DPS Meeting, Madison, October 1998
Session 5. Asteroid Observations I
Contributed Oral Parallel Session, Monday, October 12, 1998, 9:30-10:20am, Madison Ballroom D

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[5.03] Refining the Composition of the Vesta Family

F. Vilas (NASA/JSC), A. L. Cochran (U. Texas)

Asteroid 4 Vesta, the putative parent body for the basaltic achondrites, shows spectral evidence of pyroxenes in the surface material and albedo differences across its surface, suggesting different compositional units (c.f., Gaffey, Icarus v. 127, 130, 1997). HST images show topographic features including a 460-km diameter crater near the S. Pole (Thomas et al., Science 277, 121, 1997). We have acquired rotationally-resolved UV/blue (R=600) spectra of Vesta in order to refine the identification of the pyroxene components in it's surface material, and search for spectral variations tied to surface features by observing an Fe2+ absorption feature near 505 nm. The spectral position and shape of this feature is highly diagnostic of the amount and form of calcium in pyroxene. We find that the feature is consistently centered at 506.54 nm, suggesting an augite (high-Ca) composition. Variations in the equivalent width of the feature correlate with topographically high terrain on Vesta's surface. This could be caused by the presence of another material (possibly olivine), particle size variations, or both. We have also acquired UV/blue spectra of 13 small asteroids dynamically and spectrally linked to Vesta, in order to test and explore the proposed genetic link between these asteroids and Vesta by identifying and characterizing the 505-nm absorption feature. Results will be presented.

This work was funded by NASA Grant NAG5 4208 (ALC) and the NASA Planetary Astronomy Program (FV).

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