Why Is Mercury The Hottest And Coldest Planet?

Is Mercury actually the hottest planet?

No, Venus is. Venus has surface temperatures of about 900°F (475°C) everywhere on the planet, day and night, thanks to a thick carbon dioxide atmosphere that traps heat in a runaway greenhouse effect. Mercury's daytime peak reaches about 800°F, which is hotter than Venus in spots, but Mercury's average temperature is much lower because its night side gets so cold. So Mercury has the hottest moments, but Venus is hotter on average.

How cold does Mercury get?

At night, Mercury drops to about -290°F (-180°C). According to NASA, the night side can be more than 1,000°F colder than the day side, the largest day-night temperature difference of any planet. Deep craters near Mercury's poles never see sunlight at all, and those permanently shadowed regions are even colder. NASA's MESSENGER mission found water ice frozen inside some of them, on the same planet whose surface can melt lead.

Why is the swing so extreme?

Two reasons together create the swing. First, Mercury has almost no atmosphere; just a thin exosphere of atoms knocked off its surface by the solar wind. Atmospheres trap and redistribute heat, which is why Earth's day and night temperatures only vary by tens of degrees, not hundreds. Second, Mercury rotates slowly. One full rotation takes about 59 Earth days, so each side of the planet bakes or freezes for weeks at a time without the relief that faster rotation would bring.

Why does Mercury have no atmosphere?

Mercury's gravity is too weak and the Sun's heat is too strong. Light gases like hydrogen and helium that would form an atmosphere are blown away by the solar wind before they can accumulate. The Sun's intense radiation also strips atoms off Mercury's surface, creating the thin exosphere, but those atoms eventually escape into space. Without a meaningful atmosphere to act as a thermal buffer, the surface is fully exposed to either direct sunlight or open space.