Confirming Water Ice on the Surface of Asteroid 24 Themis

We have confirmed the detection of a surprising, and so far unique, absorption feature on a main-belt asteroid, 24 Themis. Our 2 to 4 micron reflectance spectra were obtained over 7 hours (84% of Themis’ rotational period) on January 23, 2008 UT, from NASA's Infrared Telescope Facility. All our spectra confirm the shape and depth of the 3.1 micron absorption reported by Rivkin and Emery (2008) and show this absorption is present throughout the asteroid's rotation. The shape and wavelength of this absorption is interpreted by Rivkin and Emery (2008) as due to water ice, as opposed to hydrated minerals originally suggested by Lebofsky et al. (1990). Our spectra fit well the same water ice model used by Rivkin and Emery (2008), and are significantly different from spectra of hydrated silicates found in asteroids and meteorites. As Rivkin and Emery (2008) state, "If confirmed, a finding of water ice on Themis would be the first detection of ice on an asteroidal surface, and the first detection of water per se (as opposed to OH) on an asteroidal surface."

This likely presence of water ice opens up interesting possibilities that could transform current views of some asteroids. For example, since water ice is not stable on the surface of Themis at 3.2 AU over 4.5 Gy, what is its source? What does this imply about the interior of this asteroid and of other members of this family? Could impacts with this type of asteroid have been a significant source of Earth's water? The presence of water ice on 24 Themis would support arguments that the cometary activity in at least two small members of this family is driven by sublimation of water ice (Hsieh and Jewitt 2006) and that these "activated asteroids” originated in this family (Licandro et al. 2007).