CRATER RIM GEOMETRY CONTROLS ON ALLUVIAL FAN FORMATION ON MARS: IMPLICATIONS FOR CONSTRAINING GLOBAL CLIMATE IN THE LATE-HESPERIAN TO EARLY AMAZONIAN

The discovery of craters across Mars containing alluvial fans has added to a growing body of evidence that the planet was once a much wetter place, possibly into the Amazonian. Detailed studies of several impact craters and the water-formed deposits they contain have provided a characterization of paleo-hydrologic conditions. However, these few studies are unable to resolve how widespread this period of fluvial activity was, nor how episodically it may have occurred, which is a key knowledge gap in Mars’ hydrologic history. Climate is typically thought to be the main driver on martian fan location, but an additional boundary condition, the degradation state and geometry of the host crater, remains mostly unexplored. Relatively fresh impact craters may have provided the necessary high relief surfaces along which alluvial fans may form, and a pre-existing catchment may have led to a positive feedback between sediment production and transport and catchment incision.

Here, we performed a morphometric analysis of ~5 to 10 fan-hosting and nearby non-fan hosting craters within southern Margaritifer Terra, southwestern Terra Sabaea, and southwestern Tyrrhena Terra. Using MOLA and CTX-resolution DEMs, we measured crater rim height and rim slope directly upslope of each fan. We also measured the local crater wall height and wall slope near each fan. For non-fan hosting craters, we made 12 profiles across each crater, resulting in 24 measurements of rim height and slope and wall height and slope. We also collected data on each individual fan deposit and their upslope catchments, including fan area, volume, length, average slope, and azimuthal orientation. Crater degradation was quantified by the degree of backwasting and impact age was estimated from crater counts on the crater’s ejecta.

Initial analysis suggests that craters hosting fans were less degraded than nearby craters of similar diameter and age, based on backwasting distance relative to crater diameter. Craters hosting fans were also statistically more likely to have steeper rim slopes, greater rim height-to-diameter ratios, and were less infilled, supporting the hypothesis that rim topography may have some control on fan formation. Current work is focused on testing whether crater morphology is also an important control on fan morphology and its relative location within its host crater.