Detection of Hydration on Nominally Anhydrous S-complex Main Belt Asteroids

We present the results of a survey of nominally anhydrous main belt S-complex asteroids. Thirty-three observations of 29 unique asteroids were obtained using the IRTF+SpeX instrument in prism and LXD short modes. We report for the first time that S-complex main belt asteroids have 3 μm features. The majority of the observations (27 of 33) have a detectable 3 μm feature that has at least 1% band depth or greater (within error), indicating the presence of hydration. Most of the asteroids have bands of 1%-2.5% depth, but a notable fraction (nine of the observations) have band depths of >5%. These band depths are comparable to those of low albedo asteroids in the middle and outer belt that have experienced aqueous alteration. We investigate the origin of the hydration, searching for correlations with orbital, physical, and circumstantial parameters. However, we do not find any strong or moderate correlations with 3 μm band depth, indicating that multiple factors may be at play, including exogenic sources, primordial water, and/or solar wind implantation. Additionally, we report the mineralogies of the asteroids, derived from the prism observations.