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The asteroid 4 Vesta was recently found to have two large impact craters near its south pole, exposing subsurface material. Modelling suggested that surface material in the northern hemisphere of Vesta came from a depth of about 20 kilometres, whereas the exposed southern material comes from a depth of 60 to 100 kilometres. Large amounts of olivine from the mantle were not seen, suggesting that the outer 100 kilometres or so is mainly igneous crust. Here we analyse the data on Vesta and conclude that the crust-mantle boundary (or Moho) is deeper than 80 kilometres., Global mapping with high-resolution imagery by the Dawn probe revealed that the south polar depression is composed of two overlapping impact basins, Veneneia and Rheasilvia (1). This discovery is critical [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.12.012 Byline: Lissa Ong (a)(b), Erik I. Asphaug (a), Donald Korycansky (c), Robert F. Coker (d) Keywords: Impact processes; Moon; Comets; Asteroids; Ices Abstract: Impacts of comets and asteroids play an important role in volatile delivery on the Moon. We use a novel method for tracking vapor masses that reach escape velocity in hydrocode simulations of cometary impacts to explore the effects of volatile retention. We model impacts on the Moon to find the mass of vapor plume gravitationally trapped on the Moon as a function of impact velocity. We apply this result to the impactor velocity distribution and find that the total impactor mass retained on the Moon is approximately 6.5% of the impactor mass flux. Making reasonable assumptions about water content of comets and the comet size-frequency distribution, we derive a water flux for the Moon. After accounting for migration and stability of water ice at the poles, we estimate a total 1.3x10.sup.8-4.3x10.sup.9 metric tons of water is delivered to the Moon and remains stable at the poles over 1Ga. A factor of 30 uncertainty in the estimated cometary impact flux is primarily responsible for this large range of values. The calculated mass of water is sufficient to account for the neutron fluxes poleward of 75[degrees] observed by Lunar Prospector. A similar analysis for water delivery to the Moon via asteroid impacts shows that asteroids provide six times more water mass via impacts than comets. Author Affiliation: (a) Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064-1077, USA (b) Department of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721-0002, USA (c) Institute of Geophysics and Planetary Physics, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064-1077, USA (d) Los Alamos National Laboratory, Los Alamos, NM 87545, USA Article History: Received 19 June 2009; Revised 25 October 2009; Accepted 10 December 2009

We provide a fast method for computing constraints on impactor pre-impact orbits, applying this to the late giant impacts in the Solar system. These constraints can be used to make quick, broad comparisons of different collision scenarios, identifying some immediately as low-probability events, and narrowing the parameter space in which to target follow-up studies with expensive N-body simulations. We benchmark our parameter space predictions, finding good agreement with existing N-body studies for the Moon. We suggest that high-velocity impact scenarios in the inner Solar system, including all currently proposed single impact scenarios for the formation of Mercury, should be disfavoured. This leaves a multiple hit-andrun scenario as the most probable currently proposed for the formation of Mercury. [ABSTRACT FROM AUTHOR]