VESTA – A HISTORICAL PERSPECTIVE (Invited Presentation)

Asteroid (4) Vesta has been called the smallest terrestrial planet (Keil 2002), but its unique place in the study of the solar system goes far beyond that designation. At opposition Vesta is the brightest asteroid, so that over the years it has been an early target of virtually every new observational technique applied to asteroids. As early as the 1920’s, Bobrovnikoff (1929) obtained multicolor lightcurves of Vesta using an innovative photographic technique; lightcurves which showed color variations confirmed by subsequent investigators. It is thus fitting that the first detailed spacecraft investigation of a main belt asteroid should choose Vesta as its initial target, and confirm the spectral diversity of Vesta’s surface. Such diversity provides important insights into and constraints upon the geologic structure and surface processes on Vesta.

Telescopic spectra of Vesta in 1969 by McCord, Adams and Johnson (1970) indicated a differentiated basaltic assemblage, analogous to HED meteorites. Prior to that time, the dominant model for asteroids based on meteorite fall statistics was that they were almost entirely undifferentiated bodies. The basaltic surface of Vesta indicated a very different geologic history. Radiometric dating of the HED’s and other differentiated meteorites indicated formation ages within a few million years of the age of the solar system itself. Hence the basaltic crust of Vesta implied that this body had remained mostly intact over nearly the entire age of the solar system, one of perhaps three asteroids to be so preserved. When distinct meteorite parent bodies were counted, it became clear that that most of the parent asteroids represented in our meteorite collections were differentiated bodies (e.g., Keil 2000). Vesta appears to be the sole intact survivor of a class of differentiated bodies that once dominated the region of the present asteroid belt. In turn, this constrains the collisional history of the pre-terrestrial planet planetesimal population. The vast majority of those initial planetesimals were differentiated and broken up by subsequent collisions as evidenced by the large number planetesimal cores represented by the iron meteorites. This collisional process is constrained by the requirement that Vesta’s crust has survived mostly intact.