Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p31b1417h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P31B-1417
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 6250 Moon (1221)
Scientific paper
The South Pole Aitken Basin (SPA; probably the largest in the solar system) is about 2500 km in diameter and up to 13 km deep [1]. Simple scaling arguments suggest that this basin should have excavated the entire lunar crust [2]. However, remote-sensing observations indicate the floor of the SPA basin is predominantly lower crustal in origin [3-5], while gravity and topography data likewise suggest the presence of a ~40km thick lower crust [6]. This discrepancy might be due to either a low velocity impact [7] or, less likely, nonproportional scaling [6]. We use a high resolution, two dimensional hydrocode (Zeus [8]) to model vertical lunar impacts. Massless tracer particles were used to track and locate the crustal excavation cavity diameter and depth [8]. Cavity diameter scales as kinetic energy to the 0.3 power, in agreement with previous studies [9], and a kinetic energy of ~2.5e27 joules is required to form an SPA-size basin. The excavation depth: diameter ratio remained relatively constant at 0.14 +/- 0.03 over basins in the 500-2500 km diameter range [cf. 6]. For constant kinetic energies, lower velocities result in shallower excavation depths. However, even at the minimum velocity of 3 km per second, we found the excavation depth for SPA to be at least 100 km, greater than the likely crustal thickness. Shock melting products are found primarily at the surface of the transient crater. Based on our vertical impact results, we conclude that the entire crust was stripped from the SPA during the basin-forming impact and that the basin floor is primarily a solidified impact melt-sheet, perhaps with some lower crustal material mixed in [cf. 2,10]. [1] Spudis et al. Science 1994 [2] Lucey et al. JGR 1998 [3] Pieters et al. JGR 2001 [4] Jolliff et al. JGR 2000[5] Lucey GRL2004 [6] Wieczorek and Phillips Icarus 1999 [7] Schultz LPSC 28 [8] Nimmo et al. Nature 2008 [9] Housen et al. Icarus 1979. [10] Collins and Melosh LPSC 35
Hammond N. P.
Korycansky Donald G.
Nimmo Francis
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