It's Raining on Saturn in a Way We Never Imagined

Saturn's dazzling rings have inspired centuries of star-gazers. But recent discoveries reveal the rings to be far more dynamic than we'd ever imagined. See, these massive disks ice don't just orbit the gas giant— They make it rain.

In a paper published in 2013 in Nature, scientists showed how tiny ice particles in Saturn's rings are magnetically pulled into the planet's upper atmosphere, creating a bizarre sort of rain. A study published this week in the journal Icarus builds off the 2013 paper, using spectral data to estimate how much rain is actually hitting Saturn's atmosphere from space. Turns out, it's in the neighborhood of 10^26 water molecules per second, or roughly an Olympic-sized swimming pool's worth of water every day.

Studying Saturn's elusive space rain may shed light on how the planet's stunning rings, and the gas giant itself, have evolved through time.

The second largest planet in our solar system, Saturn is basically a massive ball of hydrogen gas with a dash of helium and methane. High pressures and searing temperatures at the planet's core probably force hydrogen into a liquid metallic state. Crystalline clouds of ammonia and lightning storms a thousand times as powerful as those on Earth grace the upper atmosphere. Circling all this are the famous rings, sand to kilometer-sized chunks of ice that orbit the gas giant like a viscous liquid.

It is a strange, beautiful, and totally inhospitable place.

Scientists have suspected the occurrence of ring rain since the 1980s, but it wasn't until a few years ago that spectral data from the planet's ionosphere—an outer, electrically charged layer of the planet's atmosphere—provided direct evidence for the phenomena.

Outside the protection of an atmosphere, Saturn's icy rings are bombarded with UV radiation, which splits water into electrically charged hydrogen and oxygen ions. Some of these ions are scavenged by Saturn's magnetic field into the ionosphere, where they can recombine to form water. This process, the "electromagnetically erosion" of ring water onto Saturn, has probably shaped and sculpted the rings over time.

Astronomers have continued searching for evidence of ring rain since the 2013 Nature study, but the elusive phenomena has yet to reveal itself again.

"We've observed Saturn's ionosphere a couple of times since the 2013 paper, but we haven't seen ring rain so far," Luke Moore, lead author of the Icarus paper, told me. "It might be occurring below the detection threshold of our instruments, or water influx from the rings could be time variable. We'll keep hunting for ring rain, but there are a lot of other questions to answer in the meanwhile."

For one, where on Saturn it might be the rainiest, which Moore also attempts to hone in on in the new study. Using an atmospheric model, Moore finds that Saturn tends to be wetter at the equator and drier toward the poles, with localized patches of rainy weather at specific latitudes.

"If we can resolve how much ring rain is occurring and where, we can start to estimate the total influx of water from the rings, which can help us understand how the rings have evolved," Moore told me.

Nobody really knows how Saturn's rings formed. They could be the remnants of a moon that crumbled under to the planet's hefty gravitational pull, or leftover debris from the gas giant's accretion. Studying the processes by which the rings interact with the planet's atmosphere today may help to answer that question.

A better understanding of the rings may even shed light on the formation and evolution of the solar system at large.

"Ifthe rings formed out of the leftover solar nebular that created Saturn, then they could be seen a microcosm for the formation of the solar system itself," Moore said.

And if nothing else, ring rain is sure to provide fodder for future fiction. Imagine, if you will, human habitats floating in the gas giant's atmosphere, using sophisticated fog-water collectors to drink in the rings. Hey, one can dream, right?