Images from the spacecraft's Mercury Dual Imaging System taken in 2011 and earlier this year show that radar-bright features at Mercury's north and south poles are within shadowed regions on Mercury's surface. MESSENGER's arrival at Mercury last year changed that. But researchers weren't sure if the radar-bright patches detected by Arecibo corresponded to shadowy places in the craters. Many of these patches corresponded to the location of large impact craters mapped by the Mariner 10 spacecraft in the 1970s. The idea received a boost in 1991, when the Arecibo radio telescope in Puerto Rico detected unusually radar-bright patches at Mercury's poles, spots that reflected radio waves in the way one would expect if there were water ice.
Scientists suggested decades ago that water ice might be trapped in those shadowed areas at Mercury's poles. But the tilt of Mercury's rotational axis is almost zero - less than one degree - so there are pockets at the planet's poles that never see sunlight. Given its proximity to the Sun, Mercury would seem to be an unlikely place to find ice. (The mapping of shadows is still incomplete near the pole.) Yellow shows the locations of bright polar deposits imaged by Earth-based radar. Red denotes areas that are in shadow in all images acquired by MESSENGER to date.
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