07.11

Vesta: Impact Crater Topography from Hubble Space Telescope WFPC2 Images

P.C. Thomas (Cornell), R.P. Binzel (MIT), M.J. Gaffey (RPI), A.D. Storrs, E.N. Wells (STScI), B.H. Zellner (Ga. Southern)

Hubble Space Telescope WFPC2 images obtained in May 1996 at scales of 36 km/pixel have provided new topographic information on Vesta, the probable parent body for the HED (basaltic achondrite) meteorites. The most significant topographic feature is a 450 km diameter crater centered near the south pole, containing a large central peak. The placement of the crater and the low phase angle of observations (5 )allow limb coordinate measurements of the crater's morphologic characteristics. It is about 8 km deep, its rim rises in places an additional 8 to 14 km, and its central peak is about 13 km high. Simple gravity scaling from the Moon to Vesta (g 22 cm/s ) suggests this crater would be equivalent to a 60 km lunar crater, securely in the size range of craters with central peaks. Other craters on Vesta are up to 150 km in diameter and a few km deep. Hydrocode results of Asphaug (1997, submitted) suggest the creation of a 450 km impact crater could launch multi-km-sized fragments to escape from Vesta (Binzel and Xu, Science260, 186, 1993). Color data from the HST images show at least part of the impact crater to have a deep mafic absorption band, as measured through the 0.673/0.953- m and 0.673/1.042- m filter ratios. These ratios are consistent with plutonic basalts and/or the excavation of olivine mantle material. Based on observations made by the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy Inc., under NASA Contract NAS 5-26555. Work supported in part by STScI grant GO-6481.