Mini-moons form in a matter of months in Saturn’s chaotic F ring


In the chaotic fog of Saturn’s F ring, scientists believe mini-moons come together and get ripped apart in a constant dance of moon formation and destruction.

And the coolest part? These moonlets can be made and destroyed over the course of a school year the researchers say. While most processes in the universe happen on the order of millions and billions of years, the small moons of Saturn’s F ring coalesce and disperse in the matter of weeks to months.

The F ring is one of the outermost in Saturn’s icy ring system. It is much narrower than the other rings, tens to hundreds of kilometers wide, while main rings A, B and C are tens of thousands kilometers wide.


Though the more luminous rings are made of pieces of ice up to the size of boulders, the F ring is known as a dusty ring because it is made of water ice particles as tiny as specks of dust.

If you passed through the ring, it would be like walking through a faint haze of smoke.

“Imagine a very light fog that you can see through even though it is kilometers wide,” said Robert French, a researcher at the SETI Institute who recently published a paper about the F ring in the journal Icarus.

Even these tiny ice particles, if they get close together, can stick together to form small moonlets about the size of a mountain. Because they are essentially giant snowballs, they are not inherently stable and can easily be torn apart when something like the 50-mile-wide, potato-shaped moon Prometheus comes nearby.

The F ring is difficult to see from Earth because it does not reflect the sun’s light back to us very well. NASA’s Cassini spacecraft can see it; the powerful space telescope Hubble has seen it a few times; and Voyagers I and II saw it when they flew by Saturn in the early 1980s. Pioneer 11 discovered it, back in the 1970s.

From images collected by these various spacecraft, scientists noticed that the F ring is in a constant state of flux. Parts of the ring are bright, and parts are more dim. The brighter areas, called “clumps,” have more dust in them. The clumps grow and shrink on the order of days and weeks, and none lasts longer than about 3 months, said French. All this is normal behavior for the F ring.

Occasionally, however, the clumps get extra bright. Scientists thinks this extraordinary brightening occurs when one of the F ring’s temporary moonlets ventures into the dense core of the ring and knocks loose a bunch of small ice particles, making what looks like a big smoke screen.


Here’s the funny thing though: When the Voyagers flew past Saturn, they saw several of these super bright areas, but Cassini, which has been in the vicinity of Saturn since 2004, has only seen two.

So, what exactly is going on here? Scientists are not sure, but they do have some hypotheses.

One thought is that the number of extra bright clumps on the ring is directly related to the orbit of Saturn’s moon Prometheus.

Prometheus interferes with the F ring on a 17-year cycle. There are times in that cycle when the moon’s orbit takes it close to the rings and causes larger disturbances, and times when it is further away and has less of an effect. Perhaps the difference in the number of extremely bright clumps observed by Voyager and Cassini is simply due to the fact that the spacecraft captured images of the F ring at different times in that cycle.

If that is the case, then Cassini should start to see more very bright clumps soon.

Of course, it is also possible that Cassini will spot fewer bright clumps over the next few years, which would suggest something entirely different.

“If it turns out they continue to decrease over time, that implies there were a lot more in the past,” said French, “and whenever you see a trend like that, you think -- it had to have a starting point.”


The F ring could be a relatively new addition to Saturn’s ring system that could have been created when a larger ice moon got ripped apart, French said. And if that is case, perhaps the F ring is simply changing, and not changing in any cyclical way.

“Either explanation is possible,” said French. “If three years from now the ring looks the same as when Voyager saw it, that is a good argument that the F ring is old.”

French said ultimately he would love to use observations of the F ring itself to find out if it has been part of Saturn’s ring system for billions of years, or perhaps just hundreds of years.

But first, he’ll need more data. “For now, there’s nothing we can do but wait,” he said.

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