BUILDING BLOCKS FOR THE TERRESTRIAL PLANETS

Planetesimals were the raw materials from which the planets accreted. Asteroid Vesta and dwarf planet Ceres are representative of only a handful of planetestimals that survived intact from the ravages of disruptive collisions and orbital scrambling. Exploration of these bodies by the Dawn spacecraft reveals that both are differentiated. Vesta has a basaltic crust composed of volcanic flows and plutons, a layered mantle of harzburgite with olivine likely sequestered at its base, and a massive metallic core. Ceres has an upper layer of serpentine, ammoniated clay, and salts, containing varying amounts of water ice, and a rocky core; an ancient, overlying crust of water ice may have been worn away over time. Although these planetesimals and the planets that formed from bodies like them have chondritic bulk compositions, the building blocks for terrestrial planet formation had already undergone differentiation – following geochemical and geothermal pathways that varied depending on whether or not ices were initially present. Details of the magmatic evolution of Vesta are indicated by igneous meteorites (basaltic ‘eucrites’ and ultramafic ‘diogenites’) derived from it, with geologic context for these samples provided by Dawn’s spectroscopic and geophysical mapping. Although we have no meteorites from Ceres, its petrologic evolution can be compared with carbonaceous chondrites, which experienced similar but less advanced aqueous alteration. Radiogenic dating indicates that Vesta’s magmatic activity was ancient, fueled by heat from the decay of short-lived radioisotopes. Although Ceres’ differentiation must have utilized the same heat source, its internal geologic activity somehow persists to the present day, as indicated by Dawn’s discovery of recent cryo-volcanism. Both types of planetesimals, each originally derived from a different part of the solar system, were needed as feedstock for formation of the terrestrial planets.