Hydrogen concentrations on C-class asteroids derived from remote sensing

We present spectroscopic observations of 16 asteroids from 1.9-3.6 μm collected from the United Kingdom Infrared Telescope (UKIRT) from 1996-2000. Of these 16 asteroids, 11 show some evidence of a 3 μm hydrated mineral absorption feature greater than 2σ at 2.9 μm. Using relations first recognized for carbonaceous chondrite powders by Miyamoto and Zolensky (1994) and Sato et al. (1997), we have determined the hydrogen to silicon ratio for these asteroids and calculated their equivalent water contents, assuming all the hydrogen was in water. The asteroids split into 2 groups, roughly defined as equivalent water contents greater than ∼7% (8 asteroids, all with 3 μm band depths greater than ∼20%) and less than ∼3% for the remaining 8 asteroids. This latter group includes some asteroids for which a weak but statistically significant 3 μm band of non-zero depth exists. The G-class asteroids in the survey have higher water contents, consistent with CM chondrites. This strengthens the connection between CM chondrites and G asteroids that was proposed by Burbine (1998 . We find that the 0.7 μm and 3 μm band depths are correlated for the population of target objects.