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Extraterrestrial raindrops surprisingly like rain on Earth


IMAGE: Infographics comparing the predicted size of raindrops on different planets. Please note that Titan and today’s Mars are too cold for liquid water raindrops. view More ▼

Credit: Image credit: AGU

WASHINGTON – Raindrops on other planets and moons are close to the size of raindrops on Earth, although they have different chemical compositions and fall through significantly different atmospheres, a new study finds. The results suggest that raindrops falling from clouds are surprisingly similar in a wide range of planetary conditions, which could help scientists better understand the climate and precipitation cycle in other worlds, according to researchers.


7;s raindrops are made of water, but in other worlds in our solar system, rainfall is made by more unusual things. Venus has sulfuric acid; on Jupiter it rains helium and mushy hailstones. It’s snowing on Mars, carbon dioxide or dry ice. On the moon of Saturn Titan rains methane or liquefied natural gas. And on Neptune, scientists suspect that it rains pure carbon in the form of diamonds. It may even rain iron or quartz on some planets if the conditions were right.

A new study examining the physics of the behavior of liquid droplets as they fall from clouds finds only droplets in clouds in a limited range of sizes – between about a tenth of a millimeter to a few millimeters in radius – can reach the surface of rocky planets like rain. This is a fairly narrow range of sizes, given that raindrops increase about a million times in volume during their formation in the cloud.

The results also show the maximum size of liquid droplets that fall, as rain is similar under different planetary conditions. The different types of liquid droplets would reach a maximum of about half to six times the size of the Earth’s water rain, depending on the strength of the planet’s gravitational pull (the stronger the gravitational pull, the smaller the raindrop). Find here an infographic comparing the size of raindrops on Earth, Mars, Jupiter, Saturn and Titan.

“There’s quite a small range of stable sizes that these different raindrop compositions can have; they’re all fundamentally limited to roughly the same maximum size,” said Caitlin Loftus, a planetary scientist at Harvard University and lead author of the new study at AGU’s. Journal of Geophysical Research: Planets, which publishes research on the formation and evolution of the planets, moons and objects of our solar system and beyond.

Rain in other worlds

In the new study, Loftus and his colleague Robin Wordsworth used the principles of mathematics and physics to model how droplets of liquid water fall through the planets’ atmospheres. They wanted to determine the possible limits of the size of the droplets falling from a cloud on a planetary surface. Excessive raindrops break down into smaller ones, while too small raindrops evaporate before hitting the ground.

They first determined the possible limits of the size of water raindrops on rocky planets such as Earth and Mars, taking into account atmospheric conditions such as temperature, air pressure, relative humidity, distance from the cloud to the earth and the gravitational pull of the planet.

They found that raindrops with a radius of less than a tenth of a millimeter evaporated before reaching the surface at all, and raindrops larger than a few millimeters in radius disintegrated into smaller droplets when they fell. .

They then looked at how water raindrops would fall on much larger planets such as Jupiter and Saturn, which have significantly different atmospheres. Comparing modern Earth, ancient Mars, and these larger planets, they found that raindrops move water in the air in a similar way, although what constitutes “air” varies greatly between planets.

Even when different liquids make up raindrops, these extraterrestrial raindrops are not as different from known water raindrops, according to researchers’ calculations. For example, Titan’s largest methane raindrops would be about twice the size of Earth’s water rains. Loftus is not sure why the maximum size of a raindrop is so uniform, but she suspects that this may be due to how the surface tension of the droplet is related to its density.

The findings will help scientists better simulate the conditions of other planets, as precipitation is a key component in the planet’s climate and food cycles, Loftus said. Modeling what precipitation might look like in the distant world could also help researchers interpret observations of exoplanetary atmospheres made by space telescopes, said Tristan Guillaume, a planetary scientist at the Cote d’Azur Observatory in Nice, France, who is not involved. the new study.

“Now with tools like [the James Webb Space Telescope]”We hope to be released soon, we will have the ability to detect really fine spectra of exoplanetary atmospheres, including those that are much cooler than those we can normally characterize, in which clouds and rain will appear,” he said. Guillaume. “So these types of instruments, as developed, will be very useful and important for interpreting these spectra.”


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Notes for journalists

This research study will be freely available for 30 days. Download a PDF copy of the paper here. Neither the newspaper nor this press release is under embargo.

Download the multimedia that accompanies this edition from the AGU editorial office.

Title of the report: “The physics of falling raindrops in a diverse planetary atmosphere”


  • Caitlin Loftus, Harvard University, Cambridge, Massachusetts
  • Robin D. Wordsworth, Harvard University, Cambridge, Massachusetts

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