Why does Mercury have such extreme day-night temperature swings?

Mercury’s temperature swings are so extreme mainly because it has only a very thin exosphere instead of a substantial atmosphere. Without air, there is little greenhouse warming at night and little heat transport from the day side to the night side. Mercury also has a slow rotation period of about 59 Earth days, so any given spot is exposed to long periods of intense sunlight and then long periods of darkness. The planet’s closeness to the Sun makes the daytime heating especially severe, but the lack of insulation is what allows the nights to become so cold. NASA’s MESSENGER mission helped refine these measurements during its orbital study of Mercury.