HELL FROZEN OVER: THE POSSIBILITY OF POLAR ICES ON MERCURY AND THE MOON

The detection of anomalous, bright radar echoes from the poles of Mercury in 1991 re-awakened a dormant idea that water ice may be cold-trapped there and at the poles of Earth's moon. Even before the first planetary missions, it was realized that the small tilt of those planets' spin axes might result in regions within their polar topography that receive little or no direct illumination. Without heat transport by a thick atmosphere, the temperature of these regions would be determined only by scattered visible and thermal radiation from nearby illuminated areas. The results of a three-dimensional simulation of the energy balance within polar topography indicate that water ice can be thermally stable in such locations over the age of the solar system. Such results not only validate the hypothesis that Mercury's radar features are indicative of polar ice deposits, but imply that these deposits may contain a record of volatile transport (e.g., comets) in the inner solar system. The model helps explain the sizes and latitudinal extent of the radar features and predicts that the lowest-latitude ice deposits are insulated by a regolith lag. Very recent Arecibo radar data support this prediction. Model results for the lunar poles are consistent with recent hydrogen detections from the Lunar Prospector spacecraft, although no radar features have been detected. The absence of lunar radar features (which require pure slabs of ice) reveals clues about the volatiles' origin or history--perhaps the lunar ice has accumulated slowly and mixed with regolith, while on Mercury ices were catastrophically emplaced by a recent comet impact.