HR: 16:30h
AN: P34A-02    [Abstracts]
TI: The Role of Volatiles in Martian Oblique Impact Ejecta Emplacement
AU: * Hessen, K K
EM: khessen@gi.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, P.O. Box 757320, Fairbanks, AK 99775 United States
AU: Herrick, R R
EM: rherrick@gi.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, P.O. Box 757320, Fairbanks, AK 99775 United States
AB: We have performed a detailed comparison of the geomorphology of oblique impacts in a dry vacuum (lunar and experimental) with those on Mars, a body with a tenuous atmosphere and apparently substantial volatiles in the crust. The ejecta from oblique impacts in a dry vacuum follow a consistent pattern as the impact angle decreases. Starting at angles of $\sim$20 degrees, an area uprange of the crater develops that has no ejecta (a "forbidden zone"). The shape of the forbidden zone is an outward curving "V" with its apex at the crater rim. As the impact angle becomes more oblique, the amount of ejecta downrange decreases. Between 5 and 10 degrees there is an abrupt transition to a complete lack of downrange ejecta (a second forbidden zone). This downrange forbidden zone is a straight-edged wedge that is always a wider angle than the uprange forbidden zone. There is also a narrow ray extending from the crater wall through the downrange forbidden zone that we interpret as ricocheted impactor material. All of the ejecta in the dry-vacuum craters was emplaced ballistically and exhibits no post-emplacement flow. While all but the smallest Martian craters have rampart ejecta, their ejecta planforms closely resemble those for craters in a dry vacuum. As impact angle decreases, the "curving V" forbidden zone appears uprange and the extent of downrange ramparts decreases. At the lowest impact angles there is an abrupt transition to a downrange straight-edged forbidden zone in the ramparts. Unlike the dry-vacuum craters, Martian craters maintain an elevated uprange rim at even the lowest impact angles. In all cases the downrange rim appears "blown out" by ricocheted material for the lowest angle impacts, but no Martian craters were observed that preserve a downrange ray, presumably because this feature is easily eroded on Mars. The similarity in ejecta patterns between the Martian rampart craters and the ballistically emplaced dry-vacuum craters suggests that Martian crater ejecta are first ballistically emplaced. Ramparts then form as a result of modest, post-emplacement lahar-like flows that preserve the basic ejecta planform.
DE: 5420 Impact phenomena (includes cratering)
DE: 5470 Surface materials and properties
DE: 5499 General or miscellaneous
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting