ICE ON THE MOON AND MERCURY

Observational data suggest that the ice deposits in permanently shaded regions on the Moon are relatively impure, being highly mixed with the regolith. In contrast, the deposits on Mercury appear to be pure and covered by dry regolith. The differences in the deposits on the two bodies may be the result of differing weathering rates, their unique impact histories, or selection effects from the observation methods. We discuss the available data and results from modeling. We find that the most suitable explanation for the Mercury observations is that a comet deposited an ice layer at least 50 cm thick in the northern cold traps less than 50 Myr ago. The lunar model suggests that if a relatively pure ice layer exists, it is old and buried or was originally thin and is now impure.