DPS 34th Meeting, October 2002
Session 4. Asteroids II
Oral, Chair(s): A.S. Rivkin and H. Scholl, Monday, October 7, 2002, 11:30am-1:00pm, Ballroom

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[4.07] The Cornell Mid-Infrared Asteroid Spectroscopy (MIDAS) Survey: Results from 2001

L. F. Lim (Cornell University and NASA/GSFC), J. F. Bell, T. H. McConnochie (Cornell University), B. E. Clark (Ithaca College), T. L. Hayward (Gemini Observatory)

The minerals thought to be major constituents of asteroid surfaces exhibit diagnostic emissivity features in the thermal infrared. Here we report new results from the Cornell Mid-IR Asteroid Spectroscopy (MIDAS) survey, a long-term program of ground-based observations designed to characterize the 8-14 micron spectral properties of a statistically significant sample of asteroids from a wide variety of visible to near-IR spectral classes. MIDAS is conducted at Palomar Observatory using the Spectrocam-10 (SC-10) spectrograph on the 200-inch Hale telescope.

We have measured spectra of varying quality for sixteen asteroids to date: 2 Pallas, 3 Juno, 4 Vesta, 9 Metis, 10 Hygiea, 11 Parthenope, 19 Fortuna, 22 Kalliope, 24 Themis, 40 Harmonia, 54 Alexandra, 89 Julia, 95 Arethusa, 145 Adeona, 498 Tokio, and 704 Interamnia. We derive temperature estimates from our data that are consistent with the predictions of the standard thermal model, and we use the derived temperatures to generate estimated emissivity spectra for our objects. In no case yet do we find emissivity features with spectral contrast greater than 5%, although a few of our spectra suggest emissivity variations at the 2-3% level.

Published spectra of the small number of asteroids studied by ISO (4 of which are also included in our survey), which appear to exhibit much stronger emissivity features, are difficult to reconcile with our measurements. Laboratory work on mineral and meteorite samples has shown that the contrast of mid-IR spectral features is greatly reduced at fine grain sizes. Moreover, the NEAR mission found that 433 Eros is covered by a relatively thick fine-grained regolith. If small bodies in general possess such regoliths, their mid-IR spectral features may be quite subtle, and may explain the apparent lack of strong emissivity variations in the MIDAS results so far.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.