Three-Micron Survey of E- and M-class Asteroids: Final Results

A. S. Rivkin, L. A. Lebofsky, D. T. Britt (LPL/U. of Arizona), E. S. Howell (U. of Puerto Rico)

In 1988, Jones et al. discovered two M-class asteroids that had strong absorption features at 3 tex2html_wrap_inline11 m. A systematic survey of M asteroids was begun in 1991, with the results through 1993 summarized in Rivkin et al. (1995). These results confirmed the existence of ``wet M'' asteroids, and upset the interpretation of these objects as all being fragments of iron-nickel cores. Similarly, observations of hydrated E-class asteroids at 3  tex2html_wrap_inline11 m by Rivkin et al. (1995) call into question the association between this asteroid class and the igneous, anhydrous aubrite meteorites (Bell et al. 1989).

Further observations of M asteroids undertaken through 1996 give a total of 26 M-class asteroids observed, with 10 of these bodies showing a water of hydration absorption. These data show a size dependence with regards to hydration state of M-class asteroids. The only M-class asteroid surveyed larger than 200 km was 16 Psyche, which is anhydrous. In the 65-200 km diameter size range, 12 asteroids were observed, of which 8 are hydrated, and 3 are anhydrous. In the smallest size range (0-65 km), 13 asteroids were surveyed of which 2 were hydrated, 10 are anhydrous. The remaining two asteroids are of uncertain hydration state.

Although the E asteroids are much less common than M asteroids, and consequently only six of these elusive asteroids were observed, a similar trend is recognizable. Four of the six E-class asteroids are observed to have a 3- tex2html_wrap_inline11 m water-of-hydration feature, while two are anhydrous. The two anhydrous E asteroids are the two smallest asteroids in the group observed.

We will present spectra of these asteroids and discuss the implications of these data with regards to their composition and thermal history.