Mercury has almost no atmosphere, so its surface temperature swings between extremes.

The longer version

Mercury lacks a real atmosphere, so there is no air to store heat or spread it around. Sunlit ground soaks up solar energy and becomes extremely hot, while the night side radiates that heat straight into space and plunges in temperature. Close proximity to the Sun adds to the intensity of incoming sunlight.

Its slow rotation makes the contrast even starker, because each location spends long stretches in daylight and in darkness. Polar craters that never see sunlight can stay cold enough to preserve water ice. The thin exosphere present is constantly replenished by the solar wind and impacts.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.