CLASSIFICATION AND MAPPING OF MID-LATITUDE POST-IMPACT CRATER FILL MORPHOLOGIES ON MARS

Many mid-latitude craters on Mars exhibit morphologically distinct post-impact “fill” material on their floors. This material is inferred to be (or have been) ice-rich based on morphological characteristics such as evidence for flow and removal, and their morphologic similarity to ice-rich lobate debris aprons and lineated valley fill. These latter features possess subsurface reflectors with dielectric properties consistent with nearly pure water ice based on the Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD) instrument. However, the vast majority of fill-bearing craters are too small for SHARAD to resolve the presence or absence of ice. We have identified 8 main classes of mid-latitude post-impact crater fill material based on morphology and apparent preservation state. Our preliminary mapping of the global distribution of crater fill using images from the MRO Content Camera (CTX) has not revealed any relationship between fill class and location with the exception of Utopia Planitia, which has a concentration of class 1 (concentric crater fill). Fill material is concentrated between ~25–60°N and ~25–50°S, with occurrence dropping off with the onset of the so-called latitude-dependent mantle. A cluster of craters bearing fill material with morphologies indistinguishable from mid-latitude fill is also observed in Terra Sabaea from ~5–10°S; the presence of fill in this location was previously documented by Shean (GRL, 37, L24202), who noted that general circulation models (GCMs) predict deposition of surface ice in this region at high obliquity. The global distribution of fill-bearing craters in both the mid-latitudes and Terra Sabaea is consistent with an origin related to ice deposition during periods of higher obliquity based on multiple GCM results. This is significant as this fill material is geologically youthful based on crater retention ages, and hence informs us on Mars’ recent (Amazonian) climate history.