Constraining the average fill densities of Mars' lowlands and fluvial erosion of Titan's polar regions.

Abstract

Other than Earth, Mars and Titan are the only bodies in our Solar System where we have observed widespread fluvial activity. In this thesis I present two approaches for constraining the extent of multiple resurfacing processes in order to gain insight into the early history of Mars and Titan. One of the most distinctive features of the Martian surface is the dichotomy between the heavily cratered southern highlands and the relatively smooth northern lowlands. The northern lowlands appear smooth because many of the craters in the north have been partially or completely buried beneath volcanic and sedimentary fill of unknown relative proportions. In Chapter 1, we use the Mars Orbiter Laser Altimeter (MOLA) topography data, the Mars Reconnaissance Orbiter (MRO) gravity model and a Wiener filter to map these buried craters and estimate minimum fill thickness and volume as well as maximum fill density. The overall trend observed for the northern lowlands is more sedimentation near the dichotomy and less sedimentation further north and near the Tharsis region, which is consistent with the geology of the region. Titan has few impact craters, suggesting that its surface is geologically young. In Chapter 2 we evaluate whether fluvial erosion has caused significant resurfacing by estimating the cumulative erosion around the margins of polar lakes. Images of drowned fluvial features around the lake margins, where elevated levels of hydrocarbon liquids appear to have partly flooded fluvial valleys, allow us to map topographic contours that trace the fluvially dissected topography. We then used a numerical landscape evolution model to calibrate a relationship between contour sinuosity, which reflects the extent of fluvial valley incision, and cumulative erosion. We find that cumulative fluvial erosion around the margins of Titan's polar lakes, including Ligeia Mare, Kraken Mare, and Punga Mare in the north and Ontario Lacus in the south, ranges from 4% to 31% of the initial relief. Additional model simulations show that this amount of fluvial erosion does not render craters invisible at the resolution of currently available imagery, suggesting that fluvial erosion is not the only major resurfacing mechanism operating in Titan's polar regions.