Is Vesta Cratering a Record of Primordial Bombardment

Introduction: The mission Dawn will provide the first, detailed images of Vesta surface and supply crucial information to constrain its mineralogical and elemental composition through VIR, its imaging spectrometer. Thanks to these data, we will be able to study in depth the crater record on the surface of Vesta and get an insight on its place in the context of Solar System history. Vesta is believed, based on the Hubble observations and ground based observations, to be the parent body of HEDs meteorites. HED meteorites whose dating is very old, to have underwent early to a substantial differentiation. Blichert-Toft et al.[1] have investigated the Sm–Nd and Lu–Hf systematics of 21 whole-rock eucrites, including five cumulates, by MCICP-MS. A statistically significant Sm–Nd isochron was obtained on 18 samples with an age of 4464±75 Ma. This means that Vesta can be used as a reasonable model for the early stages of the terrestrial planets formation. Therefore on Vesta we shall find the record of primardial processes affecting old planetary surfaces just after crustal formation. The primordial bombardment can be related in turn to the final phases of Jupiter and Saturn accretion. In the framework of our studies on the origin of Jupiter[2] [3], we evaluated the accretion rate during the final phases of Jupiter formation and, through an N-Body code developed on purpose, we evaluated the flux of impactors on Vesta keeping track of their formation zones, which can bear information on their composition. We also evaluated the flux of impactors in the case Jupiter formed earlier than Vesta and underwent to a mild displacement, as hypothesized by the Nice model, and in the case the planet formed later but still radially migrated, to estimate the relative importance of the different processes. We have also considered the importance of the Saturn and other outer planets presence. We will describe the different scenarios and their implications for the evolution of Solar System to provide a reference frame for future studies of Vesta's cratering history.