THERMOPHYSICAL PROPERTIES OF MARTIAN LAYERED CRATER EJECTA: RELATIONSHIP TO EMPLACEMENT PROCESSES

A subset of Martian impact craters exhibit fluidized ejecta blankets, sometimes with multiple layers. These ejecta deposits are thought to form as the result of volatile interaction during the impact process - most likely from a subsurface reservoir (either ice or liquid). This interaction forms an ejecta layer that flows away from the crater and forms a cohesive deposit that may have a clearly defined steep margin (or rampart). Evidence of emplacement mechanism identified in visible images suggests that the processes involved in forming these craters includes flow deposition, ballistic emplacement, and/or modification of the surface by airburst. The results of these processes should also be evident in variations of thermophysical properties, particularly those related to particle size and packing density. Thermophysical variations have been used previously to identify low thermal inertia crater rays (likely due to small particle sizes) and to examine the processes that form these features. In addition, thermophysical variations related to debris flow have been previously identified on ice-related geomorphologies (e.g. lobate debris aprons).

An initial survey of relatively fresh layered ejecta deposits on Mars suggests that variations in nighttime temperature recorded by the Thermal Emission Imaging System (THEMIS) are related to these impact processes. Low temperature materials (appearing dark in THEMIS nighttime infrared images) lie on top of the lobate ramparts: these apparently fine-grained deposits have morphologies that suggest ballistic emplacement. Clear evidence of flow-related processes are not identifiable in layered deposits, but some lobes exhibit thermophysical variations along the margins that may be related to emplacement processes. Additional complications include post-depositional modification of ejecta - ballistic facies are difficult to identify in thermal data at some craters, and this modification may be masking flow-related thermophysical variations within lobate deposits.

Initial results suggest that coupled thermal and high-resolution image studies of layered crater deposits will prove valuable for understanding the role subsurface volatiles played in the formation of these craters on Mars.