DPS 35th Meeting, 1-6 September 2003
Session 22. Asteroid Physical Studies I
Oral, Chairs: Clark and R.P. Binzel, Thursday, September 4, 2003, 10:30am-12:00noon, DeAnza III

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[22.07] Mineralogical Constraints on Silicate-Rich Asteroids from SpeX

J. M. Sunshine (Science Applications International Corporation), S. J. Bus (U. Hawaii, Institute for Astronomy), T. H. Burbine (NASA GSFC), T. J. McCoy (Natl. Museum of Natural History, Smithsonian Institution), R. P. Binzel (MIT)

We have recently begun a new survey of silicate-rich asteroids using SpeX, a low- to medium-resolution infrared spectrograph, at the Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. In its low-resolution mode (R ~ 100), SpeX can produce spectra of faint asteroids from 0.8 to 2.5 Ám with S/N comparable to data typically collected with visible wavelength CCDs.

This rich dataset of near-Earth and main-belt objects contain subtleties in their spectral signatures that are well suited to analysis with the Modified Gaussian absorption band model. Among our results are clear unambiguous evidence for the presence of both high- and low-calcium pyroxene (HCP and LCP) as well as olivine and plagioclase. The quality of these data allow us to use the proportion of HCP relative to LCP, to constrain the petrologic history of these bodies. Very primitive bodies (e.g. primitive achondrites) have < 10% HCP, while moderately evolved bodies (e.g. ordinary chondrites) have 15-20% HCP, and highly evolved bodies (e.g. basaltic achondrites) have >25% of their pyroxene as HCP. Our ability to infer the presence (or absence) of HCP with SpeX data is a new tool for examining and mapping igneous processes on asteroids.

For example, our current dataset contains several asteroids, including members of the Merxia and Agnia families, with spectra that show little to no evidence for olivine, but instead are dominated by pyroxene absorptions and include significant (>40%) proportions of HCP. This HCP content implies a history of partial melting and silicate differentiation. As such we are actively examining smaller members of the Agnia and Merxia families to further constrain the petrologic history of these bodies.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.