsequent resurfacing (Frey et al., 2002), have been identified in the northern lowlands of Mars using MOLA data, indicating that the northern lowlands are much older than previously thought (e.g., Fig. 14). The northern and southern hemispheres of Mars are now thought to have relatively similar ages, constraining the formation of the hemispheric dichotomy to early Martian history.
Both MRO and the Mars Express (MEX) mission, currently in Mars orbit, are addressing the need for more 3-D image data directly through the production of stereo images and DEMs from HiRISE data, and the High Resolution Stereo Camera (HRSC) on board MEX. Many of the initial results from HiRISE were presented at the 38th annual Lunar and Planetary Science Conference, including studies of volcanic and tectonic landforms and observations of specifically targeted regions (see e.g., http://www.lpi.usra.edu/meetings/lpsc2007/pdf/sess322.pdf). The HSRC is designed to acquire multispectral stereo images of the Martian surface to support the study of a variety of surface processes including volcanism, tectonism, impact cratering, erosion, and deposition (e.g., Fig. 10). HRSC context images typically have spatial resolution of tens of meters, whereas the additional Super Resolution Channel (SRC) images embedded within the context images have typical spatial resolutions of only 2 or 3 m, enabling us to develop detailed understanding of Martian surface processes in 3-D (Neukum et al., 2004).
Recent studies (e.g., Edgett, 2005; Malin et al., 2006) do successfully investigate Martian geology in 3-D. However, the instruments used in these studies can only access the rocks exposed at the surface. For example, the total vertical section exposed in the Sinus Meridiani region is less than 900 m. In the next section of this paper, we look at instruments that are designed to probe the subsurface.
WHAT ABOUT THE SUBSURFACE?
The first instrument to produce a global data set of observations of the near subsurface of Mars, to depth of tens of centimeters, is the Gamma Ray Spectrometer (GRS) on board Mars Odyssey (Boynton et al., 2004; Evans et al., 2006). The GRS global data set provides information on the spatial distribution of elemental abundances, which have been used to further our understanding of the bulk composition and early differentiation, crustal and atmospheric evolution of Mars (e.g., Newsom et al., 2007; Taylor et al., 2006; Sprague et al., 2007). Thus, the GRS data span the gap between the wide variety of data sets produced by instruments designed to investigate the surface and those that are designed to investigate the deep subsurface of Mars.
To access the deep subsurface, instruments need to use techniques that can penetrate to depths of several kilometers. Currently, the most appropriate technique is sounding radar. There are two radar instruments currently operating in Mars orbit: the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on board MEX and the Shallow Subsurface Radar (SHARAD) instrument on board MRO. Both MARSIS and SHARAD are designed to answer one of the fundamental questions about the history of Mars: Where did all the water go? There is abundant evidence that liquid water was once present on the surface of Mars, but we don't know where the water is now. Some water may have escaped the planet along with the majority of the Martian atmosphere, but the rest may still be trapped in the subsurface in solid or liquid form. MARSIS is mapping the subsurface structure to a depth of a few kilometers, using low-frequency radio waves that are reflected by any subsurface contacts between layers with different dielectric properties, enabling us to distinguish between layers of different materials that may be present, including water and ice (Picardi et al., 2005). Early results from MARSIS include the investigation of the north polar layered deposits, indicating that these deposits cover a total thickness of 1.8 km, and that they are composed of nearly pure water ice (Fig. 15). A buried impact basin that is potentially partially filled with ice was also identified (Picardi et al., 2005). More recently, the detection of a large number of buried impact craters in the northern hemisphere of Mars by MARSIS confirms that the crust beneath the plains is similar in age to the Martian southern highlands, placing constraints on models of the origin of the Martian crustal dichotomy (Fig. 14) (Watters et al., 2006). SHARAD is mapping the subsurface structure at shallower depths than MARSIS (Seu et al., 2004; 2007). Preliminary analysis of early results from SHARAD is also focused on the north polar layered deposits (Phillips et al., 2007). Given the possible identification of deposits formed recently by water on Mars (Malin et al., 2006), SHARAD may provide exciting results.
GRAVITY AND MAGNETIC DATA
In addition to the novel radar sounding techniques described above, more traditional geophysical approaches can also provide data that constrain the nature and structure of the Martian subsurface (e.g., Figures 16 and 17). Models of the Martian gravitational field are used to constrain the nature and evolution of the Martian crust (e.g., Smith et al., 1999b; Neumann et al., 2004; Wieczorek and Zuber, 2004) and models of the internal structure and thermal evolution of Mars (e.g., Zuber et al., 2000), as well as contributing key data to models of the Martian atmosphere and the seasonal changes in the location of carbon dioxide deposited at the surface (e.g., Smith et al., 1999c). Similarly, magnetic data for Mars are used to constrain the nature and evolution of the Martian crust (e.g., Acuna et al., 1999) and models of the internal structure and thermal evolution of Mars (e.g., Connerney et al., 1999).
Each new mission to Mars provides additional gravity data: for all orbiting spacecraft, the Doppler shift in the radio communications signal can be used to determine the Martian gravitational field (e.g., Lemoine et al., 2001). Each new mission provides a subtly different data set that can be used to enhance our understanding of the Martian gravitational field. For example, the gravity data from MRO will be used to understand the subsurface structure of Mars on the scale of several hundred kilometers, and the rigidity of the planet as a whole. In contrast, MEX can provide gravity data at relatively small wavelengths because of the relatively low periapsis of the spacecraft orbit, and can therefore provide new constraints on the local structure of the Martian crust and lithosphere. Thus, the gravity data from the MEX mission have been used to validate existing global gravity models developed using data obtained at longer wavelengths during earlier missions (Beuthe et al., 2006).
The combination of data from a variety of planetary science instruments that have been obtained at a range of scales is being used to help us to address significant geological questions, such as what geological processes operate on each planet (and how and when do they operate)? These extensive data sets address the fundamental aim of planetary science: to understand the formation and evolution of Earth in a wider context.
Imaging instruments at Mars such as those on board MEX and MRO are designed to provide 3-D data sets to complement DEMs provided by MOLA, but many of these data sets are still limited to the observation of surface outcrops. It is necessary to access the subsurface to assess geological problems in three dimensions. New sounding radar instruments on board MEX and MRO are now helping us to unlock the third dimension and develop a better understanding of the formation and evolution of the Martian surface. The ability to integrate subsurface information, such as that provided by MARSIS and SHARAD, is of fundamental importance to our understanding of planetary geology.
Our experiences in combining Mars data from a range of instruments over a range of scales will be applied to our studies of other planets. For example, our understanding of the Saturnian system is rapidly improving through the combination of different data sets from the Cassini-Huygens mission (e.g., Lebreton et al., 2005; Stofan et al., 2007; Witasse et al., 2006).
A major challenge in future planetary studies is the integration of the fourth dimension: time. Over relatively short time periods, this is already being achieved. For example, the MGS spacecraft collected data over a sufficiently long time period (almost a decade) to detect recent activity in some Martian gullies (e.g., Fig. 18), illustrating the fact that the Martian surface is still active to some degree (Malin et al., 2006). However, current planetary geological timescales are based on assigning ages to planetary surfaces depending on the number of impact craters identified on that surface. By unlocking the third dimension, the assumptions of this dating technique are called into question (e.g., by our new understanding of the mechanisms of burial and exhumation of impact craters (e.g., Arvidson et al., 2003), and the identification of many buried impact craters in the northern hemisphere of Mars (Watters et al., 2006)). Collecting data over sufficient time periods to quantify the scales, extents, and rates of planetary geological activity (e.g., active volcanism on Io and lake level fluctuations on Titan) is of fundamental importance.
Building on the current legacy of planetary exploration, several further planetary missions are planned for the future. For more information on NASA's vision for space exploration, see: http://www.nasa.gov/mission_pages/exploration/main/index.html. More information on ESA's program “Expanding Frontiers,” and their long-term strategy for human exploration of space, called the “Aurora Exploration Programme,” is available at, respectively: http://www.esa.int/esaCP/Expanding.html and http://www.esa.int/SPECIALS/Aurora/index.html. More information on plans for space exploration from the Japanese Aerospace Exploration Agency (JAXA), the China National Space Administration (CNSA), and the Indian Space Research Organization (ISRO), is available from, respectively: http://www.jaxa.jp/index_e.html, http://www.cnsa.gov.cn/n615709/cindex.html, and http://www.isro.org.