Russell, Christopher - The Smallest Terrestrial Planet Seen in the Light of Dawn

Kepler, Titius, Bode and von Zach were certain that a planet lay between Mars and Jupiter, a conviction that was confirmed with the discovery of Ceres on January 1, 1801.  However, Ceres was very small and, even when combined with the mass of Pallas, Juno and Vesta, discovered later, they were dwarfed by the other planets.  Thus, these bodies were demoted to ‘minor planet’ status and later to ‘asteroid’ status.  Only recently was Ceres, the largest of these bodies, ‘raised’ to dwarf planet status.  But what really defines a planet?  Can a planet be defined by its surroundings, or is it an essence from within, as Klaus Keil implied with his sobriquet for Vesta, “the smallest terrestrial planet”?  Dawn orbited Vesta for over a year with cameras, spectrometers, and radiometric tracking.  It confirmed our zeroth-order expectations that Vesta was the parent body of the howardite, eucrite and diogenite meteorites.  Dawn showed the ravages inflicted by the collisional environment of the asteroid belt as Jupiter formed and the outer planets migrated.  But Vesta is not a simple battered rock.  It is a largely intact protoplanet that underwent igneous differentiation, similar to the terrestrial planets.  While much of the structure on the surface of Vesta results from exogenic processes, there is also evidence for endogenic processes, possibly recently in geologic history.  Curvilinear gullies in crater walls suggest that the crater forming event resulted in the release of flowing water into the crater contributing to the formation of pitted terrain on the floors of these craters as the floor devolatilized.  At a minimum the gullies and pitted terrain suggest the presence of ice locked up in some regions of Vesta’s crust.  Impacts could produce transient atmospheres and transient flows on the surface.  These may have been produced as recently as 50 Ma ago.  Vesta, if judged from the processes occurring on the surface and below, very strongly deserves Klaus Keil’s sobriquet.