VESTA AND CERES EVOLUTION BEFORE AND AFTER DAWN CONTROLLED BY GEOCHEMISTRY

The geochemistry of a planet, the minerals and molecules in it, and the temperature and pressures, control the way the planet develops and evolves and the final physical state. Very important is the volatile and radioactive elements contents, as they control the temperature achieved, and therefore the minerals formed, and these control the land forms and the interior structure. We discuss two very different examples of this. Vesta and Ceres are the largest and third largest bodies between Mars and Jupiter, in the region occupied by the asteroid belt. They are not asteroid-like but are intact, geochemically evolved bodies that have experienced planetary processes. They are located relatively close together but have followed very different evolutionary paths. Vesta is a dry, basaltic object that reached high temperatures and mostly or completely melted and physically differentiated. Ceres contains something like 25% water by mass, also is highly evolved, differentiated both physically and chemically, but by cooler aqueous processes, and appears to have been active within recent geologic time. Thus, the geochemistry of these two small planets is vastly different. The Dawn mission has studied both objects in detail from orbit, motivated by the prior telescopic observations, meteorite studies and thermodynamic modeling, confirming and vastly extending the earlier findings. The HED meteorites almost certainly are pieces of Vesta and allow a detailed investigation of Vesta’s chemical evolution. However, we apparently do not have meteorites exactly like Ceres, but study of the chemistry of the various carbonaceous chondrites allows us to bracket its possible evolution sequence. Mysteries still remain. Why did Vesta apparently accreted dry and Ceres wet when we find them so close together in the Solar System? Where are Ceres’ meteorites and family members that we find in abundance from Vesta? Why don’t we find more olivine on Vesta? Did Vesta evolve from a magma ocean or was there only partial (although certainly global) melting? Why are there ammoniated phyllosilicates on Ceres? Is the ammonia of internal or external origin? What are the transient exposed surface water ice and salt deposits on Ceres telling us about composition of the interior? Could there remain pools of liquid water within Ceres – even life?