ICE ON THE MOON AND MERCURY - Advances in Geosciences - page 93

ICE ON THE MOON AND MERCURY
DANA H. CRIDER
Department of Physics, Catholic University of America
106 Driftwood Dr., Gibsonville, NC 27249, USA
[e-mail]
ROSEMARY M. KILLEN
Department of Astronomy, University of Maryland
College Park, MD 20740, USA
[e-mail]
RICHARD R. VONDRAK
Solar System Exploration Division, NASA Goddard Space Flight Center
Greenbelt, MD 20771, USA
[e-mail]
Observational data suggest that the ice deposits in permanently shaded regions
on the Moon are relatively impure, being highly mixed with the regolith. In con-
trast, 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.
1. Introduction
Both the Moon and Mercury have regions near their respective poles where
the low obliquity of the planet combined with topography provides perma-
nent shadow from direct sunlight. Thermal modeling 1 predicts that tem-
peratures are low enough that water ice would be stable to sublimation in
a permanently shadowed region (PSR) for billions of years. 2 The amount of
ice near the surface is important as both a historical tracer of processes and
conditions in the solar system and as a potential resource for solar system
exploration.
Observational evidence supports the existence of volatiles in PSRs at
both the Moon and Mercury, 3 , 4 however, the interpretation of the data as
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