Scientific research finds alien raindrops are eerily similar to raindrops on Earth

Raindrops on other planets and moons are close in size to those on Earth, even though they have different chemical compositions and land in vastly different atmospheres, a new study finds. These results show that raindrops falling from clouds are strikingly similar across a wide range of planetary conditions, which could help scientists better understand climate and precipitation cycles on other worlds, the researchers said.

Raindrops on Earth are made of water, but precipitation on other worlds in our solar system is made of something more unusual. On Venus, sulfuric acid falls; on Jupiter, helium and viscous ammonia hail. On Mars, it's carbon dioxide, or dry ice. On Saturn's moon Titan, below is methane, or liquefied natural gas. And on Neptune, scientists suspect it's beneath pure carbon in the form of diamonds. It could even drop iron or quartz on some planets if the conditions are right.

A new study examines the physics of droplets as they fall from clouds and finds that only droplets in clouds, in a limited range of sizes, with radii ranging from about a tenth of a millimeter to a few millimeters, can be As rain reaches the surface of rocky planets. This is a fairly narrow size range, as raindrops increase in volume by about a million times as they form in clouds.

The findings also show that the maximum size of droplets that fall as raindrops is similar under different planetary conditions. The maximum size of the different types of droplets is 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 raindrops). The graph below compares the raindrop sizes of Earth, Mars, Jupiter, Saturn, and Titan.

Caitlin Loftus, a planetary scientist at Harvard University and lead author of the new study in the AGU journal Geophysical Research, said these raindrops of varying compositions can have a fairly small range of stable sizes, and they are fundamentally are limited to the same maximum size. The journal publishes research on the formation and evolution of planets, moons and celestial bodies in the solar system and beyond.

In the new study, Loftus and colleague Robin Wordsworth used mathematical and physical principles to simulate how liquid water droplets fall through a planet's atmosphere. They wanted to determine the range of possible sizes of water droplets falling from the clouds to the planet's surface. Raindrops that are too large will break up into smaller raindrops, while those that are too small will evaporate before hitting the ground. They first determined the range of possible sizes of water droplets on rocky planets like Earth and Mars, taking into account atmospheric conditions such as temperature, air pressure, relative humidity, distance from clouds to the ground, and the strength of the planet's gravity.

They found that raindrops with a radius of less than a tenth of a millimeter evaporated before reaching the ground, while raindrops with a radius of more than a few millimeters broke up into smaller droplets as they fell. They then studied how the droplets land on planets with huge atmospheres like Jupiter and Saturn. Comparing modern Earth, ancient Mars and these more atmospheric planets, they found that raindrops move similarly through the air, although the factors that make up "air" vary widely between planets.

According to the researchers' calculations, these alien raindrops are not too different from the familiar raindrops of water, even if different liquids make up the raindrops. For example, the largest methane raindrops on Titan are about twice as large as those on Earth. Loftus isn't sure why the largest raindrops are so uniform in size, but she suspects it may be due to the relationship between the droplet's surface tension and its density.

Loftus said the findings will help scientists better model conditions on other planets, where precipitation is a key component in a planet's climate and nutrient cycles. Modeling what precipitation on a distant world might look like could also help researchers interpret observations of exoplanet atmospheres by space telescopes.