Evidence for carbonates on Mars has remained elusive. For example, in spite of remote sensing instruments such as OMEGA and THEMIS that are sensitive to infrared emissivity spectral features of carbonates, carbonate outcrops have not been detected[1] at 100 m or coarser spatial scales[2]. Though ubiquitous, carbonates dominated by Magnesite (MgCO3) in Martian dust have mass fractions less than 5% and can form under current atmospheric conditions[3]. Furthermore, with the exception of the surface dust component, carbonates have not been detected by any in situ mission, even though mineralogic modeling does not preclude small amounts of calcium carbonate in Independence class rocks of Husband Hill in Gusev crater[4] (note: An IAU naming convention within Gusev is not yet established).
The absence of carbonate deposits on Mars may be due to low pHaqueous environments. Even the least soluble carbonate, siderite (FeCO3), precipitates only at a pH greater than 5[5]. The chemical and mineralogic observations by the Mars Exploration Rovers at both Columbia Hills and Meridiani are consistent with low-pHaqueous conditions at the two sites[6] during Noachian[7] and Noachian-Amazonian[8] times, respectively. Sophisticated olivine dissolution models can utilize the presence of olivine bearing outcrops on Mars to predict temporal constraints for such brines (e.g., at a pH of 3.5 and 273 K a 1 mmforsterite grain may survive as long as 140 ka[9]). The chronologic resolution will improve once Martian physicochemical conditions are better constrained.
The emerging view of the Martian surface indicates at least three chronological eras of chemical alteration: clays (phyllosian), sulfates (theiikian), and anhydrous ferric oxides (siderikian)[1]. If, as postulated by Bibring et al.[1], the Noachian era of clays was spatially widespread with formation in equilbrium with the atmosphere, thermodynamics predicate an early Martian atmosphere that is low in carbon dioxide (partial pressure less than 10³ Pa)[10]. Therefore, it may be that the atmosphere of early Mars simply lacked enough CO2 to yield volumetrically significant carbonate deposits. Furthermore, the Hesperian era of sulfates would have been dominated by the low-pH conditions that inhibit carbonate formation as evident in the Columbia Hills of Gusev and the sedimentary outcrop in Meridiani Planum. Alternatively, clays may have formed on early Mars without sustained atmospheric contact, while large water bodies may have always been acidic inhibiting the formation of carbonates even in a CO2 rich atmosphere[11].
Nevertheless, it is intriguing that Shergotty-Nakhla-Chassigny type meteorites from Mars contain evidence for carbonates, albeit at volume fractions less than 1%[12]. Furthermore, the enrichment of P - a moderately lithophile element on Mars[13] - and depletion of Si in the Wishstone class float rocks, which potentially dominate the northwest flank of Husband Hill[7], suggests that they may be associated with carbonatitic magmas[14]. While the bulk of extrusive rocks from corresponding alkaline magmas would contain amounts of carbonates too small to be detected or survive acidic alteration, carbonatite melts in the Martian mantle would contribute to the carbon inventory of bulk Mars[14]. Consistent with the volatile-rich accretion of Mars[15], such a scenario could imply a greater carbon inventory than the current planetary-mass-adjusted 10-4 to 10-3 estimate[16] relative to that of bulk Earth.
^ abc Bibring et al. (2006). "Global Mineralogical and Aqueous Mars History Derived from OMEGA/Mars Express Data". Science312: 400–404. doi:10.1126/science.1122659.
^ Bandfield et al. (2003). "Spectroscopic Identification of Carbonate Minerals in the Martian Dust". Science301: 1084–1087. doi:10.1126/science.1088054.
^ Catling (1999). "A chemical model for evaporites on early Mars: Possible sedimentary tracers of the early climate and implications for exploration". Journal of Geophysical Research104: E06S01. doi:10.1029/1998JE001020.
^ Chevrier et al. (2007). "Early geochemical environment of Mars as determined from thermodynamics of phyllosilicates". Nature448: 60–63. doi:10.1038/nature05961.
^ Fairen et al. (2004). "Inhibition of carbonate synthesis in acidic oceans on early Mars". Nature431: 423–426. doi:10.1038/nature02911.
^ Halliday et al. (2001). "The Accretion, Composition and Early Differentiation of Mars". Space Science Review96: 197–230. doi:10.1023/A:1011997206080.
^ Grady and Wright (2006). "The carbon cycle on early Earth—and on Mars?". Philosophical Transactions of the Royal Society (Biological Sciences)361: 1703–1713. doi:10.1098/rstb.2006.1898.
