When scientists bounced radio waves off Mercury in 1991, they received an unusual bright return from the North Pole. Scientists concluded that the radio waves bounced off ice, just under the surface of the North Pole.
How could such a hot planet harbour ice? Mercury's rotation is almost perpendicular to its orbital plane, So the North Pole always sees the Sun just above the horizon. The insides of its craters would never be exposed to the Sun, and scientists suspect that they would remain very cold.
The freezing temperatures would favor the trapping of water out- gassed from the planet, or ice brought to the planet from impacts with comets. These ice deposits might be covered with a layer of dust, but they would still show bright radar returns.