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SYFY WIRE Bad Astronomy

Are Saturn's rings dying?

By Phil Plait
Wide image of Saturn and its rings.

One of the most spectacular sights through a telescope's eyepiece is the planet Saturn. Its iconic rings are so obvious even in small telescopes that they cause gasps from first-time viewers. I can't even tell you how many people have asked if they're real when I show them Saturn through my own 'scope. The look of awe and joy on their faces is one of the biggest reasons I love showing off the planet.

The rings are so huge and bright and beautiful that they seem like a permanent fixture of Saturn, and certainly its single most important feature. It's hard to imagine Saturn without them.

Yet that may be its ultimate fate. A new paper predicts that in the future — perhaps as soon as 100 million years — Saturn's rings may be gone. Or, at least, severely depleted and not at all like they look today.

The culprit here is "ring rain": Literally, Saturn's rings are eroding, and the material — water ice — is pulled down into Saturn's atmosphere. Measuring the rate of this rain has been a big goal for planetary scientists for a long time, and in this case the astronomers reanalyzed data taken from Earth in 2011 and, using new modeling techniques, were able to show that the rings are losing between 400 to nearly 3,000 kilograms of mass per second. Given the total mass of the rings, this means the clock is ticking. In a few hundred million years they may be gone.

The astronomers used data from the Keck infrared telescope in Hawaii to look at light emitted by a peculiar kind of molecule, consisting of three hydrogen atoms stuck together in an uneasy alliance. This trihydrogen molecule is created in Saturn's atmosphere in a couple of ways. One is when electrons are accelerated to high speeds by the planet's magnetic field, and they slam into hydrogen molecules — this is basically the same process that creates an aurora. The resulting debris can reform into the odd three-atom molecule. The other way is when high-energy ultraviolet light from the Sun breaks down hydrogen molecules, which again reform as trihydrogen.

This has been well known for some time. But here's the interesting bit: Water falling in from Saturn's rings can affect the amount of trihydrogen in the atmospheres, sometimes very strongly. So by measuring the amount of light seen from the trihydrogen molecule, they can measure the rate at which water is falling.

This too has been done before, but what's new here is how they applied physical models to the process. What they want to determine is the temperature and density of the hydrogen since these depend on the amount of rain, and that wasn't well known before. Newer models are able to find these much better, allowing them to better measure not only the amount of water coming in from the rings, but where it's coming from.

Some of the final science Cassini performed including mapping Saturn’s magnetic field, an inner radiation belt, and the rain of ice and dust falling from the rings on to the atmosphere. Credit: NASA/JPL-Caltech

What's happening at Saturn is complicated! But in a nutshell, the rings are made of ice particles — literally frozen water, and nearly pure. They get hit by tiny meteoroids flying around space, which creates a cloud of vapor around them (ultraviolet light from the Sun adds to this as well). This vapor is ionized, meaning it has an electric charge, and this means it can be affected by Saturn's magnetic field, which pulls them it of the rings. In most cases the gravity of Saturn takes over, pulling the water molecules down into the atmosphere, where they interact with the trihydrogen already there, changing the amount of the molecule in the atmosphere. That's how the rate of ring rain (an apt term, given the rings are water) was found.

The effects of ring rain were seen (though not yet understood) as far back as when Pioneer 11 passed Saturn in the 1970s, and the two Voyager probes passed Saturn in the early 80s. Cassini also saw lots of evidence for it, especially in its final days when it passed physically between the rings and the planet's atmosphere. This new data hangs a number on it.

The total mass of Saturn's rings is about 1.52 x 1019 kilograms — 15,000 trillion tons. That's a lot, but at the rate they're losing water the astronomers in the paper estimate it'll take just under 300 million years for the rings to completely drain. Depending on several factors it could be some faster or slower, with a range of about 100 million to one billion years.

So is that it? Are Saturn's rings dying?


I'm not so sure. There's still a lot we don't know about the rings. Their age is still a contested topic, for example. If they formed with the planet 4.5 billion years ago, it seems really unlikely that we just so happen to be seeing them just before they go away completely (in the last few percent of their lifetime). It's possible, but unlikely. If instead Saturn's rings are much younger, perhaps a few hundred million years old, then it's more palatable… though then we have to contend with the coincidence of them being around at all as we are too.

But Jupiter, Uranus and Neptune also have rings, though very weak compared to Saturn's. Maybe huge glorious rings are an ephemeral thing, a bit of transient structure that forms somehow and fades rapidly compared to the lifetime of the planet itself. Circumstantially, that is consistent with everything we see.

And while 15,000 trillion tons sounds like a lot, it's equivalent to an ice ball only 300 km across. Saturn has many moons larger than that, so it's not at all ridiculous to think a couple of small moons collided in recent geologic time creating the magnificent ring system, which has been eroding away ever since. And maybe this has happened several times over Saturn's life, making it even less of a coincidence that we happen to be around to see the results.

Bear in mind, too, that this paper only looks at how the rings are draining; it doesn't discuss sources of material for it. We know that the moon Enceladus, for example, has geysers that supply water to Saturn's faint and fuzzy E ring. That's nowhere near enough to resupply all the rings, but the point is there are ways the rings can be "refilled", including giant collisions.

Saturn's rings may be dying, but they may also be capable of resurrection, even episodically. We've only just begun to observe this planet and understand even a part of the immensely complex physics and mechanics of its behavior.

So don't cry over the demise of the rings just yet; these moments may be lost in time, like tears in ring rain.