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SYFY WIRE Bad Astronomy

Weaving Saturn’s E Ring

By Phil Plait

I get a lot of email from readers, generally either pointing out some interesting bit of news, or asking a question. Every now and again I get a question that really sends me down a hole of research: looking up numbers, observations, reading journal papers, what-have-you. Those are fun!

I got one of those from Missy Kent (which I choose to pronounce Miss Secant for reasons which are tangential to this article), who asked, “How long would it take particles from the geysers on Enceladus to make Saturn’s E ring?”

To give you the background, Enceladus is an icy moon of Saturn, orbiting it about 238,000 kilometers out. A few years ago, the Cassini spacecraft flew past it and discovered several geysers, literally jets of water ice, shooting out from near its south pole. Saturn’s gravity squeezes the moon, heating it up, melting the interior. That water is under a lot of pressure and finds its way up to the surface through cracks in the ice. The resulting geysers reach heights hundreds of kilometers from the surface … and in fact some of the geyser spray leaves the moon altogether.

Saturn, of course, has magnificent rings. They’re composed of gazillions of tiny particles of ice, some of them microscopic, some a few meters across. You might think of the rings as very flat—and for the most part, they really are. But some of the rings are not flat, and are actually downright puffy. One of these is the E ring, which extends from about 180,000 to 480,000 kilometers from Saturn.

Enceladus is right in the middle of it, and in fact orbits in the thickest, densest part of the E ring. That led astronomers to suppose it was actually somehow supplying the particles in the ring … a conjecture that turns out to be correct!

Is that possible? Do enough particles leave Enceladus to actually inject themselves into the E ring?

It turns out Cassini observations give the answer. I found a very cool scientific journal paper that gives the details. The ice particles are small, only a few microns across (a human hair is about 100 microns thick), but there are a lot of them. The paper gives the geyser ejection rate as about 50 kilograms per second, though I’ve seen other estimates as high as 360 kg/sec. Let’s start with 50. The paper authors estimate that only about 10 percent of the particles in the geysers actually have enough speed to leave Enceladus, so let’s say they blow about 5 kg/sec that actually escape the moon. 

By looking at the particles in the E ring, astronomers can determine the density of particles per cubic centimeter (by the way it scatters, reflects, and transmits light). From that, plus the size of the ring, they can get its total mass: about 1.2 billion kg.

To get the time it takes to blow out the same mass as the ring from Enceladus, just take the total mass of the ring and divide it by the rate the moon’s blowing out ice: 1.2 billion kg / 5 kg/sec = 240 million seconds. In more acceptable units, that's about eight years.

Whoa. That’s fast. And if we use the higher estimates of the ejection rate it would take even less time.

Mind you, there’s more going on here we need to think about. If we assume that the E ring is old (a likely guess; if it’s only a few years old that would be a pretty big coincidence that we happened to see it, given that Saturn is 4.55 billion years old!), then there must also be something removing material from it, or else it would be far, far more massive than it is now. It’s far more likely that it’s achieved a steady state, with creation and erosion balanced.

An obvious conclusion is that the erosion is due to collisions with moons—the E ring is amazingly broad and stretches across the orbits of several moons. Another likely cause is sputtering; collisions with other particles that eject tiny bits of icelike shrapnel.

We also know that some of the icy particles that leave Enceladus go places other than the E ring; for example there’s water in Saturn’s upper atmosphere, most likely rained down from space by the moon’s geysers.

The takeaway point is that it probably doesn’t all that long to create a diffuse ring around Saturn like the E ring using just geysers on Enceladus, even if you include all the losses. Even if the numbers are off by an order of magnitude, we can easily see changes in the E ring over a single human lifetime.

And here’s another question for you, in the same spirit as the one that started all this: Given the rate the geysers are erupting, how long would it be before Enceladus disappears, using up its entire mass?

Enceladus has a mass of about 1020 kg—100 septillion kilos. That’s a lot … even at the top end of the geyser scale, it would take about 300 quadrillion seconds to use up the entire mass of the moon. That’s nine billion years, twice the current age of the solar system. So we’ll have Enceladus around for a while yet, merrily spewing away and keeping Saturn’s E ring supplied with ice.

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