Alien raindrops are surprisingly similar to rain on Earth

WASHINGTON – Raindrops on other planets and moons are nearly the size of raindrops on Earth, despite having different chemical compositions and falling through vastly different atmospheres, a new study finds. The results suggest that raindrops falling from clouds are surprisingly similar across a wide variety of planetary conditions, which could help scientists better understand other worlds’ climates and precipitation cycles, the researchers said.

Raindrops on Earth are made of water, but in other worlds in our solar system, precipitation is made of more unusual material. Sulfuric acid rains on Venus; Helium and mushy ammonia hailstones rain on Jupiter. Carbon dioxide or dry ice is snowing on Mars. On Saturn’s moon Titan, methane, or liquefied natural gas, rains. And on Neptune, scientists suspect that pure carbon is raining in the form of diamonds. It could even rain iron or quartz on some planets if conditions were right.

A new study looking at the physics of how liquid droplets behave as they fall from clouds finds that only droplets in clouds with a limited range – between about one-tenth of a millimeter to several millimeters in radius – can reach the surface of rocky planets as rain. This is quite a narrow range, as raindrops increase in volume about a million times as they form in a cloud.

The results also show that the maximum size of liquid droplets that fall as rain is comparable under different planetary conditions. Different types of liquid droplets would be up to about half to six times the size of water rain on Earth, depending on the strength of the planet’s gravity (the stronger the gravity, 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 number of stable sizes that these raindrops of different compositions can have; they are all fundamentally limited to roughly the same maximum size,” said Kaitlyn Loftus, a planetary scientist at Harvard University and lead author of the new study in AGUs. 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 on other worlds

In the new study, Loftus and colleague Robin Wordsworth used mathematical and physical principles to model how liquid water droplets fall through planetary atmospheres. They wanted to determine the possible size ranges for droplets that fall from a cloud to a planetary surface. Raindrops that are too large disintegrate into smaller ones, while raindrops that are too small evaporate before they hit the ground.

They first determined the possible size ranges for water raindrops on rocky planets such as Earth and Mars, given atmospheric conditions such as temperature, air pressure, relative humidity, distance from the cloud to the ground, and the force of the planet’s gravity.

They found that raindrops with a radius smaller than about one-tenth of a millimeter evaporate before ever reaching the surface, and raindrops with a radius larger than a few millimeters break up into smaller droplets when they fall.

Then they looked at how water raindrops would fall on much larger planets such as Jupiter and Saturn that have vastly different atmospheres. When they compared modern Earth, ancient Mars, and these larger planets, they found that raindrops move water through the sky in the same way, even though what ‘air’ is differs greatly between the planets.

Even if raindrops are made up of different liquids, these alien raindrops aren’t that different from known water raindrops, the researchers’ calculations said. For example, the largest methane raindrops on Titan would be about twice the size of water rain on Earth. Loftus isn’t sure why the maximum raindrop size is so uniform, but she suspects it may be due to the relationship between a drop’s surface tension and its density.

The findings will help scientists better simulate conditions on other planets, as precipitation is a key component in a planet’s climate and nutrient cycling, 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.

“Now with tools such as [the James Webb Space Telescope], which will hopefully be launched soon, we will be able to detect very fine spectra of exoplanetary atmospheres, including those that are quite cooler than the ones we can usually characterize, which will include clouds and rain, “said Guillot.” So these kinds of tools as they are being developed will be very useful and important for interpreting those spectra. ”

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Notes for journalists

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Paper title: “The Physics of Raindrops Falling in Different Planetary Atmospheres”

Authors:

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