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Crucible for Moon Formation in Saturn’s Rings

Hard to believe that it’s been ten years for Cassini, but it was all the way back in January of 2005 that the Huygens probe landed on Titan, an event that will be forever bright in my memory. Although the fourth space probe to visit Saturn, Cassini became in 2004 the first to orbit the ringed planet, and since then, the mission has explored Titan’s hydrocarbon lakes, probed the geyser activity on Enceladus, tracked the mammoth hurricane at Saturn’s north pole, and firmed up the possibility of subsurface oceans on both Titan and Enceladus.

I mentioned the Galileo probe last week, its work at Europa and its fiery plunge into Jupiter’s atmosphere to conclude the mission. Cassini has a similar fate in store after finishing its Northern Solstice Mission, which will explore the region between the rings and the planet. As discussed at the recent European Planetary Science Congress in Cascais, Portugal, the spacecraft’s final orbit will occur in September of 2017, taking Cassini to a mere 3000 kilometers above the planet on closest approach. A final Titan encounter will then provide the gravitational muscle to hurl the craft into Saturn’s atmosphere, where it will be vaporized.

But of course we’re not quite through with Cassini yet. New work out of the SETI Institute delves into the creation and destruction of moons on extremely short time-scales within the Saturnian ring system. Robert French, Mark Showalter and team accomplished their studies by comparing the exquisite photographs Cassini has produced with pictures made during the Voyager mission. The upshot: The F ring has taken on a completely new look.

“The F ring is a narrow, lumpy feature made entirely of water ice that lies just outside the broad, luminous rings A, B, and C,” notes French. “It has bright spots. But it has fundamentally changed its appearance since the time of Voyager. Today, there are fewer of the very bright lumps. We believe the most luminous knots occur when tiny moons, no bigger than a large mountain, collide with the densest part of the ring. These moons are small enough to coalesce and then break apart in short order.”

Short order indeed. It appears that the bright spots can appear and disappear in the course of mere days and even hours. Explaining the phenomenon is the nature of the F ring itself, which is at the critical point known as the Roche limit. This is the range within which the gravitational pull on a moon’s near side can differ enough from that on its far side to actually tear the moon apart. What we seem to be seeing is moon formation — objects no more than 5 kilometers in size — quickly followed by gravitationally induced breakup.


Image: Cassini spied just as many regular, faint clumps in Saturn’s narrow F ring (the outermost, thin ring), like those pictured here, as Voyager did. But it saw hardly any of the long, bright clumps that were common in Voyager images. Credit: NASA/JPL-Caltech/SSI

Mark Showalter compares these small moons to bumper cars that careen through moon-forming material, taking form and then fragmenting as they go. Adding to the chaos is the moon Prometheus, a 100-kilometer object that orbits just within the F ring. Alignments of this moon that occur every 17 years produce a further gravitational influence that helps to launch the formation of the tiny moonlets and propel them through their brief lives. Prometheus, then, should cause a periodic waxing and waning of the clumps of moon activity.

Further work with the Cassini data should help the researchers firm up this theory, for the Prometheus influence should cause an increase in the clumping and breakup activity within the next few years. Cassini has a sufficient lifetime to test that prediction. What adds further interest to the story is the fact that we’re seeing in miniature some of the elementary processes that, 4.6 billion years ago, led to the formation of the Solar System’s planets. Consider Saturn’s F ring, then, a laboratory for processes we’d like to learn much more about as we turn our instruments to young stars and the planets coalescing around them.

The paper is French et al., “Analysis of clumps in Saturn’s F ring from Voyager and Cassini,” published online in Icarus on July 15, 2014 (abstract). This news release from the SETI Institute is also helpful.


Comments on this entry are closed.

  • Andrew_W September 17, 2014, 12:34

    Now, if Robert Farquhar was in charge Cassini would be going to chiron or some other object way out there, instead of doing the death plunge.

  • stephen September 17, 2014, 13:10

    Analog SF magazine had an article, I think in 1983, called
    “The Blivet in the B-Ring”, by Richard C. Hoagland, about some very unusual detail in the rings, suggesting something artificial might be there. I wonder if anybody has followed up on that recently, and what the findings are? Thanks.

  • Galacsi September 17, 2014, 15:13

    Electrostatic agregation ? Gravitational fragmentation ?

    I smell sulphur !

  • ljk September 17, 2014, 16:16

    Anything by Richard C. Hoagland should be taken with several dump truck fleets full of salt. I refuse to link to his main site on principle but Google his name and you will see why his ideas are to be questioned thoroughly. Sadly he is not alone in this concept, but it is all fringe stuff with very liberal interpretations of selected Cassini images.

    Just so no one says I am being too closed minded, perhaps it is not impossible that the rings were made deliberately, but as we now know they are as ancient as Saturn itself and not relatively recent as recently thought, one would have to ask what aliens were doing in our Sol system and specifically at a forming Saturn 4.65 billion years ago. I will stick with Occam’s Razor on this one unless and until some far more solid scientific evidence comes along.

    Side note:

    In Arthur C. Clarke’s novelization of 2001: A Space Odyssey, it is said that the rings were produced by the Monolith ETI tearing apart at least one moon of Saturn for resources in order to produce the Stargate. Perhaps this is where some people got the spark of the idea and then went off into the Twilight Zone.

  • andy September 17, 2014, 16:44

    That would be the same Richard C. Hoagland who made the claims about the artificial nature of Iapetus and certain hills in the Cydonia region of Mars, and that there are giant glass skyscrapers on the Moon?

  • Rob Henry September 18, 2014, 5:26

    stephen, I also believe such things should be investigated. Unfortunately, to even begin the process we must establish a baseline. If we think something looks suspiciously artificial and THEN try to test it as such, we would suffer conformation bias that would invalidate our test UNLESS the bases of that test was subsequent data that was independent of our prior knowledge. IMHO Richard C. Hoagland is as hard working, honest, and smart as many of his detractors, but unfortunately he was never trained in the subtitles of scientific method. This in no way makes him wrong, but it does mean that he can give us no confidence in the truth of his speculations.

  • ljk September 18, 2014, 10:40

    Cubesats to the Moon (Mars and Saturn, Too)

    The next generation of planetary explorers.

    By Mark Betancourt

    Air & Space Magazine

    September 2014

    Full article here:


    To quote:

    Case in point: Saturn’s rings. The $3 billion Cassini spacecraft, which has gathered the most detailed information about Saturn to date, can’t risk coming close enough to the rings to study the icy particles they’re made of. One wayward particle and boom, there goes Cassini. Some researchers have suggested sending a large spacecraft to hover above the rings and take detailed images, but the fuel requirements for such maneuvers would be enormous, says Matthew Tiscareno, who works with Cassini’s imaging team at Cornell University in New York.

    That’s why he and other colleagues at Cornell propose something simpler, smaller, and about 1,000 times cheaper. A swarm of tiny spacecraft, not unlike Vermont Tech’s Lunar CubeSat, could be injected directly into Saturn’s rings, where they would orbit along with the ice particles. Those CubeSats could then release hundreds of even smaller spacecraft, called chipsats, that would “tag” individual ice particles, recording basic information about their composition, density, and motion within the rings.

    In some ways, this would be the ideal way to study the rings: gathering lots of data from lots of little sources. You actually don’t want something bigger, says Tiscareno.

    “If you had a spacecraft the size of a Volkswagen or—you know, Cassini’s the size of a schoolbus—it would disrupt the environment that it’s trying to measure a lot more than a CubeSat would.”

    Not all the tiny spacecraft would likely survive bumping around inside Saturn’s rings, but even if only one in four sent back data, he says, the mission would be a success. It’s the buckshot approach to planetary science.

  • ljk September 18, 2014, 10:50

    I had a similar idea about exploring Saturn’s rings back in 2003. If you can read through the somewhat clunky post here:


  • Alexander McLin September 18, 2014, 14:48

    @ljk Just curious, how were the rings dated?

  • ljk September 18, 2014, 16:08

    Dust reveals ancient origin for Saturn’s rings

    Data from the Cassini spacecraft suggest rings formed 4.4 billion years ago.

    Alexandra Witze

    19 August 2014

    Saturn’s spectacular ring system may date back some 4.4 billion years to the time when the planet itself formed, new findings suggest. The work could help to resolve a long-running debate about whether the rings are ancient or formed much more recently, on the order of hundreds of millions of years ago.

    For the first time, NASA’s Cassini spacecraft has measured the rate at which dust from outside the Saturn system is falling on the rings and polluting them. That rate turns out to be about 40 times lower than previously thought, which eliminates a major argument against the ‘old rings’ theory: that if the rings had been around for billions of years, they should have gotten coated with a dark spray of other particles and look a lot dirtier than they do.

    “The rings can be three to ten times older than we used to think,” says Larry Esposito, a planetary scientist at the University of Colorado Boulder.

    Full article here: