Determinative Mineralogy: An essential component of Planetary Exploration – Decadal White Paper of the Extraterrestrial Mineralogy Panel

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 Jupiter’s 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 Europa’s 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 Earth’s 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 Earth’s oceans are indeed a legacy of cometary influx. This original hypothesis — that the world’s 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 Moon’s 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 21^st 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.

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.

THE FUTURE

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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