LAYERED CRATER EJECTA ON MARS AS POTENTIAL EVIDENCE FOR SUBSURFACE ICE

Many Martian impact craters are similar to those found on the Moon and Mercury. A subset of Martian craters, however, have continuous ejecta deposits, some with multiple layers, in addition to discontinuous radial deposits. This study focuses on this subset, referred to as layered ejecta craters. Layered ejecta is deposited as a continuous unit surrounding the crater, typically with steep slopes along the outer margin. These ramparts, when observed, clearly define the layers of ejecta. Layered deposits are hypothesized to have formed due to interaction of reservoirs of sub-surface ice during the impact process, creating fluidized ejecta that flows away from the crater. Radial discontinuous deposits are more consistent with ballistic ejecta emplacement (as observed in lunar craters).

The temporal relationship between the layered and radial ejecta is not always clear, although it appears in some cases that the layered deposit overlies the radial. Better insight into these deposits is critical for developing a better understanding of their formation mechanism.

Layered ejecta craters were examined using images from the Thermal Imaging System (THEMIS, onboard Mars Odyssey) and High Resolution Imaging Experiment (HiRISE, onboard Mars Reconnaissance Orbiter). THEMIS thermal infrared data (100 m/pixel resolution) can be used to determine surface temperature, which is controlled by the thermal inertia of the material. Thermal inertia depends on physical properties of the surface such as particle size and packing density. Surface morphology is examined in detail using THEMIS visible (18 m/pixel resolution) and HiRISE (meter scale and smaller resolution) images.

Initial results indicate that layered and radial ejecta deposits have different thermophysical properties expressed as variations in nighttime surface temperature, although it is not always consistent that the layered ejecta has a higher thermal inertia than the radial (as might be expected). Where observed, ramparts have distinct thermophysical signatures that may be related to emplacement processes. Later modification by eolian and glacial processes sometimes complicates analysis, however. Results from relatively fresh craters should allow for a dramatic increase in the understanding of the formation of complex ejecta deposits on Mars.