"Because the crater is deep, and the Sun never gets very high off the horizon at the pole, there are parts of the crater floor that are permanently in shadow; t

"Because the crater is deep, and the Sun never gets very high off the horizon at the pole, there are parts of the crater floor that are permanently in shadow; they literally are never illuminated by the Sun. Those spots can be very cold; well below the freezing point of water" from this article http://www.slate.com/blogs/bad_astronomy/2012/11/29/_mercury_has_ice_buried_beneath_its_surface_in_craters_at_its_north_pole.html http://www.slate.com/blogs/bad_astronomy/2012/11/29/_mercury...