Paper No. 2
Presentation Time: 8:20 AM
THE CRATERING HISTORY OF VESTA (Invited Presentation)
O'BRIEN, David P.1, MARCHI, Simone2, SCHENK, Paul M.3, JAUMANN, Ralf4, MCSWEEN, Harry Y.5, REDDY, Vishnu6, DENEVI, Brett W.7, VINCENT, Jean-Baptiste6, RUSSELL, Christopher T.8 and RAYMOND, Carol A.9, (1)Planetary Science Institute, 1700 E. Ft. Lowell, Suite 106, Tucson, AZ 85719, (2)Southwest Research Institute, Boulder, CO 80302, (3)Lunar and Planetary Institute, Houston, TX, (4)German Aerospace Center (DLR), Institute of Planetary ResearchGerman Aerospace Center (DLR), Berlin, Germany, (5)Earth and Planetary Sciences Dept, University of Tennessee, Knoxville, TN, (6)Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany, (7)Johns Hopkins University Applied Physics Lab, Laurel, MD, (8)Earth Planetary and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095-1567, (9)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, obrien@psi.edu
The Dawn mission has completed its orbital mapping phases at Vesta, bringing into focus the interesting and complex cratering history of Vesta, first hinted at by the detection of a large south polar impact structure in HST imaging [1]. The south polar impact basin Rheasilvia is ~500 km wide and ~22 km deep [2,3]. Analysis of the topography showed that there is also a second ~400 km diameter impact basin, Veneneia, that lies partly beneath Rheasilvia, as well as numerous smaller basins in the north [2,3,4]. Rheasilvia has a remarkably fresh appearance, and from crater counts its age has been estimated to be only ~1 Gyr [2,4]. Veneneia's age is poorly constrained, but it is estimated to be older than 2 Gyr [2]. Mapping of the surface shows an increase in crater density between the southern and northern hemispheres, suggesting that the formation of these basins reset most of the southern hemisphere [4].Smaller craters on Vesta are seen at all stages from fresh to highly eroded. Some craters expose atypically bright or dark material, often as what appear to be layers in the crater walls [3]. Most craters do not show signs of significant impact melt, but several craters show flat floors and 'pitted' terrain that may be due to impact melt and possibly the release of volatiles [5]. A significant number of craters have formed on regional slopes, leading to an asymmetrical morphology [3].
Vesta's cratering record can help discriminate between different models of its impact history, and hence provide a strong constraint on the early impact history of the Solar System. One approach is to use a lunar chronology curve scaled to Vesta [6]. Another approach is to calculate the impact rate based on the best current models of the primordial depletion and dynamical evolution of the main belt [7]. Both approaches will give approximately the same result for surfaces younger that ~3 Gyr. However, the scaled lunar curve predicts an early impact rate orders of magnitude too large, while the theoretical curve is broadly consistent with the number of observed basins on Vesta.
[1] Thomas et al. (1997), Science 277, 1492. [2] Schenk et al. (2012), Science 336, 694. [3] Jaumann et al. (2012), Science 336, 687. [4] Marchi et al. (2012), Science 336, 690. [5] Denevi et al. (2012), Science, submitted. [6] Schmedemann et al. (2012), LPSC, 2544. [7] O'Brien et al. (2012), LPSC, 2688.
