Raindrops Are Surprisingly Similar on Other Planets

Whether they are made of water, methane or liquid iron, raindrops’ size and shape are limited by the same equations

A photograph of Jupiter's swirling clouds and the curve of the planet's horizon
NASA’s Juno spacecraft captured this image of swirling clouds in Jupiter's northern latitudes on November 3, 2019 NASA/JPL-Caltech/SwRI/MSSS/Björn Jónsson, © CC NC SA

When it rains, it pours—but at least there’s a theoretical limit on how big each raindrop can be, even on other planets.

New research published last month in the journal JGR Planets presents calculations of the sizes and shapes of raindrops on planetary bodies beyond Earth, and finds that the constraints on alien rain are pretty similar to those here at home. The researchers found worlds with a stronger gravitational pull had smaller maximum-size raindrops than those with weaker gravity. Air density, on the other hand, doesn’t play a big role in shaping the droplets.

“They are proposing something that can be applied to any planet,” says Tristan Guillot, an astronomer at Observatory of the Côte d’Azur who was not involved in the study, to Science News’ Lisa Grossman. “That’s really cool, because this is something that’s needed, really, to understand what’s going on” in other worlds’ atmospheres.

Some of the extraterrestrial destinations analyzed in the paper have rain that’s made of remarkably different stuff than Earth’s water droplets. On Saturn’s moon Titan, for example, liquid methane falls from the sky, and Jupiter’s forecast features a wintery mix of ammonia “mushballs.”

Beyond our solar system, the exoplanet WASP-76b has storms with iron rain. Scientists studying the exoplanet’s weird weather have found that the days on WASP-76b are so hot, they vaporize iron. When winds carry the fumes to the planet’s night side, the iron condenses into iron-droplet clouds and rain, Ashley Strickland writes for CNN.

In the new study, Harvard planetary scientists Kaitlyn Loftus and Robin Wordsworth wanted to understand the characteristics of the droplets themselves.

“The lifecycle of clouds is really important when we think about planet habitability,” says Loftus in a statement. “But clouds and precipitation are really complicated and too complex to model completely. We’re looking for simpler ways to understand how clouds evolve, and a first step is whether cloud droplets evaporate in the atmosphere or make it to the surface as rain.”

If a droplet is too small, they found, it will evaporate before it hits the ground. And if a droplet is too large, it will break apart into smaller droplets. Worlds with stronger gravity have smaller maximum-size droplets. On Jupiter, droplets can be about one quarter-inch wide at most. On Earth, the largest raindrops are about 0.4 inches across, which is just larger than an Advil tablet.

On Titan, which has the weakest gravity of the worlds analyzed in the study, the largest methane-drops can be over an inch wide.

The calculations apply well-known physical equations to raindrop properties like their common half-circle shape, regardless of what they are made of, and the rate of evaporation, which depends on the drop’s surface area. They also took into account the strength of gravity, the atmosphere’s temperature, pressure and humidity, and the distance between the world’s clouds and the ground, per another statement.

“This is basically fluid mechanics and thermodynamics, which we understand very well,” says Loftus to Science News.

The results could help other scientists understand observations of other worlds made with space telescopes or help them create simulations of other climates and nutrient cycles. And in the future, Loftus hopes to extend the precipitation-predicting research to include phenomena like snowflakes and hail.

But solid precipitation is more complicated to describe mathematically. As Loftus tells Science News, “That adage that every snowflake is unique is true.”