MORPHOLOGIC CHARACTERISTICS OF THE BEST-PRESERVED MARTIAN CRATERS: THERMOPHYSICAL MAPPING OF GASA AND ISTOK

The purpose of this study is to characterize ejecta deposits of Martian craters in order to better understand both impact and modification processes. Maps of Istok and Gasa craters and ejecta were created using mosaics of quantitative thermal inertia (TI) derived from nighttime infrared images taken by the Thermal Imaging System (THEMIS). Using the JMARS interface (http://jmars.asu.edu), THEMIS and MOLA images of were identified and downloaded; images were imported and mapped in ArcGIS. Map units identified include the crater wall, floor, continuous, and discontinuous ejecta. Continuous ejecta is typically coarse material surrounding the crater marked by a thermally distinct boundary, while the the discontinuous ejecta is characterized by ray-like deposits. Thermal inertia data of these units were extracted so patterns related to thermophysical properties could be examined.

At Istok, the discontinuous ejecta forms two deposits: an outer unit of low overall thermal inertia and an inner unit with higher TI. Most likely this shift is due to the particle size, as dust sized ejecta were able to travel farther than coarser material. The continuous ejecta lacks a distinct thermophysical signature. Overall, the crater walls have high TI, consistent with large blocks exposed in the steep walls. The floor of the crater shows the greatest variation in TI and most likely contains a range of sizes from dust to broken-up bedrock. 

Gaza crater also has two thermally distinct discontinuous ejecta units, with the outer having a lower TI (e.g. dustier) and the inner unit higher (sand and larger rocks). The continuous ejecta lacks a thermophysical signature, possibly due to the location (inside a larger crater). The crater walls exhibit the highest TI, grading down to lower inertia deposits (talus) towards the crater floor. The floor of this crater seems to be made up of a combination of fine grained material and larger rock fragments. 

Future work includes comparison between thermophysical maps and visible high-resolution images to identify correlations in thermal units and surface morphologies. Additional craters of differing sizes will also be mapped to compare processes at both simple and complex craters, and to characterize the effects of modification.