What is the weather on Mercury?

Since there is no real atmosphere, just a low, thin collection of ionized gases, there is no real "weather". The surface temperature, however is extremely hot when you're on the sunny side, and extremely cold when you're on the shady side.

Mercury has two characteristics that make its surface temperature interesting. One is a highly elliptical orbit that takes the planet from 46 million km at perihelion to 70 million km at aphelion. That means the solar intensity varies from 4 times the intensity at Earth to over 10 times. The other aspect is that it rotates only about one and a half times per Mercury year. This makes a solar day on Mercury (sunrise to sunrise) apparently 156 Earth days long. A point that gets hammered by the maximum solar intensity will be in the frigid dark 88 Earth days later. The day side temperature can reach 700 Kelvins (about 430°C, or about 800° F.) Curiously, Venus is hotter at the surface than Mercury, even though it is farther from the Sun. The dense atmosphere of Venus holds in what little solar radiation makes it through the clouds, while Mercury efficiently radiates back to space. On the dark side, Mercury can be as low as 90 K (-183°C).

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