On December 14th, 2017 we learned about

Despite our original estimates, Saturn’s iconic rings are a relatively recent addition to the solar system

As cool as Saturn’s hexagonal poles, massive wind storms and tantalizing moons may be, the planet is clearly defined by its rings. Even though it’s not the only planet to have rings around it in our solar system alone, the orbiting ice and rock are so pronounced that it’s hard to imagine the planet without them. In fact, for a long time the leading hypothesis was that Saturn had always had its rings, as they were thought to be formed from the same cosmic debris that formed the rest of the solar system. However, the evidence is stacking up against that idea, and it now seems that the sixth planet’s most famous feature is actually one of its more recent additions.

Not measuring enough mass

The first doubts about the rings’ age started in the 1980s, when the Voyager spacecraft took some quick measurements of Saturn as it flew towards the edge of the solar system. The data from Voyager suggested that the rings didn’t have nearly the mass they were expected to, especially if they had somehow been formed alongside the gas giant 4.6 billion years ago. These data weren’t considered conclusive, but fortunately Cassini arrived at Saturn in 2004 to offer a second opinion.

Cassini obviously gathered a lot of information about Saturn in its 13 years orbiting the planet, but the data most relevant to this study were gathered during the mission’s Grand Finale. Once the spacecraft had been fated for a fiery decommission in Saturn’s atmosphere, it wasn’t considered to be a huge risk to fly it through the space between the rings and the planet. The ring of most interest was the B ring, which was expected to have the mass of a small moon like Mimas. Instead, readings of the gravitational pull from the B ring were weaker than the ancient-rings model predicted, with the rings being just 40 percent of what they should be, supporting Voyager’s earlier measurements.

Missing billions of years of build-up

The size of Saturn’s B ring isn’t the only reference point in this study— there’s also plenty of dirt on other rings as well. Literally, the icy, reflective rings have been getting progressively dirtier over the ages thanks to a continual bombardment of sooty comets from the outer edges of the solar system. By measuring the rate of contamination, researchers have determined that they don’t have 4.6-billion-years of grime built up on them. They’re basically too clean to be that old, meaning the rings of Saturn have only been around for a few hundred million years, which isn’t long on a cosmic scale.

Why form when they did?

This presents a new challenge for researchers. While some had championed the idea of ancient rings, the evidence for much younger rings is convincing. The issue is coming up with a new model that can explain why they turned up when they did. Did something large hit Saturn, kicking debris into its orbit? Did one of the planet’s many moons get shattered? With rings this new, is there a chance that they’re not as stable as we’d assumed, leaving them primed to eventually collapse back to Saturn’s surface? Even with new information on the rings’ age, there are a lot of questions that need to be examined, and researchers feel like they’re basically at square one for an explanation. Sounds like a good excuse to send another robot.

Source: Saturn’s rings are a recent addition to the solar system, Cassini observations show by Paul Voosen, Science

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