Volatile-rich Crater Interior Deposits on Mars: an Energy Balance Model of Modification

Introduction: Several craters on Mars are partially filled by material emplaced by post-impact processes. Populations of such craters include those in the circum-south polar cap region, in Arabia Terra, associated with the Medusae Fossae Formation, and in the northern lowlands proximal to the north polar cap. In this study, crater fill material refers to an interior mound, generally separated from the interior walls of the crater by a trough that may be continuous along the crater's circumference (i.e. a ring-shaped trough), or may only partially contact the crater walls (i.e. a crescent-shaped trough). The fill deposit is frequently off-center from the crater center and may be asymmetric, (i.e. not circular) in plan view shape. Such craters associated with the Medusae Fossae Formation are likely partially exhumed and contain remnants of Medusae Fossae material that may be ig-nimbrites [1], eolian materials [2, 3], true polar wander deposits [2], volatile-rich deposits [4], or volcanic ash accumulation [5]. In Arabia, such crater fill has also been interpreted as polar layered deposits associated with true polar wander [2] or lacustrine sedimentary deposits [6]. The fill of craters around the south pole is contiguous with south polar layered material, which argues for a similar process of deposition [7] with possible later exhumation of or flow into the crater [8]. Two craters at high northern latitudes contain fill material but are well separated from the north polar layered terrain: Korolev (73°N, 195°W) and an unnamed crater (77°N, 145°W). This configuration suggests this fill material was also deposited in a similar manner to the polar cap materials [7] and may or may not be remnants of a formerly more extensive polar cap [9]. An alternative origin is that impact disruption of the subsurface cryosphere allowed effusion of sub-cryosphere confined groundwater into the crater under artesian-like conditions [10, 11]. This process would be aided in a globally interconnected hydrosphere-cryosphere system [10] by the low planetary elevation of the northern lowlands craters [12]. By the same reasoning , such an explanation would not be likely for filled craters in high-elevation, circum-south polar regions. However, filling by groundwater effusion seems a less likely explanation than polar-like depositional processes because the only large craters (> 40 km diameter) in the northern lowlands containing significant fill material are the two most proximal to the north polar cap [12]. Given these theories of crater fill formation , fill material in the north and …