Identification of large (2-10 km) rayed craters on Mars in THEMIS thermal infrared images: Implications for possible Martian meteorite source regions

Four definitive and three probable rayed craters have been identified on Mars using 100-m resolution thermal infrared images obtained by the Mars Odyssey Thermal Emission Imaging System (THEMIS). These seven craters are similar to the previously discovered rayed crater Zunil and are best recognized by a distinct thermal contrast with respect to their surroundings. Martian rays, unlike their lunar counterparts, only exhibit minor contrasts in visible albedo. As a consequence, their presence on Mars most likely went unnoticed until substantial global coverage of THEMIS thermal infrared was achieved. Their presence has since been discerned in the coarser-resolution Thermal Emission Spectrometer (TES) data set, which preceded THEMIS. Observations in visible images of the primary cavities, secondaries, and rays suggest that, like lunar ray counterparts, Martian rays are invariably young geomorphic features. Martian rays are typically greater than hundreds of kilometers in length and consist of numerous densely clustered secondary craters, and thereby are a physical manifestation of high-velocity ejecta. Spallation accounts for a small fraction of the high-velocity ejecta that experiences low-shock compression due to interference from the rarefaction wave with the free surface. Spallation is currently the favored mechanism responsible for ejecting meteorites from Mars and is likely responsible for some of the ray-forming secondaries. Additional observations and inferences based on Martian rayed craters are compared with current Martian meteorite delivery models and the Martian meteorites themselves. The correlations presented here suggest that Martian rayed craters are the most plausible candidate source craters for the Martian meteorites to date.