HR: 11:50h
AN: U22A-07    [Abstracts]
TI: Maximum Velocity of a Boulder Ejected From an Impact Crater Formed on a Regolith Covered Surface
AU: * Bart, G D
EM: gbart@seti.org
AF: SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, United States
AU: Melosh, H J
AF: University of Arizona, Dept. of Planetary Science, 1629 E. University Blvd, Tucson, AZ 85721, United States
AB: We investigate the effect of regolith depth on boulder ejection velocity. A "boulder" refers to an apparently intact rock or rock fragment lying on a planetary surface, regardless of emplacement mechanism. Boulders appear in planetary images as positive relief features --- bright, sun-facing pixels adjacent to dark, shadowed pixels. We studied 12 lunar craters in high resolution (1~m) photographs from Lunar Orbiter III and V. Local regolith depth was measured using the method of small crater morphology. Ejection velocities of boulders were calculated assuming a ballistic trajectory to the final boulder location. A plot of regolith depth/crater diameter vs.\ maximum boulder ejection velocity shows that craters formed in deeper regolith (with respect to crater size) eject boulders at lower velocities. When ejection velocity (EjV) is in m/s, and regolith depth (Dr) and crater diameter (Dc) are in meters, the data fit the relation Dr / Dc = 1053 × EjV_max^-2.823. To explain the data, we turn to impact cratering theory. An ejected particle will follow a streamline from its place of origin to its ejection point (the Z-model), and then follow a ballistic trajectory. Material ejected along more shallow streamlines is ejected at greater velocities. If shallow regolith covers the surface, the most shallow (greatest velocity) streamlines will travel only through the regolith. Boulders, however, must be ejected from the bedrock below the regolith. Thus, the boulder ejected with the greatest velocity originates just below the regolith, along the most shallow streamline through the bedrock. If the regolith is deeper, the most shallow streamline through the bedrock will be deeper, and the maximum velocity of an ejected boulder will be lower. Hence, the regolith depth and maximum ejection velocity of a boulder are correlated: greater boulder ejection velocities correspond to thinner regolith. We observe this correlation in the data.
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 5464 Remote sensing
DE: 5470 Surface materials and properties
DE: 6250 Moon (1221)
SC: Union [U]
MN: 2007 Fall Meeting