Biology
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p44b..05e&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P44B-05
Biology
[5220] Planetary Sciences: Astrobiology / Hydrothermal Systems And Weathering On Other Planets, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Impact craters provide one of the best means of probing the composition of the mostly buried ancient Martian crust by exposing rocks from the subsurface in the ejecta, walls, and central peaks of impact craters. One of the principal discoveries of the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) is the enormous mineralogic diversity exhibited in the rock units associated with impact craters. Some craters serve as probes of the primary mafic crust of Mars, exposing rocks bearing olivine and low- and high-calcium pyroxenes, which show little evidence of alteration. In other craters, the presence of secondary minerals such as Fe/Mg smectite, chlorite, prehnite, kaolinite, illite/muscovite, hydrated silica, or analcime testifies to aqueous alteration under varying geochemical conditions. Can subsurface stratigraphies be reconstructed by examination of these Martian impact craters? To what extent can the geochemical conditions and environmental settings of aqueous alteration be inferred? Do alteration minerals indicate aqueous alteration prior to impact or post-impact processes (e.g. near-instantaneous phase transformations by shock, hydrothermal activity driven by the heating of materials in and around the crater, or preferential weathering of impact glass)? These questions will be examined with examples drawn from CRISM images of the southern highlands of Mars from west of the Isidis Basin, north of the Argyre Basin, Terra Tyrrhena, Noachis Terra, and Arabia Terra. In general, the ability to precisely determine subsurface stratigraphy is rare. However, in several cases, the environmental setting of aqueous alteration—specifically, alteration at hydrothermal or low-grade metamorphic conditions—can be determined definitively by means of indicator minerals and characteristic mineral assemblages. In particular, we find prehnite in several impact craters, which serves as an indicator of aqueous alteration under hydrothermal conditions at temperatures of 200-400C and pressures <3 kbar. Assemblages with chlorite/prehnite/illite and smectite/silica/analcime are also characteristic of low temperature hydrothermal alteration. In most cases, these materials are found in the ejecta as well as crater walls and central peaks, likely indicating they are excavated products of pre-impact hydrothermal alteration.
Buczkowski D.
Clark Roger Nelson
Ehlmann Bethany L.
Mro-CRISM Team
Murchie Scott L.
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