Since Galileo Galilei discovered Saturn’s stunning rings some 400 years ago, scientists have wondered how and when they formed. Now, as Lisa Grossman at ScienceNews reports, researchers analyzing data from the Cassini spacecraft suggest they are actually remnants of at least one of Saturn's moons—and they're a surprisingly recent addition.
Saturn’s rings have been something of a puzzle for astronomers. Some have previously suggested that the discs of rock and ice chunks could be leftover material from the planet’s formation and have been there for billions of years. But past scientists have pitched the idea that they could alternatively be remnants of saturnian moons.
But neither source entirely fits. Researchers have argued if the rings had been around since Saturn's formation some 4 billion years ago, the rings should be darker. The constant cosmic radiation bombarding since that early formation should have darkened the rock and ice, they argue. On the other hand, others argue that the ring system is likely pretty dense. If they did not form in Saturn’s early days, where did all that material come from in our relatively clean Solar System?
Now new insights from the Cassini spacecraft, presented at the Fall Meeting of the American Geophysical Union in New Orleans, may offer some clues. The craft performed 22 daring dips through the gap between Saturn and its rings before smashing into the planet this past September. And as Paul Voosen at Science reports, during those final swoops, Cassini was able to determine the density of the B-ring using the Doppler effect. What researchers found is that the ring, which makes up about 80 percent of the mass of the rings, is not nearly as dense as initially believed. Its mass is only equivalent to about 40 percent of Saturn’s tiny moon Mimas.
A second line of evidence comes from 12 years of data collected by Cassini’s Cosmic Dust Analyzer. As Voosen reports, Saturn and its rings are constantly pelted by a rain of dust and micrometeorites from the edges of the solar system, which should darken the bright water ice found in its rings over time. Until Cassini, researchers did not know how much of this material bombarded the planet, but using the craft data, researchers estimate the rings are around 150 million to 300 million years old—or even younger.
Planetary scientist Larry Esposito from the University of Colorado, Boulder, who was not involved in the study, previously believed the rings were on the older side. But he tells Voosen that the evidence is compelling. “These two Cassini results really argue strongly that the rings are young, maybe under 200 million years,” he says.
There were previous hints that the rings were not as dense as believed. As Espisito explains, data from the Voyager mission in the 1980s suggested a similarly low mas, but researchers doubted the result. He tells Grossman that, though scientists do not know exactly how the rings formed, it’s possible that a moon half the size of Mimas was ripped apart some 200 million years ago, producing the material that coalesced into the ring.
It’s even possible that Saturn goes through a ring cycle. In 2016, researchers modeled the various orbits of some of Saturn’s 62-known moons. By examining Saturn's tidal forces, they found that its inner moons are likely very young, less than 100 million years old, forming during the age of the dinosaurs. The researchers believe that these young moons and the ring system were created when the orbit of a previous generation of saturnian moons was disturbed, causing them collide into one another; the remnants were then shredded into rings. Over time, the rings coalesced into new moons.
In a way, Saturn’s beautiful rings are an ephemeral gift to astronomers. “We’re just really lucky to have developed intelligent life on Earth and launched a spacecraft to Saturn during the 200 million years when it happens to have rings around it,” Esposito tells Grossman.
This isn’t the first awesome insight from Cassini’s Grand Finale dives. Earlier this week researchers also announced that the rings impact the density of the planet’s ionosphere.