Modification of impact craters in the northern plains of Mars: Implications for Amazonian climate history

We measured the depth, wall steepness, and ejecta roughness and surveyed the wall and floor morphology of all craters 10-25 km in diameter within the typical Vastitas Borealis Formation in the northern lowlands of Mars north of 52 degrees N. Two of the 130 craters have unusually rough ejecta; they are deep, have steep walls, and are apparently the youngest in the population. Icy mantles filling the local subkilometer-scale topographic lows is the main contribution to ejecta smoothing, which occurs at a time scale on the order of tens of Myr. Wall degradation and crater shallowing generally occur at longer time scales, comparable to the duration of the Amazonian period. Many craters are shallow due to filling of the crater with specific ice-rich material of uncertain origin. We use our collected data to infer the nature of the past climate back through the Amazonian, a period prior to similar to 10-20 Myr ago, when orbital parameter solutions are chaotic and one must rely on geological data to infer climate conditions. We conclude that moderately high obliquity and wide obliquity variations were probable during the last 40-160 Myr. We tentatively conclude that high obliquity peaks (>40-45 degrees) may have occurred episodically through the last 210-430 Myr. A sharp step in the frequency distribution of wall steepness at 20 degrees may indicate a geologically long period prior to that time where obliquity never exceeded 40-45 degrees.