GEOMORPHOLOGY AND CRATER PRESERVATION OF HIGH THERMAL INERTIA, NOACHIAN-AGE UNITS IN THE HIGHLANDS OF MARS

Noachian-age, high thermal inertia (TI) units in the highlands of Mars are priority targets for exploration. The thermal characteristics at locations in Terra Cimmeria for example indicate surface exposure of bedrock with a discontinuous or thin mantle of fines. Mineralogical analyses indicate a mafic composition, sometimes with olivine enrichment. While the high TI and mafic mineralogy have been used to suggest an effusive volcanic origin, a possible clastic origin has also been proposed. Here we evaluate a suite of high TI units in the highlands to describe their geomorphology, stratigraphy, and preservation of 10 to 100-m diameter (D) impact craters. High Resolution Stereo Camera (HRSC) images (~12 m/pixel) were co-registered at each location to the daytime Thermal Emission Imaging System (THEMIS) global mosaic (100 m/pixel). Each area covers approximately 10,000 km². Higher resolution images from the Context Camera (CTX) (6 m/pixel) were georeferenced to HRSC and High Resolution Imaging Science Experiment (HiRISE) (25-30 cm/pixel) images to the CTX. Terrain maps were constructed at a scale of 1:10,000 using CTX. Low and moderate TI, Hesperian-age, ridged lava plains in Hesperia Planum and Syrtis Major were also mapped for comparison. The mapping identified surface textures and landforms for the high TI units that are unique relative to known lava plains. These include lighter-toned layered materials that exhibit m-scale orthogonal joints, layered buttes/knobs that are conical or elongate, and degraded impact craters with scalloped margins and concentric, interior layered patterns of variable albedo. No lobate margins or lava flow textures were observed. Crater counts were performed on each unit at the CTX scale, including craters with D ≥ 200 m, and at the HiRISE scale, including D ≥ 50 m. The data indicate Amazonian retention ages for 10 to 100-m-scale craters and an overall lower crater size frequency at these diameters when compared to Hesperian lava plains. We conclude that many Noachian-age high TI units in the highlands represent localized areas on Mars that have experienced relatively high Amazonian-age erosion rates when compared to more typical, regolith-covered lava plains. This can be accomplished by either high surface process rates (e.g. wind erosion) or due to weaker lithologies (e.g. clastics).