A conceptual model for the formation of ramparts on Martian impact crater ejecta

Abstract

Mars Orbiter Laser Altimeter (MOLA) elevation measurements for 23 different impact craters and 12 different long runout landslides show rampart ridges on Martian fluidized ejecta flows are higher relief than those on Martian landslides. We propose a conceptual model to explain this height difference that is based on the effects of the impact and ejecta emplacement process on the development of ejecta ramparts. Our model explains the relatively high relief of the distal ramparts as well as the particle size distribution that is inferred from thermal inertia measurements. In this model, impact events produce ejecta curtains that advance radially at increasingly higher velocity outward excavating and roughening the surface as they impact. This produces an inertia-driven, ground-hugging ejecta flow composed of primary and secondary ejecta. This flow moves rapidly across the surface following closely behind the impacting ejecta curtain. The ejecta curtain continuously adds impact-generated debris to the flow front that includes large particles, some of which are overridden by the flow, but some accumulate at the flow front. These particles are pushed forward at the flow front as a high-friction “dam” with the accumulating material growing into a relatively high rampart as the ejecta curtain adds more large particles to it. In addition, the impact-roughened surface cause substantial vibrations and shear in the flows moving behind the ejecta curtain. This roughness results in kinetic sieving in the flows that brings large particles to the surface and transports them to the flow front where some are also overridden or accumulate to add the ones already at flow front. We propose that these processes combine to produce the observed high ejecta ramparts. The relatively high velocity of the ejecta flows pushes the load of coarse-grained debris to the top of even high developing ramparts. When the flow halts, it drains back from the accumulated coarse debris at the flow front, leaving a high rampart dominantly composed of large particles.