PROGRESS ON GEOLOGIC MAPPING OF THE BROKEN BITS OF ASTEROID 4 VESTA

Asteroid 4 Vesta is a target of the upcoming DAWN spacecraft mission under NASA's Discovery program. Experience gained from the NEAR mission demonstrates that a firm understanding of an asteroid's geology prior to the mission permits more sophisticated questions to be addressed during the mission. Our recent work shows that much can be learned about Vesta by studying genetically related asteroids in the Vesta region of the main belt.

The depth, width, and spectral placement of the 1- and 2-mm mafic silicate absorption features in the ground-based reflectance spectrum of 4 Vesta (the archetype V-class asteroid) indicate that orthopyroxene is a major phase in its surface mineralogy. Hubble Space Telescope observations confirmed albedo differences across Vesta's surface, and produced shape and topographic information about Vesta. Dynamical studies previously revealed that a family of mainbelt asteroids is associated with 4 Vesta. Taxonomic surveys have found numerous, smaller asteroids with Vesta-like 1-mm spectral absorption features.

Until recently, work on the surface mineralogy of Vesta was the definitive analysis for one of these V-class asteroids since no complete near-infrared spectrum of another V-type existed. The case for a Vesta-HED meteorite connection has a firm mineralogical foundation. Previous spectral data have suggested a link to additional V-class asteroids, but until now that supposed association had not been mineralogically tested.

Analysis of our near-infrared reflectance spectrum for asteroid 1929 Kollaa, which includes the critical 1- and 2-mm spectral absorption features, allowed us not only to determine that this mainbelt asteroid is a daughter of 4 Vesta, but also to constrain the location of its formation within Vesta and establish its connection to the cumulate eucrites. This is the first mineralogical link to be made between the HED meteorites and an asteroid other than Vesta, and between Vesta and another asteroid.

We recently obtained near-IR reflectance spectra for several additional Vesta region asteroids, including 3494 Purple Mountain. Reduction and analysis of these data are continuing and the results will allow us to apply a mineralogic (genetic) test to their possible connection with 4 Vesta and the HED meteorites.