Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p53c1532b&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P53C-1532
Other
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
We present an analytical model of impact excavation that predicts the relative distribution of clay abundances on crater ejecta deposits. Spectrometers have detected clay-bearing units in and on much of the ancient Martian crust. Geothermally heated aquifers in basaltic rock provide conditions conducive to forming Fe/Mg phyllosilicates at depth. Throughout the Noachian Period, a high flux of km-scale bolides excavated buried materials and distributed them over the surface. We use the Maxwell Z-model to quantify the volume and final location of excavated clay-bearing material. We focus on two potentially detectable properties: the volume of clay-bearing material ejected as a fraction of total ejected volume, and the volume percent of clay-bearing material in the ejecta as function of distance from the crater's rim. Generally, the volume percent of clays in the ejecta is greatest for craters less than 25 km diameter. Larger crater sizes incorporate a higher fraction of clay-poor material because they excavate to greater depths, at which clays are likely absent. Specific trends in bulk clay volume fraction and the distribution of clay fraction across the ejecta deposit as a function of crater size depend on the depth to the clay-rich layer and its thickness. Impact excavation likely explains clays associated with ejecta deposits and may reveal clues about the volatile content and stratigraphy of the upper Noachian crust. Applying our model to the Mawrth Vallis region suggests that a clay layer a few hundred meters thick is buried at the -3000 m elevation contour. Given that clay layers are likely thin and buried in the upper 3 km of the crust we predict that small to mid-sized craters (<25 km) will best exhibit detectable amounts of clays and that these clays will be most abundant in the crater wall and rim, less so in distal ejecta.
Barnhart Charles J.
Nimmo Francis
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