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Mercury orbits the Sun closer than any other planet in the Solar System, so you’d think that the temperature of Mercury is hot, and hotter. You’d be right. The side of Mercury facing the Sun can reach 700 Kelvin, or 426 degrees Celsius.
But Mercury isn’t the hottest planet in the Solar System; that’s Venus. Here’s why. Unlike Mercury, Venus is cloaked in a thick atmosphere that traps the Sun’s energy like a greenhouse. Additional energy pours in and is also trapped, and so temperatures on the surface of Venus reach 735 Kelvin, across the entire planet. No matter where you go, the temperature is always the same.
Back on Mercury, the side facing the Sun gets to 700 Kelvin, but the planet has no appreciable atmosphere. Without this atmosphere, it can’t trap in the heat. And so, the side facing away from the Sun – covered in shadow – dips down to 100 Kelvin, or -173 degrees Celsius. That makes Mercury colder than any of the inner planets, and even colder than the cloud tops of Jupiter.
There are thought to be craters at the north and south poles of Mercury where the sunlight never reaches, and these places might even be colder than the shadowed side of Mercury.
We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.