VESTA – A HISTORICAL PERSPECTIVE
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.