INVESTIGATING SOURCES OF SECONDARY CRATER RIM THERMAL INERTIA VARIATIONS ON MARS

Impact craters across Mars exhibit lack of consistent correlations between geologic and geomorphic target lithology and thermal inertia (TI) signatures on their rims, suggesting there are underlying mechanisms responsible for the high variability in rim TI of spatially proximal impact craters. TI is an expression of how readily the temperature of a material responds to the presence of a heat source (usually insolation) in the upper few centimeters of the surface, primarily controlled by particle size variations. TI data can be used to infer information about grain size, a fundamental textural parameter of sediments that may provide information on conditions of transportation and deposition. Therefore, TI studies of the martian surface are of fundamental utility in constraining the geologic history on Mars. Previous work [e.g., Beddingfield et al., 2018] has shown that rim TI variations of primary craters on Mars could be due to regolith mantling, possibly a result of crater degradation producing fine-grained regolith over time through comminution. Here, we focus on constraining why TI variations exist by studying rim TI values of secondary crater populations (formed by fragments ejected during a primary impact event) in low dust content regions on Mars. Such populations of craters are useful in understanding variations in rim TI because all members of the same family of secondaries form essentially contemporaneously, eliminating age as a factor in accounting for variations in rim TI. To assess the significance of target lithology, impact sites with secondary crater populations covering homo- and heterogeneous geologic target units (i.e., number of units intersected) are analyzed. The data used in this study are 100 m px^­–1 nighttime IR TI-images from the Mars Odyssey THEMIS instrument, and 5 m px^­–1 Mars Reconnaissance Orbiter CTX images. Based on initial observations of the relationship between rim TI and target lithology among single populations of secondaries covering more than one geologic unit, we hypothesize that target lithology contributes to variations in rim TI across Mars. Localized surface environment differences may contribute to local TI variations. We anticipate that a larger sample set of secondary crater populations will provide further constraints on sources responsible for rim TI variability.