One of the oldest mysteries in modern astronomy is the age of Saturn’s Rings. The Cassini mission seemed to have settled the question once and for all, but now a team of researchers have just claimed that things might not be so clear-cut.
Cassini showed that the rings were mostly made up of clean ice particles. There wasn’t much of the dust and organic gunk you’d expect to accumulate over time. This meant that the rings couldn’t have been around for very long. However, a team of researchers have now suggested that the evidence has been misinterpreted. The rings are actually clean because the dirt gets swept out over time. If true, this would make the rings much older than currently believed.
An old puzzle
Back in the 1960’s there were two competing theories as to where Saturn’s rings came from. The first theory was that a moon or passing asteroid had drifted too close to Saturn. It passed close enough that tidal forces ripped it apart. The resulting debris would have spread out into the flat disk, which we see today. The competing theory was that the rings had formed along with the Planet itself. They were composed of material that had simply never coalesced into a moon. As time passed, and technology advanced, evidence was collected to both support and disprove both ideas.
It turned out that the rings were dynamically unstable, so could not possibly be ancient. The individual particles would gradually disperse over time. The rings would vanish after a few tens of thousands of years, and so couldn’t possibly be old. At the same time, it seemed highly unlikely that the rings should happen to be visible at the same time that were were around to see them. If they could only last a few thousand years, but the Solar System has been around for a million times that long, then what are the odds that they should be around right now?
Then, in 1980, the Voyager 1 space probe discovered the first of Saturn’s system of tiny Shepherd Moons. Pandora and Prometheus are too small to see from Earth, but their gravitational influence is just enough to keep ring particles from dispersing. Once we knew about them, and the other shepherd Moons, the Old Ring hypothesis was back on the table.
Cassini to the Rescue
New data kept arriving, until the Cassini spacecraft reached the end of its mission in 2017. The Grand Finale was designed as a final chance to explore Saturn and its moons. Since the spacecraft was at the end of its life, there was no longer any need to protect delicate instruments. Mission planners could finally explore regions that would otherwise have been too dangerous. One of these was the gap between Saturn and its ring system.
The spacecraft was sent to to get the closest possible look at the particles that made it up. Result: 95% of the mass of the rings is made from water ice, and the remainder is a mix of small rocks, organic materials and metals. From this, astronomers figured that the rings must be a few tens of millions of years old. This is quite young, in astronomical terms. Rocky and organic dust is pretty common throughout the Solar System, and the rings would accumulate it over time. The fact that these pollutants still make up such a small portion of the rings shows that they can’t have been around for very long.
Saturn’s Rings old after all
But according to Dr Aurélien Crida of the Observatoire de la Côte d’Azur, this might not be right. “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. 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”
According to Dr Crida and his co-authors, the polluting material doesn’t necessarily stay in the Rings when it arrives. The rings are made of particles and chunks of various sizes. They range in scale from rocks several meters across right down to micron-sized dust particles. As they flow and interact with each other, there’s a tendency for these particles to migrate towards the edges. Stuff approaching the inner edge ends up falling into Saturn, while anything leaving the outer edge drifts away into space. Meanwhile, in the E-Ring, at least, the lost material is constantly being supplied with new ice particles. One of Saturn’s more interesting moons, Enceladus, has huge geysers, fueled by underground oceans, which violently spray water ice into space. These ice crystals settle into the E-Ring and keep it supplied with brand new, clean material.
The team’s models show that the rings could stay clean and stable for billions of years. If true, this means that they could be as old as the Solar System itself. However, this new work does not prove that the rings are old – only that they could be. Still, this makes it unlikely that the rings would be as young as the previous model suggests. As Dr Crida says, “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”