Carbonates on Mars

Head (vessel) Evidence for carbonates on Mars was first discovered in 2008. Previously, most remote sensing instruments such as OMEGA and THEMIS—sensitive to infrared emissivity spectral features of carbonates—had not suggested the presence of carbonate outcrops, at least at the 100 m or coarser spatial scales available from the returned data.

Though ubiquitous, carbonates on Mars are dominated by Magnesite (MgCO₃) in Martian dust had mass fractions less than 5% and could have formed under current atmospheric conditions. Furthermore, with the exception of the surface dust component, by 2007 carbonates had not been detected by any in situ mission, even though mineralogic modeling did not preclude small amounts of calcium carbonate in Independence class rocks of Husband Hill in Gusev crater (note: An IAU naming convention within Gusev is not yet established).

The first successful identification of a strong infrared spectral signature from surficial carbonate minerals of local scale (< 10 km²) was made by the MRO-CRISM team. Spectral modeling identified a key deposit in Nili Fossae dominated by a single mineral phase that was spatially associated with olivine outcrops. The dominant mineral appeared to be magnesite, while morphology inferred with HiRISE and thermal properties suggested that the deposit was lithic. Stratigraphically, this layer appeared between phyllosilicates below and mafic cap rocks above, temporally between the Noachian and Hesperian eras. Even though infrared spectra are representative of minerals to less than ≈0.1 mm depths (in contrast to gamma spectra which are sensitive to tens of cm depths), stratigraphic, morphologic, and thermal properties are consistent with the existence of the carbonate as outcrop rather than alteration rinds. Nevertheless, the morphology was distinct from typical terrestrial sedimentary carbonate layers suggesting formation from local aqueous alteration of olivine and other igneous minerals. However, key implications were that the alteration would have occurred under moderate pH and that the resulting carbonates were not exposed to sustained low pH aqueous conditions even as recently as the Hesperian. This increased the likelihood of local and regional scale geologic conditions on Mars that were favorable to analogs of terrestrial biological activity over geologically significant intervals.

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