Alien raindrops surprisingly like rain on Earth

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

Raindrops on Earth are made of water, but other worlds in our solar system have precipitation made of more unusual things. On Venus, it rains sulfuric acid; on Jupiter, hail with helium and ammonia rains. Snow or dry ice is snowing on Mars. On Saturn’s moon, Titan, it rains methane or liquid natural gas. And on Neptune, scientists suspect that it is raining pure carbon in the form of diamonds. It could even rain iron or quartz on some planets if conditions were right.

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

The results also show the maximum size of falling liquid droplets, because rain is similar in different planetary conditions. Different types of liquid droplets would exceed up to half to six times the size of water rain on Earth, depending on the force of the planet’s gravitational attraction (the stronger the gravitational pull, the smaller the raindrop). Find an infographic here that compares the size of raindrops on Earth, Mars, Jupiter, Saturn and Titan.

“There is a fairly small range of stable sizes that these different compositions of raindrops can have; all are fundamentally limited to about the same maximum size, ”said Kaitlyn Loftus, a planetary scientist at Harvard University and lead author of the new AGU study. Journal of Geophysical Research: Planets, which publishes research on the formation and evolution of planets, moons and objects in our solar system and beyond.

Rain on other worlds

In the new study, Loftus and his colleague Robin Wordsworth used the principles of mathematics and physics to model how liquid water droplets fall through planetary atmospheres. They wanted to determine the possible size ranges for drops falling from a cloud on a planetary surface. Too much raindrops split into smaller ones, while too little raindrops evaporate before they reach the ground.

First, they determined the possible size ranges for raindrops on rocky planets such as Earth and Mars, given atmospheric conditions such as temperature, air pressure, relative humidity, distance from cloud to ground, and the force of attraction. gravitational pull of the planet.

They found that raindrops with a radius of less than about a tenth of a millimeter evaporate before they ever reach the surface, and raindrops larger than a few millimeters in radius break into smaller droplets as they fall.

Then they looked at how raindrops would fall on much larger planets like Jupiter and Saturn, which have very different atmospheres. Comparing modern Earth, ancient Mars, and these larger planets, they found that raindrops move water through the air in a similar way, even though what constitutes “air” varies greatly between planets.

Even when different liquids make up the raindrops, these foreign droplets are not as different from the familiar ones, according to the researchers’ calculations. For example, the largest drops of methane rain on Titan would be about twice as large as the rain of water on Earth. Loftus is not sure why the maximum size of raindrops is so uniform, but suspects that it may be due to the relationship of the surface tension of a raindrop to its density.

The findings will help scientists better simulate conditions on other planets, because precipitation is a key component in the planet’s climate and nutrient cycles, Loftus said. Modeling what precipitation might look like in a distant world could also help researchers interpret observations of exoplanetary atmospheres made by space telescopes, said Tristan Guillot, a planetary scientist at the Observatoire de la Côte d’Azur in Nice, France. , which was not connected to the new study.

“It simply came to our notice then [the James Webb Space Telescope], which we hope will be launched soon, we will have the ability to detect very fine spectra of exoplanetary atmospheres, including those that are rather colder than those we are used to characterizing, in which clouds will appear and rains, “Guillot said.” So these types of instruments as they are developed will be very useful and important in interpreting these spectra. ”

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Notes for journalists

This research study will be available free of charge 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 version from AGU.

Paper title: “Physics of raindrops in various planetary atmospheres”

Authors:

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