Mars is a cold, arid world with a thin atmosphere, but that wasn’t always so. In its youth Mars was wet with rivers and oceans. Presumably that also means young Mars had rainfall. But how much did it rain on Mars? New research proves not only did it rain on Mars, the rains were strong enough to shape the planet’s surface.
To study Martian rain, two geologists used the same rainfall models used for Earth. These are well studied, and are proven to work well. But there are differences between Earth and early Mars. One big difference is that gravity on Mars is about a third that on Earth, meaning that water droplets would fall more slowly and strike the surface with less energy. Then there is the fact that the atmosphere of Mars has changed significantly. Young Mars had an atmosphere four times thicker than modern Earth.
Because of these factors, there wasn’t much rain on Mars despite plenty of liquid water. Instead water vapor would tend to coalesce into small droplets to form a thick fog. This fog could make the surface of mars wet, but wouldn’t alter the terrain much. As the atmosphere of Mars thinned to a pressure similar to Earth’s, larger water droplets could form. Given the low gravity, could merge into quite large rain drops. On Earth large raindrops tend to break apart as they fall faster, which limits their size to about 6 millimeters in diameter. Falling at a slower speed, Martian water droplets could grow to about 7.5 millimeters. Torrential rains with large rain droplets created surface runoff that cut paths on the surface of Mars. These “gully washers” can be seen today, such as in the image above.
Paper: Robert A. Craddock and Ralph D. Lorenz. The changing nature of rainfall during the early history of Mars. Icarus, volume 293, (September 2017).