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Mercury is the closest planet to the Sun, and yet it’s not actually the hottest planet; that title goes to Venus. Why is Venus hotter than Mercury?
It’s not that Mercury isn’t hot. It’s plenty hot. If you were standing on the equator of Mercury at noon, the temperature rises to 700 kelvin (427° C or 800° F). Even at the poles, the temperature is 380 kelvin. But the moment you enter night on Mercury, the temperatures drop down to just 100 kelvin. That’s the same as -173° C. The reason Mercury can get so hot in the day and then so cold at night is because it doesn’t have an atmosphere to trap the heat from the Sun.
And when it comes to an atmosphere, Venus has the thickest one in the Solar System. The atmosphere on Venus is so thick that you would experience 93 times the pressure you’d experience at sea level on Earth. You’d have to dive down a kilometer under the ocean to experience that kind of pressure on Earth. Furthermore, these atmosphere is almost entirely carbon dioxide, a greenhouse gas that’s great at trapping heat from the Sun.
The average temperature on Venus is 735 kelvin (461° C, or 863° F). Furthermore, it’s the same temperature everywhere across the planet, whether it’s day or night. The thick atmosphere on Venus traps the heat from the Sun, and the weather distributes this temperature around the entire planet. No matter where you went, you’d always experience that same 735 kelvin temperature.
And that’s why Venus is hotter than Mercury.