Carlos Solis
“On Earth, rainbows form when light is refracted, internally reflected, and scattered by water droplets,” Dr. Alfredo Carpineti, a space expert at IFLScience, explained. The liquid plays an incredibly important role, he added.
Earth is the only known world with liquid water. But in theory the drops needed to make a rainbow don’t have to be water. Scientists know of a place beyond our planet that contains a different kind of liquid (more on that in a bit).
So what properties does a liquid need to form a rainbow? “You need a substance that splits light into colors with minimal absorption and, obviously, it has to be in the atmosphere,” Dr. Carpineti said.
Invisible rainbows beyond Earth
A rainbow could, for example, form on Titan, Saturn’s largest moon. It’s the only place besides Earth where liquids sit on the surface as hydrocarbons like methane and ethane. You can find those hydrocarbons in rivers, lakes, and seas there, as well as in clouds and rain.
Would rainbows made from those liquid hydrocarbons look the same as the ones we know and love? Probably not. “On other worlds, rainbows can look the same if they’re made from rainwater,” Dr. Carpineti said.
He said, “Titan has hydrocarbon rains, and that’s great because methane is transparent.” But Titan’s atmosphere is opaque to visible light, so only infrared sunlight gets through, he emphasized.
So don’t expect a dazzling riot of color. But not all is lost. “Rain on this Saturnian moon, combined with infrared light, would create beautiful invisible rainbows,” he said.
Researchers have also recorded rainbow-like bands elsewhere in the Saturn system, including above its moon Enceladus. However, rainbows on Titan remain hypothetical for now.
“We’ve seen amazing phenomena on Venus and even on an exoplanet,” he added.
What did he mean by that?
The expert was referring to the exoplanet WASP-76b — a so-called “hot Jupiter” located in the constellation Pisces.
Dr. Carpineti helped detect asymmetric light coming from that planet, and he’s been eager to explain it ever since. That discovery made a simple point clear: you don’t have to limit searches to the sort of rainbows that decorate Earth. Rainbows can be more unconventional, he said.
In his book Invisible Rainbows he explored refraction across different wavelength ranges. “For example, very energetic light near supermassive black holes can refract in the surrounding material, producing emission at different X-ray wavelengths. It’s a kind of X-ray rainbow,” Dr. Carpineti said.
An X-ray rainbow? Fascinating!
