Carbonates on mars
2019-10-25T13:30:50Z (GMT) by
Carbonates are predicted to exist on Mars, as they are a common product of the interaction of CO2, water, and Fe–Mg silicates over a wide range of conditions. They have been identified from orbit, using NIR spectroscopy, with a particular link to Noachian impact craters, sometimes clearly associated with impact-induced hydrothermal action, but in other cases, there may be excavation of preexisting carbonates. MER Spirit, Phoenix Polar lander, and Mars Science Laboratory (MSL) have all identified carbonate with a variety of techniques, but for MSL so far only in trace amounts and in eolian material rather than bedrock. Carbonate-rich terrains are also a target for future Mars rovers and sample return in the Syrtis-Terra Tyrrhena region, which is the only Martian region with extensive outcropping carbonate units. The most detailed available analyses of carbonate have been made on the nakhlite and ALH 84001 Martian meteorites. Using existing data and new mineralogical work on recently discovered nakhlites, a model for impact-induced hydrothermal alteration of the parent rocks has been established. Rather than a long duration hydrothermal process, recent experimental results suggest this occurred relatively quickly, over a few months, and the sideritic carbonate is part of a metastable assemblage, with some signs of late dissolution, as the nakhlite fluid evolved. The ALH Mg–Ca–Fe 84001 carbonate may have formed at lower temperature, but again in metastable, rapidly cooled conditions, likely associated with evaporation, with some subsequent shock-induced recrystallization. Diverse geological processes have resulted in the formation, and sometimes dissolution, of carbonate on Mars, and usually in isolated terranes rather than within continuous, thick geological formations that formed over millions of years. The dominant basaltic geochemistry will also have acted to neutralize the pH of CO2-rich fluids. However, the combined spacecraft, lander, and meteorite data suggest that Mars has average upper crustal abundances equivalent to at least hundreds of mbar CO2. This is consistent with thick pCO2 atmospheric models for ancient Mars. The meteoritic carbonates, and carbonate identified by MSL, have also recorded the isotopic evolution of the Martian atmosphere, with oxygen and D/H, C-isotopic signatures of mass independent fractionation and preferential loss of light stable isotopes as the Martian atmosphere changed from its early relatively high pressures.