The Roughness and Albedo of 4 Vesta and Vestoids

Dawn measurements of 4 Vesta represent the first detailed investigation of a V-type asteroid. When combined with ground-based observations of vestoids, which are believed to be fragments of Vesta, these data enable a comparison of the surface properties of this class of asteroid with other small bodies. Such comparisons have been made with two important photometric parameters: roughness and albedo. We find that the macroscopic roughness, which includes everything from large features such as craters, mountains, and ridges, down to small clumps of particles, differs for Vesta and vestoids. Vestoids are rougher than their parent body, and furthermore, Vesta appears to be smooth mainly at small size scales. This result suggests that small craters and clumps of particles are infilled with dust from ejecta which is kept on the surface by Vesta’s planet-like gravity. In addition, Vesta could accrete dust from its surroundings in the asteroid belt. Analysis of the global albedo of Vesta shows that the variegations on this body are far greater than any asteroid studied so far by a spacecraft. The albedo seems to be bifurcated into two distinct regimes. This bifurcation appears to be exogenous in origin.