THE GEOLOGY OF VESTA: INSIGHTS FROM UNBRECCIATED EUCRITES

The DAWN mission, due to launch Summer 2007, will orbit and analyze both 4 Vesta and 1 Ceres, two of the largest asteroids that lie in the main belt between Mars and Jupiter. They are believed to be different both in origin and composition. Vesta appears to have a basaltic surface, indicating at least partial melting. Remote sensing observations, combined with mathematical modeling and comparison to meteorites suggest this asteroid was differentiated into a core, mantle and crust, making it the largest differentiated body in the asteroid belt. A major reason for continued interest in Vesta is that it is thought to be the parent body for HED meteorites, the most voluminous class of achondrites, due to compositional and spectral similarities. This family consists of cumulate orthopyroxenites (diogenites), and basalts/gabbros (eucrites) as well as regolith breccias containing clasts of both materials (howardites).

Gaffey (1997) observed sub-hemispheric color and spectral variations across the surface of Vesta, corresponding to regions of differing mineralogies. He was able to produce a generalized lithologic map of Vesta using these results. We can build upon such work by using the HED meteorites as a spectral groundtruthing tool for DAWN in order to produce more accurate geologic maps of Vesta's surface.

This study focuses on the eucrite member of the HED family (specifically the unbrecciated eucrites), and attempts to quantify the petrologic factors that have the greatest impact upon their spectra (grain size, mineral chemistry, and texture). Eucrites are primarily composed of pigeonite and plagioclase, with varying amounts of silica and opaque phases (usually ilmenite, chromite, and sulfides). However, despite their similarities, we are able to see great petrologic variability, from quench-textured, rapidly-cooled lithologies, to coarse, exsolved, slowly-cooled, cumulate types. Such variability is also reflected within their spectra and our results show that we are able to distinguish between different types of eucrites – potentially corresponding to different lithologies on the surface of Vesta - using their spectra alone.