Role of impact excavation in distributing clays over Noachian surfaces

Abstract

[1] 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 percentage of clay-bearing material in the ejecta as a function of distance from the crater's rim. Generally, the volume percentage of clays in the ejecta is greatest for craters less than 25 km in 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 and less so in distal ejecta.