IMAGE: An infographic comparing the projected size of raindrops on different planets. Note that Titan and the current Mars are too cold for raindrops in liquid water. regard more
Credit: Image Credit: AGU
WASHINGTON – Raindrops on other planets and moons are close to the size of raindrops on Earth, despite having different chemical compositions and falling through many 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 scientists say could help them better understand the climates and precipitation cycles of other worlds.
Raindrops on earth are made of water, but other worlds in our solar system have deposits of unusual things. On Venus it rains sulfuric acid; on Jupiter it rains helium and porridge ammonia hailstones. On Mars, it snows carbon dioxide, or dry ice. On Saturn’s moon Titan it rains methane, or liquefied natural gas. And on Neptune, scientists suspect that it is raining pure carbon in the form of diamonds. It can even cause iron or quartz to rain on some planets if the conditions are right.
A new study looking at the physics of how liquid droplets behave as they fall out of clouds can only find droplets in clouds within a limited size – between about a tenth of a millimeter to a few millimeters in radius – the surface of rocky planets reached as rain. . It is a fairly narrow size, as raindrops increase in volume about a million times during their formation in a cloud.
The results also show the maximum size of liquid droplets that fall because rain is similar in different planetary conditions. Different types of liquid droplets can reach 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 here an infographic version comparing the size of raindrops on Earth, Mars, Jupiter, Saturn and Titan.
“There is a fairly small range of stable sizes that these different compound raindrops can have; they are all basically limited to 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 physics principles to model how liquid water droplets fall through planetary atmospheres. They wanted to determine the possible size ranges for droplets falling from a cloud to a planetary surface. Raindrops that are too large break up into smaller droplets, while too small raindrops evaporate before falling to the ground.
They first determined the possible sizes 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 strength of the planet’s gravity.
They found that raindrops with a radius smaller than about a tenth of a millimeter evaporate before they ever reach the surface, and raindrops larger than a few millimeters in radius break up into smaller droplets when they fall.
They looked at how raindrops would fall on much larger planets like Jupiter and Saturn with very different atmospheres. Compared to modern earth, ancient Mars and these larger planets, they found that raindrops move water through the sky in the same way, although what is ‘air’ is very different between the planets.
Even when different liquids form raindrops, these alien raindrops do not differ as much from known water droplets according to the researchers’ calculations. The largest methane raindrops on Titan, for example, would be about twice the size of water rain on Earth. Loftus is not sure why the maximum raindrop size is so uniform, but she suspects it may be due to how a droplet’s surface tension relates to its density.
The findings will help scientists better simulate conditions on other planets, as precipitation is an important component in a 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 with space telescopes, said Tristan Guillot, a planetary scientist at the Observatory de la Côte d’Azur in Nice, France. which was not related to the new study.
“Now with instruments like [the James Webb Space Telescope]”, which will hopefully be launched soon, we have the ability to detect very fine spectra of exoplanetary atmospheres, including those that are even cooler than those we can normally characterize, in which clouds and rain occur,” Guillot said. “This kind of tool will therefore be very useful and important in interpreting these spectra as they are developed.”
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Notes for journalists
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Question paper: “The Physics of Falling Raindrops in Diverse Planetary Atmospheres”
Authors:
- Kaitlyn Loftus, Harvard University, Cambridge, Massachusetts
- Robin D. Wordsworth, Harvard University, Cambridge, Massachusetts
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