Thermally distinct craters near Hrad Vallis, Elysium Planitia, Mars

Evidence of volcano–ground ice interactions on Mars can provide important constraints on the timing and distribution of martian volcanic processes and climate characteristics. Northwest of the Elysium Rise is Hrad Vallis, a ∼ 370   m deep, 800 km long sinuous valley that begins in a source region at 34° N, 218° W. Flanking both sides of the source region is a lobate deposit that extends ∼ 50   km perpendicular from the source and is an average of ∼ 40   m thick. Previous studies have suggested the formation of the Hrad Vallis source region was the result of explosive magma–ice interaction and that the lobate deposit is a mudflow; here we use newly available MOLA, MOC, and THEMIS data to investigate the evidence supporting this hypothesis. Within the lobate deposit we have identified 12 craters with thermal infrared signatures and morphologies that are distinct from any other craters or depressions in the region. The thermally distinct craters are distinguished by their cool interiors surrounded by warm ejecta in the nighttime THEMIS IR data and warm interiors surrounded by cool ejecta in the daytime THEMIS IR data. The craters are typically 1100–1800 m in diameter (one crater is ∼ 2300   m across) and 30–40 m deep, but may be up to 70 m. The craters are typically circular and have central depressions (several with interior dune fill) surrounded by ∼1 to >6 concentric fracture sets. The distribution of the craters and their morphology suggests that they are likely the result of the interaction between a hot mudflow and ground ice.