“There is something that is cleaning the rings,” said Crida. “We don’t know what it is, but it is now an observed fact, it’s not just a conjecture.”
Crida said that perhaps the ice ejected by micrometeoroid impacts tends to reattach itself to the rings, while the ejected pollutants rain out. Becker conjectures that pollution is being preferentially ejected by impacts, regardless of whether the ice is reattaching itself in this manner. And Hyodo wonders whether the geysers on Enceladus’ south pole are adding more water, diluting the rings’ pollution. But no one knows for sure.
But not everyone believes that there’s a lot of cleaning going on. “Getting the stuff dirty is easy,” said Militzer. “Cleaning is hard.”
Where They Came From
What if, said Crida, the pollution argument is correct? What if the rings have always been exposed to an unchanging influx of cosmic dust, and the rings are 100 million years old at most? Then we would have to explain how the rings formed so recently, which is a tricky prospect.
First, we have no idea what created the rings, so assigning them an origin story at any point in time is difficult. The rings may be the vestige of a comet torn asunder by Saturn’s gravitational tides, or the product of a collision between a comet and an icy moon, or the result of something that disturbed the orbit of several moons, causing them to smash into each other.
A sample-return mission to Saturn’s icy loops could find the remnants of the original bodies that were annihilated and used to forge the rings, said Militzer. But no such mission is forthcoming.
Second, the solar system’s first billion years or so were a pinball-like pandemonium, with protoplanetary objects constantly colliding. These days, said Crida, things are far more settled, so the likelihood of a catastrophic collision leading to Saturn’s rings is far lower. If they did form in a recent cataclysm, said Militzer, such an event would dramatically change our perspective: It would imply that our planetary neighborhood hasn’t entirely outgrown the bedlam of its primeval days just yet.
Linda Spilker, the Cassini project scientist at NASA’s Jet Propulsion Laboratory, said clues may lie in Saturn’s moons, as their development is somewhat linked to that of the rings. But their own stories are also riddled with uncertainties, from their origins to their ages.
A 2016 model, using the current positions of the moons to peer backward through time, suggests that the present system of rings and inner moons could have been created when a pair of midsize moons smashed into each other about 100 million years ago.
But the ability of such a collision to form the rings we see now, said Dones, is an active controversy; a much-debated 2017 study, for example, suggests that not enough material would have been available to make today’s rings. “It just doesn’t work,” said Crida, adding that the only way this two-moon impact could have created all those moons and rings is through “magic.”
“The question of whether the rings are old or young will one day be definitively answered,” said Becker. But right now, there is enough evidence on both sides that “there’s still plenty to argue about before we can say anything conclusively.”
While the past is unclear, the future seems more certain. The rings may look permanent, but the opposite is true. Observations from a telescope atop Hawaii’s Mauna Kea volcano found torrents of material raining out from the rings. When scientists add this to the material detected by Cassini, they estimate that the rings will disappear completely in 100 million years.