Physics Question #365

Citre, a 13 year old male from the Internet asks on March 26, 1998,

What kind of weather is on Mercury and Saturn? Does it rain or snow on these planets?

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The answer

Donald J. Barry answered on March 26, 1998

Mercury is a planet with no air, so weather as we know it would simply not exist. No wind, no clouds, no rain, no sleet or snow. But because Mercury is the closest to the sun, it is HOT! On the daylight side, the temperature of the surface would melt lead. During the Mercurian night, which lasts 56 days, temperatures rapidly cool off. Before dawn, the surface temperature would be quite frigid: much colder than dry ice.

Saturn has the opposite problem: too much air. It's essentially all gas, though conditions deep down no longer resemble gas as we know it, because the conditions are too extreme (too hot, too much pressure). But at the upper levels of the atmosphere, where pressures are much like what we breathe, the temperatures would not be too much colder, though the gases wouldn't let us live long - there isn't any oxygen! Saturn is much too dry to have any liquid water in its atmosphere, but snow of ammonia ice exists high in its atmosphere, at temperatures much colder than even in our arctic regions. There are very high winds, in addition, which move several hundreds of miles per hour in places. But since there's no surface to blow against, if you were riding in a balloon in the atmosphere, you'd find all the gas moving with you, and not realize you were going quite so fast. Occasionally storms brew on Saturn. Famously, it seems that about every thirty years a big storm brews up which pumps lots of ammonia high into Saturn's atmosphere, creating lots of extra white clouds around the planet. We have very little understanding of why this happens or the mechanisms that power the storm. We're still figuring out weather on Earth!

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