An Exobiological Strategy for Mars Exploration (Part 2)
SEARCH FOR EVIDENCE OF PREBIOTIC CHEMISTRY
INTRODUCTION
It is possible that all martian chemistry is prebiotic, in the sense that it took place without the agency of a pre-existing biota. Whether it is prebiotic in the sense of having led to a martian biota is less clear. In either case, however, determination of the extent to which such organic chemical evolution proceeded on Mars is of fundamental importance to the study of life’s origins. The focus here is on processes leading to the formation of substances that might ultimately have combined to yield self-replicating, self-sustaining organisms. Such reactions must have run their course somewhere, sometime, at least once; presumably on the early Earth before 3.5 Gyr ago. Because Earth’s primitive biota would have quickly eliminated the organic chemistry from which it sprang, to say nothing of the degradative effects of tectonic activity, no extant terrestrial records allow reconstruction of that chemical pathway. The most likely site where such a record may have been preserved is the planet Mars.
It follows that, although popular interest is generally associated with the possibility of life, either extinct or extant, on Mars, we will consider here an alternative that is of comparable scientific importance: that the prebiotic chemical processes which led eventually to life on Earth got started on Mars but never ran to completion. A very wide range of possibilities is open. A few or many organic compounds might have formed before conditions became unfavorable. But the idea is attractive because conditions early in martian history appear to have been at least somewhat similar to those on Earth. Chemical processes on Mars might then have resembled those preceding the development of life on Earth. Subsequently, however, the surface histories of these planets have differed significantly. While the Earth’s earliest sediments have been destroyed or heavily altered, much of the martian surface appears to have been relatively undisturbed, perhaps since about 3.5 Gyr ago. To whatever extent minerals and other compounds have been preserved, examination of them can provide at least a view of conditions prevailing on Mars and, possibly, unique information about prebiotic processes in the solar system. This would be of unparalleled value and interest.
Two mistakes can be imagined: first, focusing inappropriately and prematurely on prebiotic chemical issues; second, ignoring them. Before investigations of martian prebiotic chemistry can be intelligently designed or any conceivable results placed in context, much must be learned about the general chemistry of Mars. But as more general measurements are defined, it would be well to consider features that would provide information bearing particularly on prebiotic chemistry. The evolution of the volatile inventory and the chemistry of carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus are of particular interest. These elements form the reactants in prebiotic chemical reactions. Most are also the volatile elements which exchange between the atmosphere and crust of the planet, and are therefore central to unveiling the geologic history of Mars, the present-day climate, and other planetary-science objectives.
The detailed exploration of Mars by orbiting satellites and landers comes at a time when many new ideas concerning prebiotic chemistry and conditions on the early Earth require innovative, field-based tests. A decade and a half ago, the study of prebiotic chemistry was dominated by a single paradigm consisting of the following sequential steps: (1) synthesis of organic compounds from reduced gases in the atmosphere and transfer of these compounds to the ocean producing (2) a ‘prebiotic soup’ of a concentration high enough to lead to (3) chemical evolution of biopolymers corresponding to early versions of DNA and proteins which organize themselves into (4) a heterotrophic organism which could obtain energy from compounds in the soup and lead eventually through Darwinian evolution to (5) autotrophs capable of feeding themselves through photosynthesis. Research in recent years has challenged all aspects of this paradigm as well as the assumptions on which it is based.
Atmospheric scientists have challenged the survivability of reduced gases in the presence of water vapor in the early atmosphere. Water photodissociates, yielding hydroxyl radicals which rapidly oxidize reduced gases. On the early Earth, for example, ammonia and methane would have been converted to nitrogen and carbon dioxide. Providing an alternative, geochemists have identified sources of organic synthesis through CO2 reduction, based on the recognition of naturally occurring metastable equilibrium states which are reached between CO2 and organic compounds in sedimentary basins and hydrothermal systems. In addition, considerable attention has focused on reactions involving iron sulfides, iron oxides, gases and aqueous solutions as potential sources of energy for early metabolic systems. New theories are emerging, diverse viewpoints are finding encouragement, and there are several competing hypotheses regarding prebiotic chemical systems. New data, new observations, and new techniques are required. Although much can and will be done in the lab, the examination of natural systems is essential and Mars provides an ideal site.
THE GEOCHEMICAL CONTEXT FOR PREBIOTIC CHEMISTRY
The cycling of volatile elements through the crust and the atmosphere of Mars has enormous implications for prebiotic chemistry. Three things are essential in order for a chemical source of energy to exist and be usable by a metabolic system. There must be chemical reactions which are (1) thermodynamically favored but (2) kinetically inhibited, and (3) the reactants must be supplied continuously by normal geochemical processes. Natural disequilibrium states meeting this description are numerous and span the range from weathering of basalt to stress-generating phase transitions in the Earth’s mantle. Among the more dynamic and energy-rich sources of chemical energy are those associated with liquid water and water/rock interaction.
Fractionation of isotopes which accompanies the cycling of volatile elements also has implications for prebiotic chemistry. Isotopes of the biogenic elements (H, C, O, N) can be fractionated by non-biological, as well as biological, mechanisms. These must be recognized and understood if fractionations associated with organic syntheses or any biological processes are to be properly interpreted. The major non-biological processes involve loss of atmospheric components to space. This loss occurs from the top of the atmosphere, where diffusive separation of atmospheric species by mass causes isotopic abundances to differ from those in the bulk atmosphere. Loss to space, therefore, preferentially removes the lighter isotopes and leaves the remaining atmosphere enriched in the heavier isotope. Mixing with non-atmospheric reservoirs can diminish the total net fractionation. Loss to space can occur by thermal escape (for H) and by non-thermal escape mechanisms (for H, O, C, and N). The latter include photochemical processes, such as dissociative recombination, and sputtering by solar-wind pick-up ions. In the latter process, O ions (predominantly) that are produced in the ionosphere spiral around the magnetic field lines of the impinging solar wind, collide very energetically with atoms and molecules in the upper atmosphere, and knock some of them off into space. Measured enrichments of atmospheric D, 15N, and 38Ar are apparently due to these processes.
Compelling evidence for exchange of atmospheric compounds with crustal reservoirs comes from analysis of isotopic ratios in volatiles derived from the SNC meteorites. Enrichment of D to the degree seen in the atmosphere (~ 5x terrestrial) can only occur via escape of large quantities of hydrogen to space. That similar enrichment is seen in the SNC meteorites appears to require substantial mixing of water from the atmosphere into the crust. Similarly, oxygen isotopes in water from the SNCs depart from the fractionation line defined by oxygen in silicates, suggesting that the water has not equilibrated with the oxygen in the silicate rocks; because loss of oxygen to space can move the oxygen isotopes off the usual fractionation line, mixing of atmospheric water down into the crust can explain this difference. Given the apparent former abundance of both crustal water and sources of heat, the presence of hydrothermal systems may be a straightforward way of allowing exchange of water between the crust, the surface, and the atmosphere. In addition, there is geochemical evidence for hydrothermal alteration of the SNCs.
Atmospheric Organic Chemistry on Early Mars
The atmospheric compositions of the terrestrial planets early in their histories are poorly known. Much of the work in prebiotic chemistry, both theoretical and experimental, has been based on the view, prevalent in the middle part of the 20th century, that the primitive atmospheres of the terrestrial planets were composed mainly of methane, ammonia, molecular hydrogen, and other reducing gases. The Miller- Urey experiment and subsequent similar work demonstrated that, in such atmospheres, a wide variety of organic molecules can be synthesized by energy sources such as ultraviolet radiation, electric discharges, and shock waves. The products include amino acids and the precursors of amino acids and nucleic acid bases, hydrogen cyanide and formaldehyde.
As a better understanding of the photochemistry of atmospheres has been gained, however, it has become clear that terrestrial atmospheres composed of methane and ammonia will, over geologically short time scales, decompose to a nonreducing mixture of molecular nitrogen and carbon dioxide. Laboratory experiments have shown that the yield of organics from such atmospheres is orders of magnitude less than that from reducing mixtures, suggesting that atmospheric organic chemistry may not have been the major contributor to the organic inventories of the early Earth and Mars.
The scenario for the environment of the early Earth and Mars has been further complicated by the recognition of the so-called faint young sun paradox. Models of stellar evolution indicate that the sun was approximately 30% less luminous than today during the first Gyr or so of solar-system history. This requires increased levels of greenhouse gases in the atmosphere of the early Earth in order to maintain a surface temperature above the freezing point of water, liquid water being taken to be a prerequisite for the origin of life on Earth. Since methane and ammonia, two excellent greenhouse gases, should not have been available in large amounts as discussed above, the hypothesis has been developed that the early Earth’s atmosphere contained much higher levels of carbon dioxide than are present today, perhaps as much as 10 bar (10 times the current total surface pressure).
Since photographic evidence for surface liquid water on early Mars is available, a similar hypothesis has been developed for the composition of the primitive martian atmosphere. A 1- to 5-bar CO2 atmosphere would have kept the surface temperature of early Mars at or above the freezing point of water (273deg.K) according to some models. Modelling which includes the effects of CO2 cloud condensation at high CO2 abundances, however, indicates that it is not possible for the greenhouse effect to keep the martian surface above 214deg.K with CO2 as the sole greenhouse gas, no matter how much is present. One possible resolution to the martian greenhouse dilemma, though not the only one, is the presence of more-efficient greenhouse gases as minor components of the early atmosphere. Ammonia or methane could serve this purpose and also increase the efficiency with which organic compounds were synthesized by atmospheric processes.
Organic Synthesis During Weathering
Regardless of the timing of the erosion responsible for surface features, it is likely that liquid water has been present at depth in the crust throughout the history of Mars. Normal geothermal gradients will cross the ice-water transition, and any enhanced heat flow from igneous activity will elevate this transition toward the surface. Contact between igneous rocks and liquid water at low temperatures is a source of chemical disequilibrium which may provide the means for organic synthesis. The major manifestation of this chemical disequilibrium is weathering of the igneous minerals to yield low-temperature alteration products which, for basalt, are dominated by clays and hydroxides. In the case of Mars, based on observation of the surface, it appears that part of the overall weathering process is the oxidation of ferrous minerals to yield ferric minerals. For example, each mole of fayalite (Fe2+ olivine) which is oxidized to ferric hydroxides and ferric silicates can provide a mole of H2. It is conceivable that this H2 could be coupled to CO2 reduction to yield simple organic compounds as weathering proceeds. On the other hand, if CH4 was introduced into an environment consisting of already-oxidized minerals, then coupled redox processes could yield organic compounds. The temperature at which weathering reactions occur increases with increasing depth or heat flow, and at some elevated temperature the processes involved would overlap those which would be considered hydrothermal.
Hydrothermal Organic Synthesis on Mars
Hydrothermal systems are an unavoidable consequence of a geologically active planet which has fluid H2O at or near its surface. When molten rock solidifies it contracts and cracks. If fluid H2O is present it will move through the cracks, heat and expand, and the hot, low-density fluid will circulate toward colder parts of the system. The critical point of H2O is at 221 bars and 374deg.C, which implies that supercritical H2O fluid can be encountered in many subsurface settings in planets with active volcanic processes. The heat capacity of any substance approaches an infinite value at its critical point, and this helps to explain why H2O is such an efficient refrigerating substance for volcanic activity on wet planets. In addition, the density reaches a minimum at the critical point and the compressibility and expansivity (pressure and temperature derivative properties of the density, respectively) approach infinite values. Therefore, the transport properties of H2O in hydrothermal systems lead to dynamic, rapid fluid flow. Accordingly, if magmatic processes occur on wet planets, the hydrothermal systems which inevitably form will transport heat efficiently and rapidly away to the surface of the planet and on to space.
Given the many lines of evidence for the presence of water at the surface of Mars, at least early in its history, together with the abundant evidence for volcanic activity, hydrothermal systems would certainly have existed within the outer layers of the planet, and may have had surface manifestations as hot springs. In addition, the SNC meteorites show geochemical evidence for hydrothermal alteration, and isotopic evidence for exchange of water between surface and subsurface, plausibly via hydrothermal systems. This has several implications for the emergence of life on Mars, including: (1) the enormous impact which subsurface mineral precipitation of carbonates, sulfides and sulfates can have on any attempt to explain the volatile budget of the planet; (2) the possibility of hydrothermal organic synthesis; and (3) the presence of inorganic sources of chemical energy that are provided by these systems and which chemolithoautotrophic micro-organisms are known to use. Each of these points is addressed below.
Comparison shows that carbon is less abundant at the surface of Mars than at the surfaces of the other terrestrial planets, Earth and Venus. It is possible that, during outgassing of Mars through volcanic activity, large quantities of volatile elements were sequestered in the subsurface as hydrothermally precipitated minerals (carbonates, sulfates and sulfides). Ongoing research is testing this hypothesis to see whether hydrothermal sequestering is plausible, given what is known about hydrothermal systems on the Earth and the composition of martian rocks inferred from SNC meteorites and previous missions. Considerable work in geochemical modelling has focused on Iceland as a partial analog for martian conditions.
Such studies are not limited simply to testing the likelihood that carbonate (and/or sulfate and sulfide) minerals will form; entire assemblages of alteration minerals can be characterized. In turn, these theoretical results, tested against terrestrial analogues, provide guidelines for the types of spectroscopic analyses that will be capable of identifying any hydrothermally altered rocks at the surface of Mars. Such spectroscopic fingerprints of hydrothermal activity, which will be detectable from orbiting satellites, will help to guide the mapping of the surface of Mars and will identify likely locations to send landers for optimum results. This coupling of theoretical results with analytical data collection will be crucial to finding locations which are the most likely to hold hydrothermally altered rocks. This approach should allow a more comprehensive study than a survey of the surface for hot-spring deposits. Although appropriate minerals for hot-spring deposits will be revealed with this approach, these methods will also identify rocks altered at much higher temperatures and pressures that may have been exposed at the surface through erosion, tectonic activity or impacts.
If an atmosphere of reduced gases was short-lived, as atmospheric modeling for Earth indicates, then other pathways for organic synthesis should be explored. Research has shown that heat, UV-radiation, shock waves and ionizing radiation can lead to organic synthesis in the laboratory. Nevertheless, they have shown that there are numerous pathways and energy sources that can lead to the synthesis of organic compounds from inorganic starting compounds. In the case of hydrothermal synthesis, a combination of elevated temperatures and oxidation states buffered by mineral-water reactions provides conditions in which organic compounds might be synthesized. Unlike most prebiotic syntheses, the pathway for hydrothermal synthesis is more likely to be through reduction of CO2 (or CO) than oxidation of CH4.
Evidence in support of hydrothermal pathways comes from theoretical studies of the reactions which occur among carbon compounds (organic and inorganic) in geochemical processes. Starting from the compositions of organic and inorganic compounds in geologic fluids, recent studies have examined the extent to which these compounds have equilibrated over geologic time. Many organic compounds and CO2 are found to have attained metastable (rather than stable) equilibrium states in sedimentary basins and in hydrothermal systems. Significantly, the oxidation states that are attained in these systems are those which allow CO2 and organic compounds to coexist. These oxidation states are highly reduced relative to surface conditions, even when compared to those in most hot springs, and are generally much more highly reduced than unconstrained conditions imposed on many laboratory experiments. Importantly, there are, in these natural systems, many pathways allowing the transfer of carbon between organic compounds and CO2. Efforts are currently underway to elucidate the extent to which reactions among organic compounds, as well as those between organic compounds and CO2, are reversible at the temperatures and oxidation conditions at which natural metastable states exist.
In terms of exploring Mars, or the Earth for that matter, it is necessary to delve beneath the surface in order to identify the extent to which hydrothermal organic synthesis may have occurred. Theoretical studies of hydrothermal systems on the Earth indicate that the greatest potential for organic synthesis is not in systems which are the most obvious manifestations of hydrothermal activity. For example, submarine hydrothermal systems have received a great deal of attention as environments in which organic synthesis may occur. However, the potential for organic synthesis is not greatest at the submarine black-smoker vents at the ridge crests. The combination of temperatures and oxidation states at black-smoker vents favors CO2 rather than organic compounds. Instead, the potential is much greater in portions of the systems in the flanks of the ridges where temperatures are lower and mineral-buffered oxidation states can be considerably lower. A corollary for Mars is that exploration of hot spring deposits at the surface will not necessarily lead to evidence of organic syntheses. There are good reasons for studying hot-spring deposits on Mars, including the constraints supplied to theoretical modelling of fluid-rock reactions, but evidence for prebiotic organic synthesis might not be found there. Instead, the search should be conducted in deeper parts of the system. Until drilling becomes a real option for martian studies, it will be enormously useful to identify portions of the martian crust that contain deeper parts of fossil hydrothermal systems and which have been exposed at the surface through erosion, impacts, or tectonic activity.
Exogenous Organic Compounds on Mars
Impact delivery of organics to the early Earth has been suggested as an alternative to atmospheric organic synthesis. Large impactors strike the Earth with enough kinetic energy to generate temperatures sufficient to pyrolyze much of any organic matter contained within. On Mars, however, the lower gravity relative to Earth results in a lower impact velocity for objects of a given mass relative to that on the Earth. This translates into a significantly increased chance of survival for impactor organics delivered by large objects, even in the absence of a dense atmosphere, relative to the chance of survival on Earth. Also, small objects, those the size of recovered meteorites and smaller, are capable of delivering organic matter to the Earth’s surface and would similarly do so on Mars. Consequently, impact delivery of prebiotic organic material to early Mars is another possible source of material for martian prebiotic organic chemistry.
Related to this subject is the question of impact transfer of organic matter between Earth and Mars early in their evolution. The SNC meteorites constitute empirical evidence that transfer of intact, moderately shocked material can take place from Mars to Earth, and although the organic content of those meteorites is very low (probably nonexistent), there seems no reason why a reasonably well-indurated, organic-bearing rock could not survive the conditions experienced by the SNCs. Whether the same process could operate in reverse, delivering organics from Earth to Mars, is less likely but perhaps cannot be ruled out. In the event that organic matter is found on Mars, this mechanism would have to be considered as a potential source, but discriminating between this and other possible sources, in any other than a statistical fashion, would not be straightforward.
OBJECTIVES FOR FLIGHT EXPERIMENTS
As a result of the Viking mission, it is already known that organic compounds were not detected in martian soil at two locations. Improved instruments and alternative sites could be considered, but evidence for oxidation – and for continuing oxidative processes – is pervasive. Until more specifically promising samples and sites can be identified, therefore, efforts should be focused on broader examinations of martian surface processes generally and on the history of the biogenic elements specifically. Measurement of isotope ratios in all accessible phases can lead to information constraining mass balances both now and in the distant past. If variations in isotopic abundances over geologic time can be reconstructed, times of origin of specific organic compounds (if any are found) can be estimated and extra-martian origins of such compounds might be ruled in or out. The elemental compositions of segregated phases, inorganic as well as organic, can provide information about chemical differentiation of the planetary surface and thus about the chemical environment at times in the past. Conceivable variations in the level of oxidation of the martian surface are of particular interest since they bear not only on the survivability of organic products but, more importantly, on rates of synthesis in the first place.
Stable isotopic ratios in atmospheric species should be measured in the bulk atmosphere and throughout the upper atmosphere. Specific ratios include D/H, 18O/17O/16O, 13C/12C, and 15N/14N. In the bulk atmosphere (to an altitude of ~120 km), compositions of H2O, CO2, and N2 are of interest, water and carbon dioxide being examined separately because differences in their oxygen isotopic abundances will carry information about rates of exchange and thus present-day geochemical dynamics. Above the 120-km homopause, fractionation to the exobase at around 200 km should be measured. At the exobase, atomic oxygen is a significant component of the atmosphere and should also be examined separately. Finally, determination of the isotope ratios of species actually escaping to space would complete a satisfactory examination of the atmospheric isotopes. The significance of the observed isotopic variations and, in particular, their historical development, can be clarified only through consideration of the processes by which species escape to space or are sequestered at the surface. Parameters relevant to these processes must also be examined closely.
Continuing study of images of the martian surface has provided a wealth of information pointing clearly to chemical differentiation of the surface, but chemical compositions are known at only two sites. To whatever extent elemental abundances can be mapped on a planetary scale, it will be invaluable in providing information about the system within which any prebiotic syntheses of organic compounds would have occurred. Elemental abundances alone are, however, insufficient to identify materials which have been altered by hydrothermal fluids. Rocks altered at high temperatures and pressures may have elemental compositions which would be difficult to differentiate from unaltered, pristine igneous rocks. In such altered phases, however, the mineralogy (i. e., crystal structure as well as chemical composition) would be vastly different. Depending on temperature and pressure of alteration, chlorites, smectites, serpentines, amphiboles, micas, and other hydrous phases might characterize the hydrothermally altered rocks. In addition, the compositions of many anhydrous phases (feldspars, pyroxenes, olivines, etc.) change as a consequence of hydrothermal alteration. As an example, plagioclase in Icelandic rocks becomes enriched in Na even in systems where fresh water reacts with basalt.
Mineralogy can be determined with increasing precision as the exploration of Mars moves from orbiters to landers to sample return. From orbit, infrared spectroscopy can reveal the presence of major minerals present in the surface material. As an example, a search could be conducted for silica, carbonates, and other evidence of hot-spring deposits. Surface manifestations of hydrothermal systems, like hot springs, can represent minor volumes of material compared to the magnitude of altered materials in the subsurface. Therefore evidence for hydrothermal systems might be more abundant in regions where the interior of the crust is revealed through erosion, impacts, or tectonic activity.
Landers equipped for IR spectrometry, and X-ray diffractometry/fluorescence can further enhance knowledge of the mineralogical inventory, especially if they are coupled with devices which can obtain samples from outcrops as well as loose material at the surface. Ideally, shallow drilling, of rocks as well as into soils, coupled to X-ray and other spectroscopic analyses will provide much-needed information on the depth of weathering, extent of ground ice or ground water and possible stratigraphic relations among mineral assemblages.
No understanding of martian prebiotic chemistry can be complete without an attempt to detect and quantify organic compounds. Measurement of the amount of organic carbon in various geochemical environments will be crucial to understanding the global carbon cycle on early (and perhaps present) Mars. These environments would include sedimentary basins, in which organics produced by atmospheric processes or delivered by impactors might be buried beneath sediments. Subsurface aquifers, which could either produce organics through hydrothermal processes or perhaps collect and concentrate them by precipitation, should also be examined.
Wherever organic materials can be found, quantitative determination of total organic carbon is a minimum requirement. Beyond that, determination of the relative abundances of carbon, nitrogen, hydrogen, and oxygen and their stable isotopes would provide clues to the likely nature of the synthetic processes. Following up on those clues, identification and quantitative molecular and isotopic analyses of individual compounds, such as amino acids, nucleic acid bases, and fatty acids, would be most useful in determining the relative importance of atmospheric, impact, and hydrothermal processes in organic synthesis on early Mars. Each of these mechanisms generates a pattern of species abundances which can, in principle, be distinguished from the others. Organic compounds of ambiguous prebiotic relevance, such as polycyclic aromatic hydrocarbons delivered by impactors, could also be useful in this regard.
THE SEARCH FOR EXTINCT LIFE
INTRODUCTION
Because life on Earth appeared early in our planet’s history and therefore must have originated relatively rapidly, it is possible that liquid water persisted on Mars long enough for life to begin there as well. Thus, the fundamental question of whether life exists or existed elsewhere in the universe may be answered in our own galactic back-yard. This mandates that we search for a martian fossil record. The current surface environment of Mars is hostile to life as we know it, and an ancient biosphere might have become extinct. The discovery of an extinct martian biosphere would be of immense scientific importance: the demonstration that life originated independently in two places in our solar system would have far-reaching implications for the distribution of life elsewhere in the Universe.
Below is a concept to search for a biosphere which became extinct perhaps a few Gyr ago. The search is naturally allied with investigations of prebiotic chemistry and extant life because all three efforts will be largely chemical studies and because the locations to be targeted on Mars will overlap extensively. Such an investigation inevitably draws heavily on our experience with studies of earth’s early biosphere.
EXPERIENCE WITH EARTH’S EARLY FOSSIL RECORD: RELEVANCE FOR MARS
Tectonic activity has altered, concealed or destroyed most of Earth’s early crust. Careful regional geologic mapping has therefore been necessary to locate and characterize older Precambrian sequences. Many early Archean aqueous sediments occur typically as relatively thin-bedded units within thick sequences of volcanic rocks. These sequences have been explored for those sediment types most likely to contain fossils. Fossils have typically been found in cherts, which are impermeable siliceous rocks that resist weathering. The best-preserved specimens are found in rocks having fine-grained, stable mineral textures with well-preserved (and/or abundant) organic matter or other reduced chemical species. These samples have been searched for morphological, chemical or isotopic evidence of ancient life, and they have been interpreted in the context of their preservation and paleoenvironment. It is significant that the antiquity of the fossil record (3.5 Gyr) corresponds with the age of the oldest sediments which have been sufficiently well-preserved to have retained conclusive evidence of life. Thus, the antiquity of the fossil record is probably limited by the preservation of rocks favorable for fossil preservation, and life is likely to have appeared significantly earlier than the oldest sedimentary sequences.
Certain concepts have emerged from Precambrian studies which seem relevant to a search for life on ancient Mars. For more than five-sixths of our own biosphere’s history, life existed predominantly as single-celled organisms. Thus the most ancient and ubiquitous life form in a putative martian biosphere would presumably have been microbial. Also, mineralization and/or rapid sedimentation which occurs in close proximity to microbial communities will enhance their fossilization and preservation. This is because decomposition proceeds to completion unless the cells have been isolated from decay. Short-term isolation can occur in aqueous environments inhospitable to microbial communities that cause decomposition; however, long-term fossilization additionally requires entombment in an impermeable mineral matrix. Some of the best preserved examples of Precambrian fossils were probably rapidly encased in primary silica precipitates before decay could occur.
Because multiple lines of evidence (morphologic, sedimentological and chemical — including isotopic) have been crucial for interpreting Earth’s own Archean fossil record, they will probably be required to prove that a prospective martian fossil deposit is indeed biogenic. Our ability to identify the remains of hypothetical martian organisms depends ultimately upon comparisons with extant terrestrial analogs, and such comparisons may prove to be inaccurate. Furthermore, biological information preserved in sediments is often altered or destroyed by biological degradation, elevated temperatures or pressures, or oxidation.
Morphologic evidence includes forms which are visible at various size scales, ranging from microscopic cells to macroscopic microbial constructs, such as stromatolites. “Chemical fossils” include those biologically produced substances which can be conclusively distinguished from nonbiological ones. Most notable among these are distinctive organic compounds (e.g., certain lipids or amino acids), although inorganic substances, such as certain phosphate minerals, can also be diagnostic. Differences in isotopic composition, such as the contrast observed on Earth between sedimentary organic carbon and carbonate, can retain the signature of biological isotopic fractionation.
Fossil evidence can be quite abundant in sedimentary rock sequences which have escaped extensive degradation. One illustrative example is the 600 to 900 Myr-old sequence from Svalbard, an island located midway between Scandinavia and the North Pole. About 25 percent of all carbonates in these rocks contain fossil stromatolites formed by microbial communities. (Note that macroscopic evidence of stratiform or domal laminated rocks is not sufficient to prove microbial genesis. Abiotic structures as various as calcretes, tufas, stalagmites, and even malachite bodies share similar physical characteristics. A role for microbial mat communities must be demonstrated on the basis of detailed petrologic study.) About 25 percent of all fine-grained siliciclastic samples contain organic-walled microfossils. About 5 percent of all siliciclastics contain diverse, well-preserved fossils that have been invaluable for taxonomic studies. About 10 percent of all carbonates contain microfossils; however, more than 50 percent of silicified carbonates contain fossils. Essentially all carbonates and fine-grained siliciclastic sediments that contain organic matter provide carbon isotopic evidence indicating biological discrimination.
The Svalbard rocks offer perspectives which are relevant to the Mars exploration effort. The likelihood for retaining evidence for life in well-preserved rocks of the right mineralogy is high. The best preservation is found in cherts and unoxidized fine-grained siliciclastic rocks, although carbonates also provide important information. The best strategy for finding fossils should combine visual with chemical and isotopic observations. Based on experience with the Precambrian on Earth, the detection of morphological microfossils has an inherently lower probability of success than the detection of chemical evidence, but it offers great rewards if successful. Chemical and isotopic observations greatly improve the overall probability for locating ancient evidence of life. In either case, success rests largely on targeting appropriate lithologies.
Compared to the Earth, the ancient martian crust has been less affected by destructive tectonic forces, thus the discoveries of remarkably abundant microbial remains in Earth’s well-preserved Precambrian carbonates and shales are encouraging for Mars exploration. However the fraction of ancient sediments deposited in aqueous environments is likely to be smaller on Mars than on Earth, particularly during the past 3 Gyr. Thus, it should be easier to find well-preserved ancient martian crust, but it will be perhaps more challenging to locate and sample aqueous sediments within crustal sediments. The principal strategy, then, is to locate and analyze aqueous sediments, particularly those that are good repositories for a fossil record.
KEY SITES ON MARS
Life’s fundamental requirements for liquid water, energy and nutrients form the basis of a search for sites on Mars which are most prospective for locating a fossil record. If we allow that life might have been initially autotrophic rather than heterotrophic, then it follows that all plausible energy sources which could drive metabolism should be considered. Solar radiation is likely to have been the major, reliable source of energy, but access to it requires elaborate photochemical systems for conversion of physical (light) energy into chemical energy. It is presently unknown whether such systems evolved early on Mars or the Earth. On the other hand, it is conceivable that an early metabolic system may have used sources of chemical energy. Rather than requiring the machinery with which to capture light energy, early microbes could have been chemoautotrophs and perhaps chemolithoautotrophs. Therefore all potential sources of chemical energy that could arise at or near the surface of Mars or the early Earth should be considered.
Thermal-spring deposits
Subaerial thermal-spring deposits have been identified as important targets for locating a martian fossil record because such springs might have been oases in the literal sense, and they also provided reduced gases to serve as sources of energy and reducing power for organic synthesis. Thermal-spring waters also can sustain the high rates of mineral precipitation which, on Earth, typically occur in the presence of microbial communities. Volcanic terrains are widespread on Mars, and some include outflow channels of simple morphology that may have formed by spring sapping. The association of such features with potential heat sources, such as volcanic cones or thermokarst, provides evidence for near-surface hydrothermal systems. Minerals most commonly deposited by subaerial thermal springs include silica, carbonates and Fe-oxides. Siliceous sediments are particularly favored because they tend to be fine-grained and are relatively stable during post-depositional changes. Organic-rich cherts (fine-grained deposits of silica) provide some of our best examples of microbial preservation in the Precambrian. Although rates of organic matter degradation appear to be quite high within thermal environments, a great deal of biological information is retained in the macroscopic biosedimentary structures (stromatolites) and biogenic microfabrics of spring deposits. Many primary biogenic features of subaerial spring deposits survive diagenesis (e.g. recrystallization, phase changes). Spring deposits are excellent targets for fluid inclusions, which may preserve samples of original liquid and volatile phases and, potentially, microorganisms and
biomarkers. Such deposits are also prime targets for prebiotic compounds.
Lakes
Lake beds are important targets, because ice-covered lakes might have been sites for life’s “last stand” on the martian surface. Subaqueous spring carbonates (tufas) and sedimentary cements deposited at ambient temperatures often precipitate rapidly in the presence of microbial communities. Such deposits form when fresh water emerges from springs at the bottom of alkaline lakes. Mineral precipitation is apparently driven primarily by inorganic processes, although microbial mats may also influence precipitation during periods of peak productivity in areas where rates of inorganic precipitation are lower. Tufa deposits often contain abundant microbial fossils and organic matter. Sublacustrine springs commonly occur in volcanic settings, in association with crater and caldera lakes, and can include thermal deposits. Volcani-lacustrine deposits are frequently strongly mineralized and comprise some of our best examples of well-preserved microbial communities in terrestrial settings
Evaporite deposits
When a lake shrinks or disappears by evaporation, the more-soluble salts can precipitate and capture other constituents. When evaporites crystallize from solution, they commonly entrap large numbers of salt-tolerant bacteria within brine inclusions. Evaporites have been suggested as potential targets for extant life on Mars, although considerable debate exists regarding the long-term viability of microorganisms within salt. Still, brine inclusions in evaporites may provide excellent environments for preserving fossil microbes and biomolecules. The disadvantage of evaporites is that their high solubility limits them to comparatively short crustal residence times. Consequently, most Precambrian evaporites are known from crystal pseudomorphs preserved by early replacement with stable minerals, such as silica or barite. On Mars, the most likely places for evaporites are terminal lake basins where standing bodies of water existed perhaps intermittently. The central portions of terminal lake basins, including impact craters and volcanic calderas, are potential targets for evaporites on Mars. The typical “bulls eye” distribution pattern for evaporites within such settings indicates that carbonates are normally found in marginal basin areas, with sulfates and halides occurring progressively more basinward. Such basins also might contain fine-grained, clay-rich siltstones and shales. These rock types are significant in that, on Earth, they sequester the bulk of sedimentary organic carbon and other reduced species such as biogenic sulfides. Although these rocks are usually less resistant to weathering than cherts or carbonates, their sometimes high organic contents on Earth emphasize their potential importance on Mars.
Cemented regolith
As surface water percolates downward through soils, more-soluble compounds tend to be dissolved from the upper horizons and redeposited at depth as mineralized “hard-pans” (e.g., calcretes, silcretes). Mineralized soils commonly contain microfossils of the soil microbiota entombed in hard-pans or duricrusts. Mineralized horizons within paleosols may be widespread on Mars and should not be overlooked as potential targets for exobiology. Indeed, images from the Viking landers indicate that, in places, soils are indurated and form surface crusts which are resistant to ablation by wind. Rock varnish, dark coatings observed in arid environments, often reflect biological processes, and therefore should be searched for on Mars.
OBJECTIVES FOR FLIGHT EXPERIMENTS
It will be important to achieve a global perspective of the extent to which liquid water has altered the chemical composition of the martian crust. Elements such as calcium and aluminum respond quite differently during aqueous weathering of igneous rocks. This creates a wide range in the Ca/Al elemental abundance ratio among the various products of aqueous weathering. Similar, albeit somewhat less dramatic, patterns are also observed for the elements magnesium, sodium and potassium, relative to aluminum. Correlations between the ratios of these elements and other morphological indicators of the activity of liquid water could be informative. Gamma-ray spectroscopy can perform elemental analysis of the martian crust from orbit and thus is important for evaluating elemental distributions on a global scale, although the prevalence of a widespread aeolian layer comparable in thickness to the gamma-ray penetration depth could be a complicating factor. The best information will likely be obtained from areas of aolian erosion where bedrock is exposed at the surface, and such areas should be targeted for orbital spectroscopy.
On Earth, high local concentrations of certain elements are particularly diagnostic of biological processes. Phosphorus is notable among these elements; it is a major constituent of bone and its deposition as phosphate-rich rock often reflects the decomposition of sedimented organic matter. Thus, in addition to the ability of phosphate to entomb and preserve fossil materials, an elevated abundance of phosphate minerals might be a key indicator of past biological activity.
A major effort should be made to locate rock types which are favorable for the preservation of fossils. These key rock types include carbonates, phosphates, evaporites, and silica-rich precipitates such as cherts. This effort should be pursued from orbiters, landers or rovers. These key minerals typically have characteristic spectral signatures in the near- and mid-infrared (IR). In addition, a number of diagnostic siliciclastic minerals, including clays, are formed by the aqueous alteration of igneous rocks. These minerals can also be identified using IR spectroscopy. Epithermal hydrothermal deposits have also been detected using airborne magnetometers.
Aqueous minerals that are both fine-grained and diagenetically stable are favored for good preservation. These minerals can be identified in near-IR reflectance spectra or mid-IR emission spectra. The presence of reduced chemical species such as organic matter may also be detected spectroscopically. Because such rocks might be relatively rare at a given landing site, rover-based spot spectroscopy would be useful for surveying and quickly evaluating numerous rocks in order to assess the chemical, mineralogical and petrologic diversity of a site. Imaging at visible and infrared wavelengths is fundamentally important for locating favorable rock types for a chemical or morphological fossil record. Camera capabilities should allow not only panoramic views but also telephoto and close-up “hand-lens” options for evaluating rock textures. Microscopy should be done with a range of light sources, including UV to detect fluorescence. This can provide important information about mineralogy and also be used to search for organic matter. For rocks found to be promising, more detailed analyses of their elemental composition, mineralogy and volatile contents are warranted. Because weathering and other processes can alter rock surfaces, an accurate assessment of mineralogy or volatile content might require that rock interiors be sampled. Thus, a drill or other device must be developed for penetrating rocks.
SAMPLE RETURN
Virtually all investigators in the Precambrian paleontology community believe that definitive discovery of martian fossils will require observations made in Earth-based laboratories. Thus, the exploration activities described above might constitute only a critical prelude to the actual revelation itself. The need for sample return is based on the conviction that fossil evidence will be challenging to obtain, and that the analytical capabilities of Earth-based laboratories are indeed much more formidable than flight instrumentation. Secondly, the latest technologies can be applied and new methods can even be developed and tailored to the task. Perhaps most importantly, the fullness of human perception, flexibility and insight can be brought to bear on returned samples.
Parts [1][2][3][4]
