Saturn's Rings Might Be Ancient After All

The latest portrait of Saturn by Hubble. NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley)

Saturn’s rings are the most striking, and certainly defining, characteristic of the second-largest planet in the Solar System. For many years it was believed that the rings were primordial, formed billions of years ago but recent analysis from the Cassini mission suggested that they are only 100 million years old.

Cassini showed that material from the rings is raining down on the planet and, given the rate, such a ring system could only really exist for a short time. While the argument is compelling, other researchers are now arguing that this debate is far from settled. In a paper published in Nature Astronomy, they point out evidence also from Cassini suggesting that the rings are almost as old as the Solar System.

“We can’t directly measure the age of Saturn’s rings like the rings on a tree-stump, so we have to deduce their age from other properties like mass and chemical composition," lead author Dr Aurélien Crida, of the Observatoire de la Côte d’Azur, said in a statement. "Recent studies have made assumptions that the dust flow is constant, the mass of the rings is constant, and that the rings retain all the pollution material that they receive. However, there is still a lot of uncertainty about all these points and, when taken with other results from the Cassini mission, we believe that there is a strong case that the rings are much, much older.”

Using Cassini’s data, it has been estimated that the rings weigh 15 billion billion kilograms, similar to a medium asteroid or a very small moon. The team focuses on the mass and viscosity of the rings to try and estimate the time elapsed from their formation until now.

The idea is that the rings will evolve over time. The inner edge rains down on the planet, the outer edge is lost to the moons and space. More massive rings lose mass faster. Interestingly the models employed show that no matter what the original mass is, over billions of years they will converge to a value consistent with what Cassini has measured.

“From our present understanding of the viscosity of the rings, the mass measured during the Cassini Grand Finale would be the natural product of several billion years of evolution, which is appealing. Admittedly, nothing forbids the rings from having been formed very recently with this precise mass and having barely evolved since. However, that would be quite a coincidence,” added Dr Crida.

In the study, the team differentiate about the formation age of the rings, the age of structures within the rings, and finally the exposure age given by the presence of material foreign to the ring. Confusion between these ages might be suggesting that the rings are younger than they actually are.

 

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