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Summary
One of the more startling discoveries in planetary science was that the poles of Mercury feature deposits that are extremely bright at radar wavelengths (1), interpreted to be due to the presence of thick water ice. Because Mercury's rotation axis is almost normal to the plane of its orbit, the temperature of polar craters largely or completely shaded from the Sun should be very low. On the Moon, for example, where the rotation axis tilt is similarly small, the polar temperatures in permanently shadowed regions have been measured by infrared radiometry to be as low as 25 K (2). These topographic depressions might be expected to contain cold-trapped volatile material that might be introduced by comets, water-bearing asteroids, or other sources. On pages 292, 300, and 296 of this issue, Lawrence et al. (3), Paige et al. (4), and Neumann et al. (5) report on the latest results from the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission confirming the expectations that the atmosphere of Mercury is indeed a wet volatile one, as well as providing the odd surprise.
