Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p43d..05e&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P43D-05
Other
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5220 Hydrothermal Systems And Weathering On Other Planets, 5410 Composition (1060, 3672), 5464 Remote Sensing, 6225 Mars
Scientific paper
Carbonate is an expected weathering product of basalt in an aqueous environment and an atmosphere with CO2. Its absence in rocks examined by numerous orbiting and landed instruments and its presence as only a very minor phase in Martian meteorites and dust has been proposed to imply that either ancient Mars" surface was not persistently wet, pCO2 was not high, or conditions were too acidic to permit significant carbonate formation and preservation. Here we report a regional rock layer with near-infrared spectral characteristics consistent with the presence of magnesium carbonate, which has been identified and mapped in the Nili Fossae region by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Carbonate is identified by strong, correlated absorption features at 2.3 and 2.5 micron whose position and width correspond uniquely to those of magnesite. Subtle absorptions at 3.4 and 4.0 microns appear to be present, although the identified spectral class lacks strong or definitive 3-4 micron features characteristic of some laboratory carbonate samples. The relative weakness of these bands is likely due to the presence of water (also indicated by a 1.9 micron absorption) and intermixed minerals in the carbonate-bearing rocks along with instrumental artifacts in this spectral region. The carbonate-bearing unit is positioned stratigraphically above the Fe/Mg smectite clay- bearing unit but below a mafic cap unit in a position similar to that of a regional olivine-bearing unit. The carbonate likely formed during the Noachian or early Hesperian via alteration of olivine by either hydrothermal fluids or near-surface water. The Nili Fossae carbonate-bearing unit is relatively thin and thus would not have sequestered large quantities of CO2. Based on OMEGA global mapping and reconnaissance with CRISM observations, this spectroscopic unit is not observed in significant outcrops elsewhere on Mars. With the possible exception of a carbonate component in transported sediments within Jezero crater, we find no evidence of classic bedded sedimentary carbonates resembling those on Earth. Instead, our results are consistent with carbonates having formed in response to unique local conditions, which require both ultramafic rocks and their substantial interaction with water to generate Mg carbonate in sufficient abundance to enable detection from orbit at a resolution of tens of meters per pixel. The existence of carbonate suggests neutral to alkaline conditions at the time of its formation, consistent with those implied by accompanying clays. Because aqueous activity in the Nili Fossae region extended into the Hesperian era, persistence of carbonate to the present indicates that acidic weathering conditions, proposed to be characteristic of Hesperian Mars, did not destroy these carbonates and thus did not dominate all aqueous environments.
Bishop Janice L.
CRISM Science Team
Ehlmann Bethany L.
Murchie Scott L.
Mustard John F.
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