Combinations of processes responsible for Martian impact crater "layered ejecta structures" emplacement

Abstract

We utilized images and stereo-derived topographic data acquired by the High Relolution Stereo Camera (HRSC) and Thermal Emission Imaging System (THEMIS) images together with other data in order to study the geology of "layered ejecta structures" associated with relatively pristine Martian impact craters. The geomorphology and morphometric properties indicate their origin as complex combinations of a variety of impact processes. The studied (inner) layered ejecta structures often exhibit ground-hugging characteristics, and many of them do not have topographic profiles expected from simple ballistic emplacement. Such profiles include ones that are plateau-shaped or thickening outward. We think that water-rich fluidized flows driven by the momentum due to the impact and by gravity, together with ballistic emplacement and vortex produced by the atmosphere-ejecta curtain interaction, were essential to the (inner) layered ejecta structure formation. We hypothesize that the thinner outer layered ejecta structures were formed by various combinations of shockwave-induced liquefaction of water-rich near-surface sediments, ballistic emplacement of ejecta-entraining water, and strong winds (expanding vapor, vortex, base surge) related to the impact. The contribution of each proposed layered ejecta structure formation mechanism should have been variable depending on the condition of the impact. Copyright 2007 by the American Geophysical Union.