Planetary Granular Topography: Slope Angles & Crater Concentric Ridges

Abstract

In the first portion of this dissertation I examine the effect of gravitational acceleration on the angle of repose of granular features. To do this I have used HiRISE DTMs to compare the slipface angles of Martian sand dunes with those measured on Earth. In doing this I have found that the slopes of active dunes on Mars do not differ from their terrestrial counterparts, and as such I have concluded that gravitational acceleration does not effect the angle of repose. In the second, larger portion of this dissertation I examine the morphology and formation of Crater Concentric Ridges (CCRs). These features, formerly known as 'Lunar Concentric Dunes', are ridges oriented concentrically to fresh craters a few kilometers in diameter. Using LROC NAC data I have created a catalog of 77 craters that have these features in their ejecta blankets. Further, I have used this data to map and measure the CCRs around eight craters of varying diameters in order to analyze their distributions. I have also been able to characterize the morphology of these ridges and how that morphology changes with distance from the host crater. Using DTMs made from NAC images I have studied the three-dimensional topography of CCRs in order to fully describe the morphology of these features. This morphological analysis has allowed me to refute several hypotheses for the formation of these features, including the previously accepted ballistic impact sedimentation and erosion hypothesis. In order to formulate a new theory for the formation of these features I have created simulations of crater ejecta flowing over regolith using discrete element modeling. In these simulations I found that Kelvin-Helmholtz instabilities form at the interface between the ejecta and regolith. I posit that these instabilities are responsible for the formation of Crater Concentric Ridges. This hypothesis is supported by the observation that the topography produced in my simulations strongly resembles that which I have measured and described around real lunar craters.