GEOPHYSICAL EVOLUTION OF VESTA AND CERES

The Dawn mission explored two massive protoplanets in the main asteroid belt, Vesta and Ceres, that are fossils from the earliest epoch of our solar system’s formation. Dawn’s data have provided evidence that these bodies formed very early, within the first few m.y. after CAIs, yet they followed divergent evolutionary paths. Vesta formed <1.5 m.y. after CAIs of volatile-depleted chondritic materials. Dawn confirmed the HED-based prediction that Vesta melted, forming at least a partial magma ocean, and a large iron core. Gravity and spectral data support a complex magmatic evolution, resulting in a compositionally stratified mantle, with olivine sequestered in the deep mantle, and eruption of evolved melts. Such complexity can explain the apparent distinct magmatic reservoirs implied by trace elements in the HED clan. Discovery of hydrated material on Vesta’s surface implies that delivery of volatiles to the inner solar system by primitive asteroids was an important process. Thus, while the basic HED paradigm was confirmed, we learned that differentiation on a small planet is more complex than envisioned. Ceres, the only dwarf planet in the inner solar system, was known to be water-rich before Dawn arrived. However, contrary to the expected ice-rich, viscously-relaxed smooth surface resulting from physical (ice/rock) differentiation and freezing of an ancient subsurface ocean, its surface has many craters, implying a mechanically strong thick crust. The lack of large craters and Ceres’ gravitationally-relaxed shape implies that the strong crust overlies a weaker deep interior. The globally homogeneous distribution of minerals across the surface indicates that Ceres’ interior experienced pervasive alteration. The topography and morphology of the surface reveal regional variations, with smoother, apparently resurfaced areas that are generally at lower elevation and rougher areas with greater relief. Local morphology such as crater floors deposits, isolated mountains, and the enigmatic bright areas indicate recently active processes on Ceres likely driven by brine cryovolcanism.

Acknowledgements: Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration.