[Previous] | [Session 4] | [Next]
E. S. Howell (NAIC), A. S. Rivkin (U. Arizona), A. Soderberg (Bates Coll.), F. Vilas (JSC), A. L. Cochran (U. Texas)
Understanding aqueous alteration on asteroids is important, both for determination of their thermal history, and for locating water sources in space. The strong water absorption band at 3~\mum is the only unambiguous indicator of hydrated minerals on asteroids which is detectable on low albedo surfaces (Rivkin et al., 1995, Jones et al. 1990). Vilas (1994) noted a correlation for many asteroids between weak bands due to Fe2+ and Fe3+ transitions at 0.65--0.75~\mum, and the strong water absorption band at 3~\mum. Inconsistencies between the visible band and 3-\mum band observations for some objects may be due to observations at different rotational phases, implying that the surface distribution of hydrated minerals is spotty. We initiated a program to observe asteroids at known rotational phases to determine the relationship between these hydration bands. We are coordinating observations at 3~\mum from the NASA Infrared Telescope Facility, with spectroscopy at 0.5--0.9~\mum using the 2.1-m at McDonald Observatory. Preliminary results of coordinated observations of 10~Hygiea at the same rotation phase show a 12% band depth at 3~\mum, but no 0.7-\mum band deeper than 2%. Asteroid 10~Hygiea has been observed to have a variable 3-\mum band depth in the past, and we observe spectral variability with rotation in the 0.7-\mum region as well. Observations of 511~Davida at similar aspects show a 3-\mum band over at least one third of the rotation period, but no 0.7-\mum band at any rotation phase. Additional data are being analyzed, but it is already clear that the relationship of these bands is complex. One cannot use the visible band as a reliable proxy for hydration. However, taken together, these bands may provide additional constraints on the composition and may clarify the aqueous alteration process.
We gratefully acknowledge partial support for this work by NAG5-8070.