Perhaps one of the most unexpected Mercurian features to be discovered in recent times was that of water—ice at the planet's poles. The discovery of water ice on Mercury was made in 1991 by bouncing powerful radar signals off the planet's surface. The discovery of water ice on Mercury was surprising, since it was believed that the high daytime temperatures caused by the proximity of the planet to the Sun would lead to the rapidly evaporation of any ices that might chance to form.
The polar regions of Mercury can be seen from Earth with powerful radars, because of the relatively large inclination (7°) that the planet's orbit presents to the ecliptic. These same polar regions, however, are never fully illuminated by the Sun, and it appears that water ice has managed to collect in the permanently-shadowed regions of many polar crater rims. It is not clear where the ice seen at Mercury's poles comes from, but it has been suggested that comet crashes may be one source.