HR: 15:10h
AN: P33B-05    [Abstracts]
TI: Characteristics of Martian Layered Ejecta Craters and Their Implications for Near-Surface Volatiles
AU: * Barlow, N G
EM: Nadine.Barlow@nau.edu
AF: Northern Arizona University, Dept. Physics and Astronomy NAU Box 6010, Flagstaff, AZ 86011-6010 United States
AB: Fresh martian impact craters are typically surrounded by layered ejecta blankets which display one of three main morphologies: single layer ejecta (SLE), double layer ejecta (DLE), and multiple layer ejecta (MLE). Analyses of MOC and THEMIS (both visible and infrared) data are providing new insights into the characteristics and distributions of these ejecta morphologies. The SLE morphology is the most common type of ejecta morphology seen on Mars and recent numerical studies are able to replicate many of its observed features through impact into ice-rich targets. DLE craters display two layers of ejecta material with the outer layer emplaced after the inner, as indicated by scour marks across the inner layer. The inner layer is thicker, has lower sinuosity than the outer layer, and does not usually display an obvious distal rampart. The SLE pancake (Pn) ejecta morphologies occur in the same regions as the DLE craters and may simply represent DLE craters whose outer layer has been obliterated. The DLE outer layer may or may not be edged by a distal rampart, and the extent of the outer ejecta layer is among the greatest of any ejecta morphology, indicating a very fluid ejecta flow at the time of emplacement. DLE craters are concentrated in the 40 to 55 degree latitude zone in both hemispheres. In the northern hemisphere, DLE craters are located in topographic lows where layered sedimentary materials may have been deposited. The MLE morphology is usually associated with larger craters than the SLE or DLE morphology, is concentrated along the dichotomy boundary, and displays the highest sinuosity of any ejecta morphology. Central pits are commonly, but not always, associated with MLE craters and suggest the presence of subsurface volatiles. Pedestal (Pd) craters, where the crater and ejecta are perched above the surrounding terrain, are usually small craters found in many of the same locations as DLE craters. We propose that they form by impact into ice-rich fine-grained materials. The surroundings subsequently lose their ice through sublimation, leaving Pd craters elevated above the surroundings.
DE: 5400 PLANETOLOGY: SOLID SURFACE PLANETS
DE: 5420 Impact phenomena (includes cratering)
DE: 5464 Remote sensing
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting