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The great Saturn ring debate is far from settled, a new study suggests.
For years, scientists have argued about the age of Saturn's famous ringsAre they ancient, dating to the birth of the planet itself? Or did the ring system form more recently, in just the past one hundred million years or so?
This last hypothesis has been gaining steam in the last few years, with multiple papers reporting that the rings could be even younger than the dinosaurs. Such studies quotes the rings' composition – more than 95% pure water ice – and total mass, which NASA 's Cassini mission pegged at about 15.4 million trillion metric tons shortly after the probe' s epic "grand finale" at Saturn in 2017. (For perspective, 15.4 million trillion metric tons is about 40% the mass of the Saturn moon Mimas, which is 250 miles, or 400 kilometers, wide.)
Related: Saturn's Glorious Rings in Pictures
These two figures, and the bombardment rate by micrometeoroids, the rings must be young, the idea goes. After all, old rings should be much more "polluted" with silicates, organic molecules and other compounds.
But there are some serious issues with this interpretation, argues the new study, which was led by Aurélien Crida of the Côte d'Azur University and the Institut Universitaire de France. For starters, the scientists said, the bombardment rate may vary over time. Maybe it is unusually high right now – a recent hypothesis of the collision of the Kuiper Belt objects, for example – and the rings were much less intensely for their history.
Crida and his colleagues also note that, during the grand finale, Cassini spotted organics in Saturn's upper atmosphere as well as solid grains in the gap between the gas giant and its innermost ring. This stuff is almost coming from the rings, the researchers said.
"One possible explanation is that the process is responsible for the erosion of the rings and the launch of these nanograins is actually 'cleaning' the rings, preferentially removing silicates rather than water ice," Crida and his team wrote in the new study , which was published online today (Sept. 16) in the journal Nature Astronomy.
If that's the case, the rings would appear to be much younger than they actually are.
Then there's the ring system's total mass. Indeed, 15.4 million trillion metric tons "are actually consistent with the expected dynamical evolution of primordial, massive rings," the researchers wrote in the new study. "Interactions between the ice blocks that make up the rings lead to an outward transfer of angular momentum, resulting in the spreading of the rings."
At distances greater than 87,000 miles (140,000 km) from Saturn, the ring material's self-gravity trumps the gas giant's tidal forces. Crida and his colleagues said, "This is the result of the formation of moons and moonlets. The rings' current mass, as measured by Cassini, added to the original ring by spreading and moon training.
Related: In Photos: Cassini Mission Ends with Epic Dive into Saturn
So Saturn's rings are ancient – as old as the solar system.
"I have to admit that I am biased, because I like the model of satellite formation from the rings (I contributed to develop it), and I would be annoyed if it [were] impossible, "he told Space.com via email.
"But I think that, objectively, it's a much more consistent picture, with a convincing model of their training at the same time as Saturn, plus training and outward migration of the satellites in agreement with the observations," he added. "In addition, the present mass is very much about 4.5 billion years of evolution, so that everything fits together."
Nailing down the rings' age is important for a variety of reasons, he and others have stressed. For starters, if the rings are young, then most of Saturn's icy moons may be geyser-spewing Enceladus, which hosts a big ocean of salty water beneath its frigid shell.
Enceladus is the best of the world's best alien life, and the chances are much better in this regard if life has had 4.5 billion years to get there rather than 100 million. (Another potential life-hosting Saturn moon, the giant Titan, is widely regarded as ancient.)
In addition, Crida said. For example, were the most of the planet's moons indeed born from the rings, as the ring-spreading scenario posits?
"If the rings were born with their present mass, then this model can not work, and the satellites must have shaped differently (competing with Saturn, like the planets around the sun)," Crida told Space.com. "Also, this model of satellite training could have been applied to Uranus and Neptune (which would have been more general than Saturn."
Cassini's grand finale, and the many other measures the spacecraft made during its 13-plus years at Saturn, for years to come. So, the debate over the age of Saturn's rings will likely continue. Stay tuned!
Mike Wall's book on the search for alien life, "Out There"(Grand Central Publishing, 2018; Karl Tate), is out now. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom gold Facebook.
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