Earth isn’t the only world where rain sometimes falls. Eons ago, Mars had a thicker, warmer atmosphere than today, with lakes, rivers, and maybe oceans of liquid water.  Images from orbiting spacecraft show that Martian geological features dating from that distant era were eroded by rainfall.

Sulfuric acid rain falls constantly from the clouds of Venus, but evaporates before it reaches the surface. Sometimes rain falls on Saturn’s moon Titan, whose frigid surface harbors lakes and rivers of liquid methane, and shows signs of erosion by methane rainfall. Liquid helium rain may fall from the clouds of Jupiter.  Rainfall, of various substances, in different kinds of atmospheres, and under different gravitational fields may be a widespread feature of planets and moons everywhere.

Understanding clouds and precipitation on other worlds is critical to understanding their climate, and the erosion features on their surfaces. But, there are many unknowns. Do the sizes of alien raindrops vary drastically depending on how strong gravity is, or the substance that makes up the raindrops, or the composition of the atmosphere?

In 2021 two American planetary scientists published a theoretical study of the physics of alien rain. From a set of complicated equations, they deduced simple principles describing the behavior of raindrops across a range of planetary conditions. They concluded that the sizes of raindrops everywhere are no more than ten times larger or smaller than on Earth. Raindrops smaller than that, regardless of what they are made of or what kind of atmosphere or gravity they fall in, will evaporate before they reach the surface. Larger ones would break apart.  In this way, at least, alien worlds may be more familiar than we thought.