- Status Report
- Feb 4, 2023
Determinative Mineralogy: An essential component of Planetary Exploration – Decadal White Paper of the Extraterrestrial Mineralogy Panel
Our existing knowledge of extraterrestrial mineralogy has been gained largely from analyses of returned or collected samples returned samples and collected meteorites from the Moon, meteorites from Mars, and the more extensive set of meteorites that represent early solar system bodies. However, there is a large gap between what we have learned from these samples and what is inferred from remote planetary exploration. This gap can be closed by multidisciplinary efforts to extend the capabilities of remote analysis, to provide a better correlation between remotely obtained data and mineralogic reality, and to use the remotely obtained data to carefully select landing sites and to collect samples for return to Earth.
There are many tools to be used in space exploration and the exploration of extraterrestrial mineralogy is no exception. The tools that are needed include remote observations, in situ analysis with landed instruments, and analysis of samples in laboratories on Earth. In a balanced exploration program, all three sources of data are necessary. Remote sensing covers large portions of body and places mineralogy in an integrated whole-body context. Landers with instruments can provide many on-site analyses for ground-truth data on a scale that approaches the "footprint" of the orbital instruments. If appropriate instruments are used on landers, accurate information can be obtained in situ on chemistry and mineralogy, and it is likely that a combination of landed diffraction, chemical, and spectroscopic instruments can provide unambiguous identifications and detection limits comparable to those obtained during routine analyses on Earth.
Naturally, mineralogy studies, whether lander based or done with returned samples, will be no more representative than the samples chosen for analysis. However, returned samples (or fortuitous meteorites) provide the ultimate assurance of mineral identity and composition, for the ability to dissect and probe and re-analyze samples by human researchers in their laboratories will not be matched by remote methods. Clearly, sample return will be required to gain information on mineral zonation, the existence of nanofossils or indirect mineralogical evidence of biological processes, to see features such as the tiny reduced Fe metal grains in lunar samples, or to detect new phases such as Yoshiokaite, which is a product of extreme volatilization associated with impact processes on the surface of the Moon. The sequencing and emphasis placed on these three tools sets (remote sensing, landers, and samples in hand) will depend on the bodies being studied, but all three tool sets have important contributions to make in understanding extraterrestrial mineralogy and in using knowledge gained from minerals to understand planetary processes.
In this white paper, a definitive identification of an individual solid phase having a specific crystal structure constitutes confirmation of a specific mineral. On Earth, this definitive identification is typically gained using methods that are sensitive to the crystal structure of a solid phase, including optical methods, vibrational spectroscopy, and diffraction methods. The most common method of phase identification in the past eight decades has been X-ray diffraction, which relies on the fact that all crystalline substances diffract X-rays in a known, predictable fashion, producing a pattern that can be used much like a fingerprint. Other methods such as reflection and emission spectroscopy, Raman spectroscopy, and Mössbauer spectroscopy are sensitive to specific attributes of minerals and organic materials and in many cases also provide fingerprint identifications. Each of these methods has specific strengths and weaknesses, and when used in combination can provide a complementary and more complete assessment of mineralogy. Indeed, a well-designed payload for study of extraterrestrial mineralogy should consist of several instruments, operating together to provide the most complete mineralogical picture possible.
GOALS OF EXTRATERRESTRIAL MINERALOGIC STUDIES
There is a wide variety of objects in our solar system for which mineralogic studies are needed to provide true understanding:
The rocky planets Mercury, Venus, and Mars,
Rocky planetesimals, including asteroids and planetary moons,
Icy planetesimals, including Kuiper belt objects, Oort cloud comets and icy moons
What would we like to learn by studying these objects?
Origin of the Solar System. Asteroids and comets have crystallization ages which predate the formation of the planets. An understanding of their mode of formation will shed light on early solar nebula processes and planet formation.
Comparative planetology. By analyzing planetary materials from Mars, Mercury, Venus, Earth, and the Moon, information on early differentiation, plate tectonics, upper mantle and crustal processes, and the presence/absence of magnetic fields over time can be determined and compared.
Natural history of volatiles, biogenic elements. Water and other volatiles, including the entire inventory of the biogenic elements H, C, O, N, S and P may have been deposited onto the surfaces of the rocky planets as a thin veneer during the late bombardment of the early solar system. What is the source of the volatiles? What is their distribution over space and time?
Did life evolve there? Earth harbors the only example of life as we know it, but we have reason to believe that life could originate wherever appropriate conditions exist (liquid water, a source of simple carbon-containing molecules and energy). Such conditions apparently existed on Mars in the distant past, and perhaps in the water ocean thought to exist below the ice-covered surface of Europa.
Can we live there? Human missions to Mars are being planned, perhaps as early as 2018. Permanent colonization will require utilization of local resources for food and shelter, in addition to knowledge of the environmental hazards associated with surface dusts.
Much of this information is contained in the mineralogy of planetary surfaces. We have learned a great deal from remote sensing techniques, but definitive answers, particularly where mineralogy is involved, will require in-situ instruments or samples returned to Earth.
X-ray diffraction has been the workhorse and the standard for determinative mineralogy for many decades. However, improvements in the instrumentation and understanding of other spectroscopic methods that are sensitive to mineralogy and the development of extensive data bases and cross reference to the extensive XRD databases has increased their use in mineralogical studies. The development of CCD detectors and miniaturization of components has made some of the spectroscopic methods especially suitable for high-resolution mapping from orbital platforms. The rapid pace of improvements in laser technology has led to increased applications of some methods.
HISTORICAL PERSPECTIVE OF INSTRUMENTATION IN PLANETARY EXPLORATION
1) X-ray Diffraction
Between 1960 and 1968, considerable effort was expended in designing and building a practical XRD for use on the Moon (e.g., Dunne and Nickle, 1968). The first proposal for a planetary XRD was from William Parrish of Philips Electronic Instruments in 1960. A series of breadboard and prototype instruments was constructed at JPL in the following years. The final flight prototypes, which were Bragg-Brentano geometry instruments utilizing a Cu X-ray tube, Soller slits and Xe gas proportional detectors, achieved a performance equal to laboratory diffractometers. The instruments suffered from relatively unreliable power supplies, and, at 20 kg, were heavy for spacecraft instruments. The lunar XRD program was ended in 1968 without ever deploying an instrument on a lunar mission.
A number of prototype XRD instruments were built and evaluated during the years 1964-1968. Combined XRD/XRF instrument concepts were investigated as well, and it was concluded that such capabilities were feasible in a flight instrument using current-day technology. In addition to the basic instrumentation, simple drills and sample delivery devices were devised to provide powder samples to the instrument. The U.S. Geological Survey utilized one of the prototype XRD instruments as a field-operable system, and performed hundreds of diffraction analyses during two field tests.
There is a renewed interest in XRD instrumentation for space exploration. The power of XRD in mineralogical analysis is compelling because it remains the most versatile and reliable method for characterization of materials with crystalline structure (the presentation of definitive XRD data is still the key requirement in acceptance of a new mineral description by the International Mineralogical Association). Vaniman et al. (1998) present a case for XRD (combined with XRF) in the search for deposits that may host evidence of life on Mars. Sarrazin et al. (2000) provide a broader analysis of the combined XRD/XRF method and its potential applications in determination of extraterrestrial mineralogy. As with several other methods described below, XRD (combined with XRF) has benefited from and has yet undeveloped potential based on new CCD technology.
2) Reflectance and Emittance Spectroscopy
Reflectance and emittance spectroscopy relies upon the interaction of electromagnetic radiation at visible to near infrared wavelengths with particulate materials of natural surfaces. These interactions involve a combination of reflection, absorption, scattering, and emission processes, dependent on geometry of the interaction, physical character of the surface, and the wavelength of radiation (Hapke, 1993). Reflectance and emittance spectroscopy are sensitive to both crystalline and amorphous materials. A key advantage is that they can be used in the laboratory as well as remotely (e.g. from orbit). Some of the disadvantages are broad, overlapping spectral features and sensitivity of these features to small changes in the chemistry and structure of a material. Variations in material composition commonly cause shifts in the position and shape of absorption bands. Thus, with the great variety of chemistry typically encountered in natural materials, spectral signatures can be quite complex and in some cases unintelligible. However, recent improvements have increased knowledge of the natural variation in spectral features and the causes of the shifts. As a result, the previous disadvantage can and has been turned into an advantage, and extensive libraries of a wide variety of materials of known composition and mineralogy are now available.
With the advances in computer and detector technology, the new field of imaging spectroscopy is developing (reviewed by Clark, 1999). Imaging spectroscopy is a technique for obtaining a spectrum in each position of a large array of spatial positions so that any one spectral wavelength can be used to make a recognizable image. The image might be of a rock in the laboratory, a landing site from a descent image or a lander or rover on the surface, or a whole planet from an orbiter. By analyzing the spectral features, and thus specific chemical bonds in materials, one can map where those bonds occur, and thus map materials, e.g., using spectral feature analysis. Even if endmember minerals (components) are not known, variations and material diversity can be mapped in heterogeneous materials such as rocks.
3) Thermal Emission Spectroscopy
Thermal emission spectroscopy, which is currently in use in Mars orbit (MGS-TES instrument), senses infrared radiation emitted by objects in the mid to far IR wavelength region. This range covers vibrational absorption features that are diagnostic of mineral structure and composition. Each spectrum is produced by fundamental vibration bands of major covalent ionic groups, lattice vibration bands, overtones, and combination modes. In principle, mineral identification can be accomplished by direct comparison of a measured spectral pattern with spectra of pure minerals, given a suitable spectral database (e.g., Christensen et al. 2000). The strongest IR bands of most oxyanionic minerals occur in slightly different wavelength regions, and these can be used for first-order determination of the mineral type (carbonate, sulfate, phosphate). For silicates having different degrees of polymerization (e.g., orthosilicates, single-chain, double-chain, layer, and framework silicates), the major fundamental vibrations of the SiO44- group (asymmetric stretching and bending modes) show characteristic regularities in shifts of their peak positions. Such shifts can be used to classify these silicates and to estimate the rock types. A deconvolution calculation is essential in order to make mineral phase identification from the spectra of mixtures (rocks and soils), mainly due to the broad width and complicated shape of thermal emission spectral bands. Such a procedure has been developed by the TES science team and has been used for MGS-TES data and terrestrial rocks (Wyatt et al. 2001). Making measurements in the thermal IR provides the capability to penetrate through dust coatings common, for example, to the martian surface that can present problems for remote sensing observations.
4) Mössbauer Spectroscopy
57Fe Mössbauer spectroscopy is based upon the interactions between the Fe nucleus and its surrounding electromagnetic field, which is determined by the symmetry of the crystal framework and the types of atoms and bonds in the minerals where it occurs. Iron of different valence states (Fe0, Fe2+, Fe3+) and different spin states, and located in different crystallographic sites in different mineral phases, yield Mössbauer spectra with distinctive patterns. 57Fe Mössbauer spectra of pure minerals are linear combinations of these patterns. The number of components of each pattern in a spectrum of a mineral depends on the number of different types (valence and spin) of Fe in that mineral and the number of crystallographically equivalent sites where Fe occurs. From the Mössbauer spectrum of a mineral or rock, the valence, electronic configuration, coordination symmetry, site occupancy, and magnetic state of Fe cations in their host crustal structures may be determined. The relative contributions of different forms and crystallographic environments of Fe are proportional to the ratios of their Mössbauer peak areas. Mössbauer spectroscopy is thus well suited to the study of Fe-minerals including oxides, hydroxides, sulfides, silicates, and nanophase Fe metal, and should be very useful on the surface of Mars where Fe-rich mineralogy is expected on the basis of the SNC meteorites and previous remote and in-situ studies (Morris et al. 2000).
A significant challenge in the use of reflectance, emission, and Mössbauer spectroscopies is the investigation of mixtures (e.g., rocks and soils) because of the substantial overlap of spectral features of different components. It is thus usually not possible to identify different phases from mere inspection of raw spectra of rocks and soils (although in some cases, a rough estimate of assemblage can be made from the shape of a mixed spectrum). Deconvolution procedures are well developed for Mössbauer spectra. Spectral peak overlap among some minerals, however, is severe enough that they cannot be distinguished in a mixture even with the most sophisticated deconvolution procedures.
Linear deconvolution methods have been developed for thermal emission spectroscopy that can effectively discriminate some mixtures of minerals, for example, the members in a series of igneous rocks (e.g., basalt-andesite-dacite). Such methods rely on the assumption that a thermal emission spectrum of a mixture is a linear combination of the spectra of its constituent minerals and that the spectrum of a solid solution is a linear combination of the spectra of end-members, which is approximately true only for fundamental vibrations. Deconvolution of thermal emission spectra is more complicated than that for Mössbauer spectra, for which the shape of spectral peaks is fixed and well understood.
5) Laser Raman Spectroscopy
Laser Raman spectroscopy is used to study the inelastic scattering phenomena in analyzed samples stimulated by a monochromatic light source. The wavelength differences between the Raman scattered radiation and laser wavelength are totally determined by the molecular properties of the target, i.e. its structural and compositional features. Measurement of the wavelength difference yields the so-called Raman shift, and for a given mineral, many different molecular vibrational modes contribute to a "fingerprint" pattern. Laser-produced, monochromatic light of ultraviolet, visible, or infrared frequency can all be used as an excitation light source to produce a Raman signal. A visible laser is normally used for instrumentation convenience and higher Raman efficiency. Raman spectroscopy provides similar molecular vibrational information as mid-IR (i.e. TES) does, but with different selection rules, and can work in the visible region. Raman spectra of minerals contain mainly fundamental vibrational modes and lattice modes; the overtones and combination modes are 1-2 orders of magnitude weaker. Thus, Raman spectral peaks are typically very narrow, sharp, and non-overlapping for well crystalline minerals and many organic compounds. The strongest Raman peaks of oxyanionic minerals shift systematically according to their molar mass and bonding strength; the Raman spectral patterns of silicates change with degrees of polymerization. Thus phase identification can be made from the inspection of raw spectra. For poorly crystalline or amorphous material, peaks may be subdued or broad, but peak positions still reflect the dominant molecular components and their structures. The Raman effect is best for molecular structures that have high degrees of covalent bonding (oxyanionic compounds), thus Raman spectroscopy is well suited for the identification of silicates, carbonates, sulfates, phosphates, nitrates, borates, oxides, sulfides, and hydroxides, as well as organic compounds. Because of the difference in selection rules, graphitic carbon (and molecules like O2, N2) cannot be detected by IR techniques, but can be detected down to 50 ppm by using the Raman technique. Variations in composition within mineral groups typically result in predictable shifts in peak positions, but not in changes to the overall spectral pattern unless the compositional variation is accompanied by a significant change in mineral structure (e.g., Wang et al., 2001). Because the incident laser beam can be designed to provide a very small laser spot size and can be used over a significant focal range, the spectra of individual mineral grains of many multiphase materials such as rocks can be determined and interpreted without recourse to complex spectral deconvolution (e.g., Haskin et al., 1997). Analysis can generally be done without any preparation of the target material. For rocks and soils, that means characterizing their properties as they are found in nature.
IN-SITU MINERALOGICAL ANALYSIS OF THE SURFACES OF EXTRATERRESTRIAL BODIES
One of the most informative characteristics of a planetary surface is its mineralogy. Suites of surface minerals can be used to characterize past and present climatic conditions, sedimentary weathering processes or hydrothermal activity. More than elemental data, mineralogical data are linked to surficial conditions and processes and can be used to elucidate present and past conditions of the atmosphere, the surface, the crust, and occasionally the deep interior of a planet. Minerals have known ranges of stability and paragenetic relationships. Even when minerals persist out of their stability fields (as is common in sedimentary rocks), disequilibrium associations, or the presence of relict unstable phases, can be used to unravel detailed sedimentary or diagenetic histories that can be linked to climate history.
The importance of mineralogical studies extends beyond the terrestrial planets and rocky moons. XRD has been proposed for landed missions to icy bodies. The possibility of a Discovery-class mission to one of the poles of the moon to look for water ice or hydrated minerals has also been discussed (Sarrazin et al, 2000). XRD, combined with XRF in a single instrument as proposed by Sarrazin et al. (2000), would be equally suited to the task of evaluating the mineralogy of the colored fracture zones of Jupiters moon Europa, thought to be due to hydrated salts (McCord et al., 1999). An evaluation of the mineralogy of the ices and salts present in these zones would shed light on the nature of the proposed ocean deep beneath Europas icy crust. The mineralogic composition of cometary is equally important. The need for in situ mineralogical studies is particularly pressing for these icy bodies, for it is very unlikely that samples can be collected from them and returned to Earth unaltered.
REMOTE SENSING OF SOLAR SYSTEM OBJECTS
Remote spectral analysis techniques have been applied to many objects within our solar system, with the result that in general terms, we know the compositions of the solid surfaces that we can image. In some instances, high-resolution spectral data are available as is the case for the Moon, Mars, and Europa, among others. However, even these high-resolution images and analyses have lateral spatial resolutions of tens to thousands of meters, many orders of magnitude larger than the scale length of the phases in surficial deposits (e.g., minerals) which comprise the images. The elemental / chemical information obtained by remote sensing, while informative, has spawned a cottage industry of studies of analog materials and a myriad of interpretations. Remote sensing is principally useful in the formulation of hypotheses that can later be investigated by in-situ analysis using techniques that can provide more unambiguous mineralogical information.
1) The Moon
The Earths moon has been visited by us a number of times, and during the Apollo program, several hundred kilograms of rocks and soil were returned to the Earth for analysis. However, it is surprising to some to learn that we are still very much interested in returning to that body to analyze additional materials. The Neutron Spectrometer on the Lunar Prospector mission identified excess hydrogen within craters at the poles of the Moon, thought to be water ice (?) about 40 cm beneath the surface in perennially shaded regions (Feldman et al., 1998). This water is in all likelihood a vestige of the bombardment of the inner solar system by icy planetesimals. The water is presumably from the same outer solar system reservoir of icy objects that must have bombarded the Earth, and we would like to know whether the Earths oceans are indeed a legacy of cometary influx. This original hypothesis that the worlds oceans and other volatiles were delivered to a barren and rocky, volatile-free early Earth, has been challenged by recent remote analyses of a number of cometary bodies which suggest that cometary water is too high in deuterium to have contributed significantly to the ocean. The nature of the water reservoir at the lunar poles is uncertain. The Lunar Prospector fast and epithermal neutron data indicate as much as 40 cm of desiccated regolith above water-ice reservoirs in permanently shadowed polar craters. It is unlikely that this reservoir is a simple accumulation of ice within regolith pore space. Limited data on the lunar geotherm (four measurements from the Apollo missions) indicate temperatures of ~245-255 K at regolith depths of 40 cm, well below the freezing temperature for pure water ice but within a range where brines may exist as liquids.
These factors indicate that any surface exploration of the lunar poles for water should be provided with (1) an ability to drill to depths of >40 cm and (2) the capability to determine the mineral and chemical constituents of the samples obtained. The samples obtained by such a system could be readily analyzed by XRD/XRF. X-ray diffraction analysis, coupled with X-ray fluorescence can explicitly determine not only the presence of hydrous alteration phases such as clays or zeolites, but can also identify the structural variants or types of clay or zeolite present (e.g., well-ordered versus poorly-ordered smectite; chabazite versus phillipsite). Finally, if coring and analysis is performed during the lunar night, XRD can provide information on any crystalline ices that might occur in the regolith samples. Beyond an abiding scientific interest in the presence of water and its source, the identification of a significant source of water on the Moon vastly improves the chances for success of any lunar colony.
Evidence for the presence of water at the lunar poles follows many years of speculation regarding lunar polar ice, assumed to be accumulated as cold-trapped volatiles released from cometary impacts (Watson et al., 1969; Arnold, 1979). Before the Lunar Prospector data were acquired, the presence of any water in lunar regolith has been highly suspect because of the complete absence of evidence for water-rock interactions in any lunar samples. This evidence ranges from the absence of aqueous drivers in lunar pyroclastic eruptions (where sulfur and halogens dominate) to the well-documented absence of hydrous minerals in lunar samples. Of course, no samples have been obtained from the lunar poles. The Lunar Prospector data have rekindled the interest in sampling these regions.
These factors indicate that any surface exploration of the lunar poles should be provided with (1) an ability to drill to depths of >40 cm and (2) the capability to determine the mineral and chemical constituents of the samples obtained. It is important to note that successful robotic drilling operations have already been fielded on the Moon; three Soviet Luna missions were able to drill successfully to depths of 35, 27, and 160 cm into the lunar regolith. The last and deepest robotic sample (Luna 24) was based on a sophisticated but easily replicated drilling system that utilized a core-lining membrane. The samples obtained by such a system could be analyzed by a number of methods to determine whether hydrous minerals have formed in icy regolith at the lunar poles from the Moons own heat or from the heat of regolith-gardening processes.
Mars occupies a special place in solar system exploration because of widespread interest in extraterrestrial life. The importance of Mars to our exploration activities is illustrated by the fact that NASA is planning two missions to Mars, an orbiter and a lander, at each launch opportunity (about every two years). The principal emphasis of this robotic exploration initiative is first, to ascertain whether life ever existed on Mars, and second, to pave the way towards human exploration in the early-to-mid 21st century.
The Importance Of Mineralogical Analysis To Mars Paleoclimatology And The Exobiological Exploration Of Mars
Mineralogical analysis could lend insight into the early history of Martian volatiles, could establish the presence and lateral extent of hydrothermal systems, and could reveal the locations of rock types (silica sinter, travertine, etc.) which may harbor evidence of liquid water, prebiotic organic material or even extinct life. Once mineralogical surveys are completed, follow-on studies could perform in situ mineralogical analyses of rock outcrops or select the most likely candidate rock samples for return to Earth. Categories of paleoclimatological / exobiological research interest are listed below along with a description of their distinctive mineralogical characteristics.
The Presence And Lateral Extent Of Hydrothermal Systems
Abundant morphological evidence exists for early and extensive volcanic activity on the surface of Mars, and for the presence of liquid water. The juxtaposition of these features is compelling evidence that hydrothermal systems once (or have always) existed on Mars. Isotopic evidence from the SNC meteorites also suggests that there was an exchange of hydrogen and oxygen between the crust and the atmosphere. The depletion of volatiles such as sulfur and carbon is puzzling and these elements may now be contained within minerals precipitated in hydrothermal systems. A knowledge of the presence and distribution of sulfur- and carbon-containing mineral phases is important because first, they are storage sites for volatiles, and second, hydrothermal systems might have contributed significantly to the Mars atmosphere over time.
Ancient hydrothermal systems could have been eroded or exhumed, exposing minerals and mineral assemblages at the surface which were formed at depths inaccessible in presently active systems. The mineralogical characterization of such a system would provide an evaluation of the role hydrothermal processing has played in modifying the early Martian atmosphere and in altering deep-seated igneous rocks. Hydrothermally altered rocks, dissected and exposed at the surface by erosion or impact cratering, may contain evidence of the nature of the early Mars atmosphere and of the fate of its volatiles.
Evidence Of Prebiotic Organic Chemistry
On the Earth, all evidence of prebiotic organic chemistry has been erased. In the earliest terrestrial rocks for which conditions of formation and of subsequent metamorphism would permit it, evidence of life is present. Therefore, even if life never originated on Mars, it would be exceedingly valuable to find some evidence in the geologic record of Martian prebiotic organic chemistry. This would be possible on Mars more than on the Earth since, due to the apparent absence of plate tectonics, extensive regions of ancient terrain exist on Mars which have not been subject to metamorphism. Current concepts of prebiotic organic chemistry suggest that many important reactions have occurred in hydrothermal systems where some energy could have been provided by mineral hydration reactions. Hydrothermal systems also provide a means for gas exchange with the atmosphere and transport of reactants to the sites of reactions.
It is often the case that hydrated minerals and their anhydrous counterparts are compositionally very similar. Thus, elemental analyses would not easily distinguish one from the other. Mineralogical analysis, however, comprising both compositional and structural information, can provide a definitive result. Once hydrothermal terrains are identified by their distinctive mineralogy, areas or samples can be chosen for more comprehensive organic analyses or sample return.
Evidence Of Extinct Life
Evidence of life on the Earth occurs in the earliest rocks which could have preserved signs of its presence ~3.5 billion years ago (recent reports indicate that life may have been present 3.85 billion years before the present; (Mojzsis, 1996). However, much of the geologic record from the earliest sedimentary sequences has either been heated to the extent that metamorphism would have removed evidence of life, or has simply been destroyed by burial and subduction. Because of the apparent lack of plate-tectonic activity on Mars, a great deal of the sediment deposited on the ancient Martian surface probably still exists and was probably not heated to as high a temperature as equivalent age sediments on the early Earth. Impact heating could still have obliterated ancient surface material, but orbital mapping should be able to identify old, undisturbed sediments. Therefore, it is likely that if life originated early in Martian history, some record of its existence would be manifest in the earliest geologic record which should be accessible to surface landers.
Life as we know it requires liquid water, a source of energy and nutrients. Good preservation requires early burial or entombment in fine-grained clastic sediments or chemically precipitated sedimentary materials such as silica or carbonate. In the search for evidence of extinct life on Mars, we can be guided by our knowledge of the existence, characteristics and style of preservation of organisms on the early Earth. We know that for 4/5 of Earth history, life consisted of nothing more than single-celled microbes living in lacustrine or marine environments. The presence of these microbes is typically revealed by the observation of sedimentary structures (i.e., stromatolites) or trace fossils, not by the organisms themselves.
Absolute determination of the presence of prebiotic organic material or of evidence of extinct or extant life will most likely require the return of a sample to Earth. The purpose of in situ sample analysis on the Martian surface would be to survey rocks or sediments, to ensure that the very best candidates for fossiliferous strata are collected, and to provide a global or regional context for the samples brought back. A summary of mineral types and the significance of each to the history of volatiles on Mars and their Exobiological significance is presented in Table 1.
History of Volatiles on Mars
Mars is presently a dry eolian planet with a tenuous atmosphere of ~7 millibars, dominated by CO2. However, ancient surface morphological features such as stream channels and other fluvial and lacustrine features provide compelling evidence that liquid water existed on the surface of Mars in large quantity. This apparent abundance of liquid water implies that early Mars was once wetter and warmer, and had a dense atmosphere.
The ultimate fate of atmospheric CO2 is of interest to paleoclimatologists and exobiologists because this compound figures prominently in prebiotic organic chemistry and because a full inventory of the CO2 sinks is required to provide a balanced volatile budget for the planet. A major reservoir for the carbon dioxide could be in the form of carbonates deposited as chemical sediments or as hydrothermal precipitates. Alternatively (or additionally), CO2 could be stored in the form of clathrate hydrates beneath the surface or in the polar caps or solid carbon dioxide at the poles, in addition to that CO2 which is seasonally deposited there. Each of these phases can be unequivocally distinguished and characterized by diffraction techniques. In the case of carbonates, the crystal symmetry (rhombohedral or orthorhombic), the extent of cation solid solution (Ca, Mg, Fe, Mn, etc.), cation order/disorder and cation stoichiometry can all be determined through mineralogical analysis. Each of these distinguishing characteristics provides information about the specific origin of the mineral phase and its environment of formation.
The total quantity of water that apparently existed on the surface of Mars early in its history cannot be accounted for by the polar caps alone. In addition to that water lost to space, it is likely that hydrated phases exist either as a consequence of their direct deposition from aqueous solution or as products of the reaction of anhydrous igneous minerals with water. The quantity, type and degree of crystallinity of clays, micas and other hydrated phases can be determined by mineralogical analysis and their known stability relationships can constrain the conditions under which they formed.
Our current best knowledge of extraterrestrial mineralogy is based on samples in hand from the Moon and meteorites. Many of the questions we wish to answer will still require samples returned for very detailed analysis (witness the recent efforts devoted to Martian meteorites). Although the need to analyze returned samples will remain for some time to come, the state of the art in remote analytical methods is progressing at a very rapid pace. In reference again to the experience from Martian meteorite studies, it is evident that no single method of mineralogical analysis is sufficient, whether in a laboratory on Earth or from a spacecraft. Combined methods gain far more than isolated analyses and the technical capability of combining mineralogical, chemical, thermal, electromagnetic, optical, and other analyses will add a power to space exploration that holds vast potential.
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