LIMITS TO ICE ON ASTEROIDS (24) THEMIS AND (65) CYBELE

We present optical spectra of (24) Themis and (65) Cybele, two large main-belt asteroids on which exposed water ice has recently been reported. No emission lines, expected from resonance fluorescence in gas sublimated from the ice, were detected. Derived limits to the production rates of water are less than or similar to 400 kg s(-1) (5 sigma) for each object, assuming a cometary H2O/CN ratio. We rule out models in which a large fraction of the surface is occupied by high-albedo ("fresh") water ice because the measured albedos of Themis and Cybele are low (similar to 0.05-0.07). We also rule out models in which a large fraction of the surface is occupied by low-albedo ("dirty") water ice because dirty ice would be warm and would sublimate strongly enough for gaseous products to have been detected. If ice exists on these bodies it must be relatively clean (albedo greater than or similar to 0.3) and confined to a fraction of the Earth-facing surface less than or similar to 10%. By analogy with impacted asteroid (596) Scheila, we propose an impact excavation scenario, in which 10 m scale projectiles have exposed buried ice. If the ice is even more reflective (albedo greater than or similar to 0.6), then the timescale for sublimation of an optically thick layer can rival the similar to 10(3) yr interval between impacts with bodies this size. In this sense, exposure by impact may be a quasi steady-state feature of ice-containing asteroids at 3 AU.