The structure of the asteroid 4 Vesta as revealed by models of planet-scale collisions

Abstract

Asteroid 4 Vesta seems to be a major intact protoplanet, with a surface composition similar to that of the HED (howardite–eucrite–diogenite) meteorites^1,2,3,4. The southern hemisphere is dominated by a giant impact scar⁵, but previous impact models^6,7,8 have failed to reproduce the observed topography. The recent discovery that Vesta’s southern hemisphere is dominated by two overlapping basins⁹ provides an opportunity to model Vesta’s topography more accurately. Here we report three-dimensional simulations of Vesta’s global evolution under two overlapping planet-scale collisions. We closely reproduce its observed shape, and provide maps of impact excavation and ejecta deposition. Spiral patterns observed in the younger basin Rheasilvia⁹, about one billion years old¹⁰, are attributed to Coriolis forces during crater collapse. Surface materials exposed in the north come from a depth of about 20 kilometres, according to our models, whereas materials exposed inside the southern double-excavation come from depths of about 60–100 kilometres. If Vesta began as a layered, completely differentiated protoplanet, then our model predicts large areas of pure diogenites and olivine-rich rocks. These are not seen^11,12,13, possibly implying that the outer 100 kilometres or so of Vesta is composed mainly of a basaltic crust (eucrites) with ultramafic intrusions (diogenites).