Gravity constraints on origin and evolution of Vesta

Kurzfassung

The Dawn mission determined the gravity field of Vesta to degree and order 20 and mapped its shape relative to the center of mass to high accuracy. The Bouguer gravity field can be calculated as the difference between the observed gravity field and one calculated under varying assumptions of the internal density structure that minimizes the misfit with the observed field. Vesta has significant Bouguer gravity anomalies that can be interpreted as crustal thickness or density variations, and likely reflect both sources. The Bouguer anomalies are associated with structural features such has the Vestalia Terra highland and the deep Saturnalia Fossae, as well as lithological provinces, such as the extensive dark material deposits at the Veneneia impact basin rim, and the diogenitic central mound of the Rheasilvia impact basin. As such, these anomalies reflect the modification of the vestan crust and mantle by impacts that have extensively fractured and pulverized it while also exposing deep-seated material and mixing it with the original crust; they also reflect the addition of low-density exogenic material to Vesta’s surface. However, impacts alone can’t account for the all of the density (and/or crustal thickness) variations implied by the Bouguer gravity field. The presence of significant density anomalies in concert with broad compositional and geologic variations is consistent with heterogeneity in the original crust and mantle of Vesta. Features such as southern Vestalia Terra and similar high-density features appear to be primordial, and suggest intracrustal plutons consistent with evidence from the trace element geochemistry of HED meteorites, and genetic models that include multiple magma chambers.