Q: Does it rain only on Earth?
A: It does precipitate on other planets and moons in our solar system.
On Earth, when particles fall from clouds and reach the surface as precipitation, they do so primarily as rain, snow, freezing rain or sleet.
On the average, a raindrop is between 0.1 and 5 millimeters. Raindrops on Earth are made of water. Sometimes they can pick up pollen or dust suspended in the atmosphere as the rain falls toward the ground.
The rain on other planets has very different chemical compositions. On Venus, it rains sulfuric acid. On Mars it snows dry ice, which is carbon dioxide in a solid state. Saturn’s moon Titan rains methane, and on Jupiter, it rains helium and mushy ammonia hailstones. On Neptune, scientists suspect it rains pure carbon in the form of diamonds.
A recent science study simulated the maximum size of liquid droplets that would fall as “rain” under the different planetary conditions. It is a fairly narrow size range, given the large variation in the gravity of the planets and moons involved. Raindrops that are too big break up into smaller ones, while raindrops that are too small evaporate before they hit the ground.
On Earth, the maximum raindrop size is about 7/16 of an inch, a similar size as on Saturn. The maximum raindrop size on Titan is about 1 and 3/16 inches, and on Jupiter the maximum size is about 9/32 of an inch.
While cartoonists typically draw raindrops in a teardrop or pear shape, raindrops are not shaped in those forms. They are drawn as teardrops to give the image of falling through the atmosphere, which they do.
No matter the planet or moon, as raindrops fall they are flattened and shaped like a hamburger bun by the drag forces of the air they are falling through.
"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences.