Could water exist on Mercury?
Recent data show that water ice may exist in the bottoms of craters at Mercury's poles. Although Mercury is the closest planet to the Sun, and can be extremely hot over most of its surface, ice may exist at the bottoms of some polar craters because the crater floors are permanently shadowed by the crater rims.
In 1991, scientists bounced radio waves off Mercury, and found an unusual bright return from the north pole. The apparent brightening at the north pole could be explained by ice on, or just under the surface.
But, is it possible for Mercury to have ice? Yes. Because Mercury’s rotation is almost perpendicular to its orbital plain, the north pole always sees the Sun just above the horizon.
image source: wikimedia commons
The insides of craters would never be exposed to the Sun, and would remain very cold. This freezing temperature could trap water out-gassed from the planet, or ice brought to the planet from impacts with comets.
These ice might be covered with a layer of dust, and would show bright radar returns.