Quantifying the Amount of Impact Ejecta at the Mer Landing Sites and Potential Paleolakes in the Southern Martian Highlands

Introduction: Martian paleolakes have been offered as landing sites for in situ and sample-return missions because of their high probability of containing climatic and hydrologic records and potential biomarkers. Prospective paleolake sites are identified in closed craters primarily based on discrepancies between the craters' expected and measured depths and interpretation of associated fluvial and lacustrine features (e.g. [1]). For instance, the 160-km diameter Gusev crater is shallower than expected and has a clear fluvial system running into it [2], but numerous sediment sources beside fluvial and lacustrine have been suggested as being able to at least partially fill Gusev, including aeolian deposits, ashfall from Appolinaris Patera [3] and basaltic lava flows [4, 5]. This study gives constraints on the maximum thickness of ballistically-emplaced crater ejecta at several sites on Mars to examine its importance relative to the numerous other sedimentation processes operable on Mars. Calculations: The thickness of ejecta (Th) as a function of distance from an impact crater can be estimated based on terrestrial and lunar craters based on the transient-crater radius [6]. This simple scaling relationship appears to hold for the terrestrial and lunar cases and so should be applicable to many Martian craters, but this type of calculation was developed primarily for ballistically-emplaced ejecta rather than the fluidized ejecta morphologies common on Mars. Nevertheless, distal ejecta deposits from large craters are observed on the Martian surface and this simple calculation gives some insight into the order of magnitude of this type of ejecta. I used a database of 26,883 craters from The Catalog of Large Martian Impact Craters [7], after excluding craters in the stratigraphically-young northern lowlands, Hellas, Argyre, and other large basins, degraded craters, and craters smaller than 7 km (the simple-complex crater transition diameter on Mars, D tc). For each crater, I calculated the transient crater diameter based on the measured crater diameter using the equation of [8]. This formulation has the advantage of taking target differences into account through a term containing D tc. I chose 28 sites of interest on the Martian surface, including the MER landing sites, several deep craters used as control sites, and potential paleolakes in the highlands [1, 9]. The thickness at each site was summed over all 26,883 craters, doubled to reflect global symmetry, and is presented in Table 1.