Why Ceres and Vesta?
Dawn focuses on two of the first bodies formed in the solar system, the surviving protoplanets, Ceres and Vesta. Radioisotope chronology from the howardite, eucrite, and diogenite (HED) meteorites believed to be from Vesta suggests it accreted in only 5-15 million years.
Similar evidence indicates that Mars continued to accrete for close to 30 million and Earth for 50 million years. The early cessation of accretion in the asteroid belt was presumably due to the formation of Jupiter whose gravitational forcing countered the accretionary process, and today is causing the disruption of the bodies that did accrete. Although we do not have similar meteorite evidence directly linked to Ceres, it too is expected to have formed in the first approximately 10 million years. In addition the asteroid belt may have been scoured by comets, scattered by the formation of the remaining gas giants. Today only some of the largest asteroids remain relatively undisrupted. The most massive of these are Ceres and Vesta, two most complementary minor planets. The former has a very primitive surface, water-bearing minerals, and possibly a very weak atmosphere and frost. The latter is a dry, differentiated body whose surface has been resurfaced by basaltic lava flows possibly possessing an early magma ocean like the Moon. Most importantly Vesta has experienced significant excavating events, most notably indicated by the huge crater near its southern pole. Cosmic ray exposure dating of HEDs indicates that impacts have released meteoritic material at least five times in the last 50 million years. Meteorites from these impacts have been used to piece together a most probable scenario for Vesta's thermal evolution.
No meteorites have unmistakably come from Ceres. Possibly the excavating events or dynamics that provided the HED meteorites did not occur at Ceres, but also possibly, the reflectance spectrum of the surface of Ceres is not indicative of its crustal rocks. Microwave studies suggest that Ceres is covered with a dry clay, in contrast to Vesta's basaltic dust layer that reflects its crustal composition. To determine if Ceres-derived meteorites are in our collections and to understand the origin of Ceres, we must travel there and obtain spatially resolved spectra inside fresh craters.
Meteorites provide an incomplete glimpse of their parent bodies. For Vesta and Ceres we need to know their interior structure, their thermal history as manifested in the geological and geophysical record, and the processes that are acting on and affecting their surfaces. We need to determine the geologic context for the HED meteorites from Vesta, and search for similar data for Ceres. We are especially interested in contrasting dry, differentiated Vesta with its wet counterpart, Ceres, just a little further from the Sun. It appears that a rather short additional radial separation allowed Ceres to accrete wet and stay cool while early heat sources in the accreting material melted Vesta. Most importantly because they both lie near the ecliptic plane in near circular orbits we can rendezvous with and study both quite cost effectively with a single Discovery mission, one with ample reserves.