Water Ice Detected In Mercury’s Craters

NASA’s Mercury-orbiting spacecraft Messenger has sent back solid evidence that permanently shadowed craters at Mercury’s north pole contain water ice and possibly icy complex organic molecules. As the closest planet to the sun, Mercury has generally been thought to be hellishly bone-dry. But 20 years ago, Earth-based telescopes discovered reflective, bright patches at Mercury’s north and south poles that suggested the presence of water ice. The results from Messenger, reported in three papers in Science, show both bright and dark patches located within craters at the north pole, where temperatures average below 100 K (DOI: 10.1126/science.1231106, 10.1126/science.1229953, and 10.1126/science.1229764). The locations of these patches match the patches spotted earlier. The bright spots are rich in hydrogen, according to measurements from the spacecraft’s neutron spectrometer, indicating the presence of water ice. The dark areas likely harbor a layer of volatile organic compounds, the researchers believe. They suggest the water and organics were deposited during comet or asteroid impacts.