A radar image of Mercury's north polar region acquired by the Arecibo Observatory in Puerto Rico is shown superposed on a mosaic of Mercury MESSENGER images of the same area in this NASA handout photo released Nov 29, 2012. Shown in red are areas of Mercury's north polar region that are in shadow in all images acquired by MESSENGER to date. Since their discovery in 1992, these polar deposits have been hypothesized to consist of water ice trapped in permanently shadowed areas near Mercury's north and south pole, but other explanations for the polar deposits have also been suggested. [Photo/Agencies]