USING IMPACT CRATERS TO INVESTIGATE THE ROLE OF VOLATILES IN MARTIAN HISTORY

Impact craters not only provide insights into the characteristics of the crustal materials into which they excavate but also serve as repositories for materials transported by other geologic processes. They therefore can enhance our understanding of the role of both subsurface and surface volatiles throughout Martian history. Subsurface volatiles influence the crater formation process. Studies of layered ejecta morphologies reveal diameter and regional dependencies which reflect the depths and distributions of subsurface volatile reservoirs. The depths of these reservoirs appear relatively constant with time. Pitted crater deposits are interpreted as resulting from the interaction of impact melt with target volatiles, and volatiles help to weaken subsurface layers which contribute to the creation of central pits during crater formation. Obliquity cycles result in surface ice accumulations in the mid- to high-latitude regions during high obliquity phases. Impact into surface ice deposits create unique crater features, such as the very thin but extensive low-aspect-ratio layered ejecta deposits around small craters which transition into pedestal craters as the ice-rich mantle sublimates during low obliquity periods. Surface ice deposits also contribute to the formation of the double layer ejecta craters, which are distinct from other types of ejecta deposits on Mars. Glacial and fluvial activity which occurs post-impact can leave characteristic deposits inside craters, revealing evidence of these processes long after climate conditions have changed. Studies of the primary and secondary crater features demonstrate that the Arabia Terra region is an area of long-lived subsurface and surface volatile activity. Interior crater deposits reveal that glacial activity occurred at latitudes as low as 27°N in Arabia, with rivers and lakes prevalent at lower latitudes. However, no evidence for extensive oceans within or adjacent to Arabia is found within the crater record. Crater size-frequency distribution analysis of craters with and without specific glacial or fluvial deposits provides constraints on the timing of these processes. This presentation will summarize our studies of various crater morphologies across Mars to provide a synthesis of the role of volatiles in the planet’s geologic history.