On rainy days, Kaitlyn Loftus likes to imagine herself somewhere else. Not on a sun-soaked beach, but on another world in the middle of its own rainstorm. Beneath the swirling storms of Jupiter or Saturn’s hazy cloud tops, where helium drops from the sky. On Neptune, where it might drizzle diamonds. Maybe Titan, a moon of Saturn, where methane rain can fill entire lakes.
Loftus is a planetary scientist at Harvard, and for her, otherworldly rain is more than a daydream. She and her colleagues recently studied how liquid droplets might behave as they descend from the clouds of different worlds, on the planets and moons in our solar system and distant planets around other stars. The team had expected to find quite a bit of variation; the conditions on Earth, after all, bear little resemblance to the environments of the other celestial bodies we know. Of the exoplanets that astronomers have discovered so far, some of them have weird characteristics—rocky surfaces so stretchy they resemble toffee, puffy atmospheres that might as well be planetary cotton candy, toasty worlds hotter than most stars.
Instead, their research suggests that raindrops on other worlds may not be so different from those on our own. Liquid droplets, whether they’re made of water or something more unusual, fall to the ground as spherical blobs that are roughly the same size. The biggest methane raindrops on Titan, for example, would only be about twice the size of the biggest water raindrops on Earth. In one sense, the similarities across worlds are not so surprising. The universe is full of echoes, from the basic composition of atoms and molecules to the fundamental forces that shape planets and galaxies. But there is something especially intriguing, even comforting, about finding uniformity in a phenomenon that seems so distinctly Earthlike, so specific to our existence as beings on this planet.
Alien rain sounds fantastical to us earthlings, but the universe is quite capable of transforming elements that seem so undeniably not-liquid into specks that fall from the sky. All it takes is some temperature tweaks. In 2012, a spacecraft orbiting Mars detected dry-ice snow falling from a cloud of carbon dioxide over the planet’s frigid south pole. Loftus says that, under very hot conditions, a planet could have globs of quartz rain from its skies. And consider the exoplanet WASP-76b, located hundreds of light-years from here. WASP-76b is twice the size of Jupiter, hot enough to melt metal, and orbits its star in a tight 43 hours. One side of WASP-76b is perpetually illuminated and the other eternally dark, which means that two regions of the planet are always in twilight. A few years ago, a different team of scientists studied these twilight zones and noticed that they could detect the signature of vaporized iron only on the planet’s evening side—in the transition from day to night—and not on its morning side, where night turns to day. The iron vapor had mysteriously vanished as it moved across WASP-76b’s night side. To the researchers, this could mean only one thing: rain.
“Because the nightside temperatures should allow for iron to condense, we deduced that it must either rain iron, if the vapor turns into liquid droplets, or iron snow, if the vapor directly turns into crystals—snowflakes,” David Ehrenreich, an astronomer at the University of Geneva who studies the atmospheres of exoplanets, told me. “The planet’s winds swirl these iron storms around, and when they reach the dayside, they transform into vapor again.”
Where there’s rain, there could be rainbows, even on Titan, where it’s so cold that methane falls to the ground as rain and then evaporates, just as water does on Earth. “Methane and water have similar index of refraction, so they should both make vaguely similar rainbows,” Sarah Hörst, a planetary scientist at Johns Hopkins University who studies Titan, told me. The moon’s atmosphere absorbs some of the colors that shine through in our rainbows, so a Titan rainbow would be tinged with oranges and reds.
Hörst says she and her colleagues often talk about how Titan rain might smell, making predictions based on their knowledge of the chemicals in its atmosphere. Compounds made of hydrogen and carbon would smell like gasoline, and cyanides like almonds. Some elements in the raindrops might resemble the smoky scent of barbecue. The aromas on hotter worlds might be more bizarre. “You get more chemical reactions as you get hotter,” Loftus explains. “I’m sure you could get some weird-smelling rain.”
The possibilities made me wonder: Could Titan have its own petrichor too? Petrichor is that fresh, earthy scent that hangs in the air after rainfall. The fragrance belongs to a molecule produced by bacteria in the soil, and it is released into the air when raindrops hit the ground. According to Hörst, the way Titan smells after a rainstorm is harder to predict. “We don’t really know the composition of the surface yet,” she said.
Studying something as small as raindrops can help scientists understand much bigger scientific questions, such as what Mars’s climate was like billions of years ago, when it was warm and sloshing with water, or which exoplanets might be good targets in the search for life beyond Earth. The research is also just a lovely thought experiment. I know it’s cheesy to admit that I love the smell of a good rainfall, but precipitation is one of the most universal phenomena of the human experience. On rainy days, I am reminded that not only do I live in this apartment that I’m typing in, on a particular block in a particular city, but I also live on a planet with its own quirks. How wonderful and strange it is to imagine that the scent of some other, unknowable version of rain on a distant world might be just as familiar to the inhabitants beneath its clouds.