Saturn's rings younger than previously thought—just a few hundred million years

Saturn's rings are much younger than scientists once thought, according to new research from Indiana University Professor Emeritus of Astronomy Richard Durisen—and they are not here to stay.

For decades, there has been debate about the origin of Saturn's icy rings. But according to two new studies from Durisen, published in Icarus, the rings are no more than a few hundred million years old—much younger than the planet itself, which formed 4.5 billion years ago. In fact, Durisen said the rings may well have formed when dinosaurs were still walking on the Earth.

Durisen and co-author Paul Estrada, a research scientist at NASA's Ames Research Center in California's Silicon Valley, also concluded that the rings will last only another few hundred million years at most.

"Our inescapable conclusion is that Saturn's rings must be relatively young by astronomical standards, just a few hundred million years old," Durisen said. "If you look at Saturn's satellite system, there are other hints that something dramatic happened there in the last few hundred million years."

Durisen and Estrada have long argued that Saturn's rings are relatively young, because they expected the rings to be eroded and darkened by the influx of interplanetary meteoroids. However, it wasn't until data was available from NASA's 13-year-long Cassini spacecraft mission—particularly its 2017 Grand Finale, consisting of 22 orbits passing between Saturn and its rings—that they were able to use theoretical models to determine the age and longevity of the rings with confidence by computing how the rings change over long periods of time.

Particularly important for their work were Cassini's measurements of the meteoroid influx rate, the mass of the rings and the inflow rate of ring material onto Saturn.

The impact of meteoroids not only pollutes the rings, it ultimately leads to ring material drifting inward toward the planet. The theoretical models presented by Durisen and Estrada demonstrate that the rings should be losing mass onto the planet at the prodigious rate of many tons per second that Cassini observed, which means that the remaining lifetime of the rings is only another few hundred million years or so.

For the first time, the detailed computations by Estrada and Durisen combine viscous spreading—due to ring particle interactions—with meteoroid effects in simulations designed to span the full lifetime of a ring system like Saturn's. They demonstrate that meteoroid impacts are what ultimately impose a short lifetime compared with the age of the solar system, given the meteoroid influx rate measured by Cassini.

"We have shown that massive rings like Saturn's do not last long," Estrada said. "One can speculate that the relatively puny rings around the other ice and gas giants in our solar system are left-over remnants of rings that were once massive like Saturn's. Maybe some time in the not-so-distant future, astronomically speaking, after Saturn's rings are ground down, they will look more like the sparse rings of Uranus."

Durisen's decades-long research career centered mostly on the evolution and stability of rotating astrophysical systems of all types, from planets to galaxies. During the two decades before his retirement in 2010, he worked primarily on protoplanetary disks—the rotating disks of gas surrounding new stars from which planets can form. But his interest in Saturn's rings began as a postdoctoral fellow at NASA Ames in the 1970s, and he has continued to study them ever since.

"In studying the universe, we often think about origins—origins of galaxies, stars and planets," Durisen said. "But planets are incredibly active and diverse systems where new things happen all the time. If Saturn's rings are not as old as the planet, that means something happened in order to form their incredible structure, and that is very exciting to study."

Durisen is interested to see what future space missions discover about Saturn's system. Though the planet, composed mostly of helium and hydrogen, probably cannot support life, the conditions on some of its moons may have supported it in the past or even now, he said.

"If we can discover what happened in that system a few hundred million years ago to form the rings, we may just end up discovering why Saturn's moon Enceladus is spewing out from its deep ocean plumes of water, ice and even organic material," Durisen said. "We may perhaps even end up finding the building blocks of life itself on Enceladus."