Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p13a1265b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P13A-1265
Other
[5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
The spectrometers OMEGA and CRISM on board the Mars Express and Mars Reconnaissance Orbiter spacecraft, respectively, detect deep, thick and widespread clay-bearing units present in and on much of the Noachian crust (Bibring et al., 2006; Mustard et al., 2008). These units' formation mechanisms and implications for Mars's deep past, however, remain open questions. These clays, along with widely distributed valley networks and putative lacustrine deposits probably indicate that the upper 10 km of the ancient Martian crust contained significant quantities of groundwater capable of chemically altering basaltic crust to form clay-rich strata. If these clays formed at depths where background geothermal temperatures are conducive to their formation, then a means of bringing these clays to the surface is needed. Kilometer-scale bolides, in ample supply during the Noachian Era, excavate upper-crustal material roughly as deep as their diameter and ballistically deliver excavated material from depth to the surface. We use Maxwell's Z-model to quantify the volume of excavated clay-bearing material and that material's final location (Maxwell, 1977; Croft, 1980; Richardson et al., 2007). We verified that excavation depth, volume and ejecta thickness profiles are consistent with observations. Our methods 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. We vary three parameters: the final crater diameter from 10 to 330 km; the clay-bearing layer thickness from 500 m to 7 km; and the clay layer burial depth from 1 to 9 km. In general, while the total delivery of clay-rich material delivered to the surface increases with crater diameter, the volume percent of clays in the ejecta peaks at ~50% for final crater sizes between 30 and 110 km depending on layer thickness and burial depth. The ejecta of a 110 km crater in Mawrth Vallis exhibits strong signatures of Fe/Mg and Al phyllosilicates (McKeown et al., 2009). As transient crater size increases beyond ~100 km, material deeper than an assumed 10 km pore closure depth is excavated. This clay-poor material reduces the peak volume percentage of clays to ~10-30% for craters larger than ~200 km. The impact excavation method for delivering clays to the surface likely explains clays associated with ejecta deposits and may reveal clues about the volatile content and stratigraphy of the upper Noachian crust.
Barnhart Charles J.
Nimmo Francis
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