Mercury rotates very slowly, so a day on Mercury—from sunrise to sunrise—lasts 176 Earth days. This leads to huge temperature variations between the day side, which can be as hot as 427 °C (800 °F), and the night side, which can be as cold as minus 173 °C (−280 °F).

Mercury has a very thin atmosphere formed by atoms that are blasted off Mercury's surface by the solar wind. This atmosphere is constantly evaporating.

Radar signals bounced off Mercury by Earth's largest radio telescope suggest that—as on the Earth's moon—there may be ice in the craters at Mercury's poles.

Mercury has an iron core and is the only small rocky planet other than Earth with a significant magnetic field.