Reports

prevent direct sunlight from reaching substantial portions of its polar regions. Lacking major convective or conductive sources of heat, the permanently shadowed, near-surface regolith experiences temperatures similar to those of the icy Galilean satellites (2). It has long been be-lieved on theoretical grounds that such conditions are favorable to the accumulation of volatiles (3, 4). Even with Mercury’s close proximity to the Sun, extremes of daytime temperature are not expected to allow sur-face heat to penetrate regolith to substantial depth, allowing near-surface water ice, if present, to remain stable against sublimation for billions of years (2). Such hypotheses were renewed when Earth-based radar obser-vations of Mercury, at wavelengths from 3.6 to 70 cm (5–9), revealed regions of high backscatter and depolarization at both poles. Radar ob-servations suggested that deposits of nearly pure water ice up to several meters thick lie at or near the surface. Analysis of altimetry and rough-ness measurements from the Mercury Laser Altimeter (MLA) (10, 11