Mineralogy Of Mars And The Problem Of Carbonates

The question of the type of elements and minerals comprising the surface of Mars is of extreme interest for the reconstruction of the geological evolution of the planet (Baker, 2006). The mineral composition of the Martian surface may provide clues to deciphering the question of possible life on Mars. The recent missions of the NASA's exploration rovers Spirit and Opportunity and ESA's Mars Express have produced a wealth of new data about its surface composition by using a variety of instruments, including different types of spectrometers (e.g., Christensen et al., 2004a, b; Bibring et al., 2005). The most exciting discovery of astrobiological interest is the mineralogical evidence for the (past) presence of water on the Mars surface, provided by hematite-rich grains and hydrated sulfate minerals (gypsum, jarosite, and kieserite) of a possible evaporite origin (Gendrin et al., 2005). From an astrobiological point of view, it is somewhat disappointing that carbonates have thus far only been detected in low abundance, with concentrations of approx. 1 weight% in several martian meteorites (Gooding, 1992) and 2 to 5 weight% in the martian dust (Bandfield et al., 2003). Based on our terrestrial experience, calcium carbonate is common biological byproduct and its paucity, combined with high sulfur content in surface rocks and soils, is explained by the presence of acidic waters, such as possible ancient martian oceans with acidic environments (Fairen et al., 2004). They may have prevented carbonate accumulation. Small carbonate concentrations, however, may also have other explanations, such as ultraviolet photodissociation and acid-fog weathering (Bandfield et al., 2003). Mars might also have had a thicker and denser CO2 atmosphere during the

Noachian era (about 4 billion years ago), which has been stored as carbonate accumulations (Kahn, 1995). Apart from basin-wide sedimentary processes, carbonate-bearing rocks might be present on Mars as the product of local processes, as in hydrothermal precipitation (Griffith and Shock, 1995), and perhaps might not have been detected remotely yet. Hydrothermal carbonates are plausible as geological products when considering the presence of effusive vulcanite on Mars that may have produced geyser and hot spring phenomena (Walter and Des Marais, 1993). In summary, although only small amounts of calcium carbonate have been detected in martian dust and meteorites, perhaps larger carbonate accumulations will be discovered by subsurface investigations.

Most published scientific research papers agree on the abiotic nature of the bacteriomorphs from the carbonate globules described in the famous martian meteorite ALH84001 (McKay et al., 1996), however, these globules deserve some interest since they could have precipitated as a consequence of early aqueous processes on Mars (Scott, 1999; Kargel, 2004). The zoned organization of the globules is indicative of compositional variations and resembles the zoning that characterize rhombs and spheroids described from Mg-rich, terrestrial cold seep carbonates (Cavalazzi and Barbieri, 2006), as a consequence of ambient chemistry changes. Since seep carbonates can entrap micrometer-scale fluid inclusions during precipitation, direct information on ambient fluids (including methane) and organic molecules, if any, might be gained through this type of rock (Parnell et al., 2002).

5.2. METHANE ON MARS (AND ELSEWHERE)

Most of the terrestrial methane has a biogenic origin that is derived from the activity of methanogenic archaea (Kvenvolden and Rogers, 2005). Marine and continental clathrates (also known as gas hydrates) stock the largest methane amounts ever detected, which probably originate through abiotic (thermal) and biotic (microbial) processes (Kvenvolden, 1993), in which the microbial decomposition of organic material by deep-earth methanogens leads to the production of methane gas. Methane release from clathrates in the atmosphere through time depends on their dissociation, which is driven by fluctuations in the sea level and bottom water temperature (Dickens, 1999, 2003) with environmental effects registered in the sedimentary record. On Mars, unlike on Earth, vast deposits of clathrates are believed to occur as carbon dioxide clathrates because of the abundance of atmospheric CO2 and H2O, both in the atmosphere and beneath the surface in the form of water ice (Hansen et al., 2005). CO2 clathrates would be present, therefore, especially at the south polar surface, in a way similar to the terrestrial clathrate-bearing permafrost (Kargel and Lunine, 1998; Kargel, 2004). The recent detection of methane in the martian atmosphere, obtained using Earth-based spectrometers and the spectrometer onboard the ESA's Mars Express spacecraft (Formisano et al., 2004; Krasnopolsky et al., 2004; Mumma et al., 2005), has redoubled the interest for possible methane-related biological activity on Mars and other bodies of the solar system, such as Europa and Titan. Although methane is a potential tracer of biological processes, on Mars it would also have been formed by other (abiotic) processes, including magmatic/hydrothermal out-gassing and cometary/meteorite impacts (Wong et al., 2003; Krasnopolsky et al., 2004). Recently, the near surface hydration of olivine and pyroxenes (serpentini-zation) has also been proposed as a possible way for the martian abiotic production of methane (Oze and Sharma, 2005). A similar process of methane formation has been described through the reaction between upper mantle rocks (serpentinized peridotites) and ocean waters in submarine hydrothermal fields (Kelley et al., 2001). Regardless of the source of martian methane, however, its supply is assumed to be continuous or very recent because of the limited residence time of methane in the atmosphere (evaluated at about 340 years) due to the photolytic breakdown by the UV radiation (Krasnopolsky et al., 2004). The detected methane and water vapor association in the atmosphere of Mars and the large variations of methane abundance that changes with the location are both potential indicators of possible releases from clathrate destabilization (Formisano et al., 2004; Max and Clifford, 2005), if any is present. Such a subsurface, possibly deep source of methane, is compatible with the hypotheses of autotrophic methanogenesis that was potentially developed by chemosynthetic ecosystems on Mars and Europa (Boston et al., 1992; McCollom, 1999). In ice-covered planetary bodies, such as Europa and Callisto, methanogenesis and the reduction of sulfur compounds has been hypothesized as a possible, although unlikely, energy source for life (Gaidos et al., 1999). The above scenarios necessarily require that ancient or still extant microbial life in the subsurface environments of Mars (and other bodies) have stored methane for later release. A possible origin of the martian methane from an ancient (extinct) biosphere is debated. The supporters of a martian subsurface biosphere - similar to the one established on Earth, for example, by methanogenic archaea in permafrost (Wagner et al., 2002) - speculate on the presence of methanogens in permafrost aquifers and of methane and carbon dioxide clathrates that would have stored fossil methane that was developed on early Mars (Pellenbarg et al., 2003; Max and Clifford, 2005). Other estimates, based on expected low biomass production by methanogenesis (Krasnopolsky et al., 2004), assume that methane on Mars cannot originate from an ancient biosphere. Whether it formed bioti-cally or abiotically, this does not inhibit the possibility of methane to be the fuel of biogeochemical processes that consume methane anaerobically as in the terrestrial cold seeps.

The present-day harsh conditions of the martian regolith, due to factors such as intense cosmic radiation, dryness, and daily temperature excursion, combined with geothermal gradient and surface gravity that is much lower than terrestrial ones, might potentially favor a deep biological activity at depths down to 10-12 km (Kargel, 2004). In the case that it derives from mechanisms that are related to seepage, such a deep biosphere can in fact reach surface (or near surface) locations, with detectable localized spots that are similar to the ones described on Earth. The interest in seep mechanisms, together with the obvious relationship with methane, relies on the variable methane concentrations detected from localized spots in the terrestrial methane seeps, which apply to methane as is detected on Mars. Moreover, low-temperature minerals of seep deposits can contain fluid inclusions in which methane might be preserved and (assuming with the adequate analytical techniques) detected.

6. Conclusion

The recent discovery of methane in the martian atmosphere, coupled with unequivocal evidences of the activity of liquid water on its surface, has revived interest in the possibility of life on Mars. Methane seep carbonates, that are known in the geological record since the Palezoic and precipitate in modern continental slopes, can be considered a prime astrobiological target because (i) they are chemically precipitated deposits with the advantage of preserving potential traces of life; (ii) they represent good repositories for microbiologically derived features; (iii) in terms of size they may develop different types of geological bodies that are often recognizable as anomalous deposits in the rock record; (iv) they have obvious and detectable relationships with methane. Well-developed microbial features include morphologies, biominerals, chemofossils, and stable isotope signatures. The genesis and patchy distribution of the terrestrial methane seep deposits agree with the distribution of methane as has been recently depicted on the surface of Mars. The paucity of carbonate-rich sediments found to date on the martian surface, as well as the lack of carbonate outcrops, do not appear to be major limiting factors for considering methane seep carbonates a target for astrobiology. For example, one would expect greater amounts of carbonates in older (and therefore buried) successions, for which only a drilling exploration would provide solid information. Also, a patchy distribution for martian carbonates cannot be excluded a priori at the present stage of knowledge, and it might imply limitations for a clear detection of these mineral components based on remote sensing instruments, with negative consequences for field recognition and sampling during future landing missions. Future issues in a thorough investigation of methane seep ecosystems and their geological products should therefore consider their potential as possible terrestrial analogues for methane-derived deposits on Mars. For this purpose, the integrated investigation of the microbiological-mineral interactions of methane-seep limestones associated with the assessment of their spectral and geochemical characteristics would set the base for the identification of martian outcrops. This approach can be best performed by investigating extant seeps where the influence of microbial processes on the precipitation can be directly established. In addition, similar comparative studies extended to the fossil counterparts can establish a link with unique, ancient textures containing microbial imprints.

7. Acknowledgements

The authors would like to thank Russel S. Shapiro and an anonymous reviewer for their careful review.

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