GSA PLANETARY GEOLOGY DIVISION G.K. GILBERT LECTURE: IMPACT CRATERS AS A TOOL FOR MEASURING THE AMOUNT AND PROCESSES OF DEGRADATION ON MARS

Impact craters are commonly occurring landforms on Mars that range in size from m-scale to over 2000 km in diameter. More numerous, smaller, simple bowl-shaped craters transition to fewer larger, more complex structures over a diameter range of ~5-10 km. Because crater formation is geologically instantaneous, the evolution of their pristine forms is not influenced by degradation and is often predictable. Hence, studying modified crater morphology provides insight into the processes and amount of degradation occurring since their formation, which can be used to constrain active geomorphic processes over time.

Evaluation of modified simple terrestrial craters enables development of a degradation sequence where key morphologies and their scale can be used to place constraints on the amount and processes of erosion. On Earth, signatures associated with water-driven degradation dominate erosional morphologies even in arid settings.

Orbital and in situ study of modified simple craters on Mars also yields information on degradation processes over time. For example, craters in southern Ismenius Lacus preserve morphologies highlighting the importance of mass wasting and eolian processes since the Hesperian. In situ exploration of small craters in Gusev crater and Meridiani Planum indicate Hesperian and Amazonian degradation was dominated by differing rates of eolian erosion and infilling with possible lesser contributions from mass wasting. At the ~750 m-diameter Victoria crater, eolian erosion accounts for ~150 m widening and ~50 m infilling, mostly by sediments sourced outside the crater.

Because simple craters are relatively small and become increasingly modified over time, their utility in evaluating degradation is typically limited to younger geological surfaces. Therefore, evaluation of modified larger, and longer surviving, complex craters is necessary for characterizing Noachian erosion. Variable initial morphology of complex craters is due to factors that include late stage collapse of walls and complicates such studies. More recently, however, multiple models of initial complex crater form were used to constrain pristine form: the 22 km-diameter Endeavour crater in Meridiani Planum indicates 100–200 m of rim degradation by processes that included considerable Noachian-aged fluvial erosion.