Cassini spots a moon, solves a mystery

By Phil Plait | March 3, 2009 11:16 pm

The folks working with the fantabulous Cassini Saturn probe just released a very cool image indicating a very cool discovery: a moonlet embedded in Saturn’s G ring.

Saturn’s G ring is the second outermost ring (one more ring, E, is outside G, but very wide and diffuse). It’s about 8000 km (5000 miles) wide, orbiting Saturn about 165,000 km (100,000 miles) out from the planet. It’s mostly dust, which was a mystery: there was no moon associated with it to replenish the dust, the only ring like it around Saturn without such a source. These new images (click to emringen) clearly show there is a moon there indeed, though it hardly qualifies with such a grandiose name: it’s only about 500 meters (600 yards) across.

These images were taken over about ten minutes, with each image being a 46 second exposure. The camera was tracking on the ring, so the stars appear as streaks. The moonlet can be seen to be embedded in the ring, and moving with it. That makes it clearly part of the G ring, and the obvious long-sought source of dust.

Incidentally, the left image is in visible light, the middle in red, and the right in near-infrared. Taking images at different wavelengths (colors) like that gives astronomers insight in the composition of the ring material — ice and dust reflect light very differently, so looking at the relative brightnesses of the ring in different wavelengths can distinguish what material is in it.

This is a very cool thing: scientists predicted the moonlet was present because they had studied other rings and knew there had to be something in there making the dust particles, so they went looking for it, and bang! They found it.

That’s science. Observation, hypothesis, test, result. A billion kilometers from Earth, and science still works. That’s one of the many reasons I like it so much. That, plus it’s simply so cool.

Credit: NASA/JPL/Space Science Institute

Comments (36)

  1. Corey

    Tell me more of this…SCIENCE.

  2. it hardly qualifies with such a grandiose name: it’s only about 500 meters (600 yards) across.

    Besides, it hasn’t cleared its orbit!

  3. Stephen

    I’m curious about the star trails in the left image – I understand the camera was tracking with the rings, but what was the kick that caused the kink in the star trails? Is Cassini scheduled so tightly that imaging begins before the camera is really fixed after a maneuver?

  4. Crudely Wrott

    That’s science. Observation, hypothesis, test, result. A billion kilometers from Earth, and science still works. That’s one of the many reasons I like it so much. That, plus it’s simply so cool.

    And a tangible measure of progress. Courtesy of science via the human brain.

    Now, if only more people knew how coo it really is . . .

    Thanks for doing your part, Phil.

  5. Kmuzu

    Just wondering why Phil would support a racist, neo-confederate like George Noory on Coast to Coast by going on his show? George Noory has called for the overthrow of the government and has supported antisemitic and racist commentators like Jerome Corsi, Texe Marrs and Alex Jones.

    I’m very disappointed Phil would support such an organization. Being part Jewish I just don’t understand it.

    Kmuzu

  6. Mike Beavington

    I must have slept through the class. Does this mean that all rings on all these planets require a moon to generate the dust? Uranus has rings, so the rings are formed from moons?

  7. Mattness

    @Kmuzu: it’s the old “either ignore the disinformation out there and leave it unchallenged, or lend the platform it’s hosted on credence by going on.”

    I guess a good case could be made for both options, that doesn’t necessarily mean that the other is entirely false though. I may add that it is a pre-requisite to be knowledgeable about what you’re talking about and the disinformation you’re challenging (and the tactics of those spewing nonsense, ie. moving the goal post etc). Otherwise it’s a lose-lose situation.

  8. Big Al

    George Noory is the victim of a disinformation attack. It’s been going on a long time, and crops up again from time to time. He actually provides a valuable outlet for woo. The sick sad people who call his show with their saucer sightings and various fantasies need such an outlet. At worst, it’s poor entertainment, and at best, there’s gems of wisdom occasionally provided by guests such as BA. Nice thing about free radio, if you tune in and the content suits your mood, enjoy., If not, go listen to PBS.

  9. Torbjörn Larsson, OM

    Besides, it hasn’t cleared its orbit!

    Actually, Emily Lakdawalla over on the Planetary Society blog claims that moonlets associated with rings looks like accretion bodies embedded in a ring with “dust on all sizes” [apparently despite typically being very thin – on orders of meters?], and notes that there could be more of these moonlets embedded in any ring. If I understand Emily correct, even if one or more (say, after a breakup) originally seeded the ring, they are today a more fuzzy part of the mass-gravitational dynamics of the rings. Emily: “With this discovery, I think Cassini’s beginning to venture into the murky terrain that separates “moons” from “ring particles.” [I admit I piece together parts from a longer comment that isn’t set in an ordered exposition – or at least an order that I can discern. But this is the impression I get of the sum total of what she says in her post.]

    Which raises a question to me. If these seeding-aggregation processes exchange materials between dust bodies in a gravitationally herded ring, is it possible that the originally seed body can diminish or disappear while its role is more or less overtaken by a few dust aggregation moonlets? [Wouldn’t that be awesome! OB BA pun: “Ring captured by dirty tugs.” ] Or are there constraints that makes this unlikely, say collision statistics vs ring life time?

  10. Copernic

    I don’t get it. The orbital mechanics, that is.

    Why should we expect a moon to be replenishing a dusty disk that encircles a planet? It doesn’t just leave a trail as it orbits, the dust has to maintain orbital velocity as well, and instead bathe the moon in an aura of particulates. Dust would either have to speed up or slow down to bespeckle in a ring-like fashion and in doing so would either increase or decrease its distance from the planet, making it very non-ringlike…right?

    Where am I confused on this?

  11. Stephen

    @Copernic:
    I believe the moon replenishes the disk because of the immense tidal forces between the planet and the other moons. That’s the source of the energy that drives Io’s volcanism, it seems it should be the case here as well.

  12. L Ron Hubbub

    I don’t see how they could have missed it all these years. It has a giant white square around it!

  13. L Ron Hubbub

    @ John Armstrong: it hardly qualifies with such a grandiose name: it’s only about 500 meters (600 yards) across.

    Besides, it hasn’t cleared its orbit!

    That’s no moon…

  14. QUASAR

    All the rocks in Saturn’s rings are technically satellites!

  15. Copernic

    @Stephen,
    Thanks, but I understand that it can be tugged and jostled and bumped and subsequently drop off specks. My question is how these specks get 200,000 some odd kilometers on the other side of Saturn. If they are going the same velocity and direction as the “moon” when they are sluffed, then they should remain roughly in the same vicinity as the “moon”. If they careen off any any other direction, they get swallowed by Saturn or find itself in a different orbit, no?

  16. I think my question falls along the lines of Copernic’s, though I’m not sure I understand his.

    What’s the relation between the moon and the dust? How does it replenishes it, how does that work?

  17. Stephen:

    I’m curious about the star trails in the left image – I understand the camera was tracking with the rings, but what was the kick that caused the kink in the star trails? Is Cassini scheduled so tightly that imaging begins before the camera is really fixed after a maneuver?

    If the spacecraft is turning, you shouldn’t be too surprised that there might be tiny glitches in the turn. It only takes on reaction wheel to slip slightly, after all.
    (That’s my story and I’m sticking to it, anyway.)

    Why should we expect a moon to be replenishing a dusty disk that encircles a planet? It doesn’t just leave a trail as it orbits, the dust has to maintain orbital velocity as well, and instead bathe the moon in an aura of particulates. Dust would either have to speed up or slow down to bespeckle in a ring-like fashion and in doing so would either increase or decrease its distance from the planet, making it very non-ringlike…right?

    You’re assuming that the dust behaves in Keplerian fashion, for a start. Dust grains are small enough to be significantly influenced by other forces, like the magnetic field, plasma drag, and Poynting-Robertson drag. All of these non-gravitational forces can change the particles’ orbits. A small change in orbit (one that is imperceptible at the scale of our images) leads to changes in longitude (relative to the moonlet) over time.

    Stephen: I don’t think tidal forces play into it, as such. My guess is objects striking the moonlet (including interplanetary particles) simply knock stuff loose. I wouldn’t expect any tidal heating here, myself. I don’t think that the moonlet’s orbit is very eccentric, for one thing.

  18. Gary Eller

    While I appreciate Mr. Weiss’ explanation, I’m also curious about a moon seeding the rings and perhaps I need it in more layman’s terms. Also, why those moonlets and not others? From pictures I’ve seen (assuming it’s not just Arizona sand.. Kidding!) our own moon has quite a bit of ‘dust’ rather than a solid rock. Though it would make more sense, I suppose, if it is because our Moon is so much more massive.

  19. Question: does the small size of the moon indicate that this ring may be shorter-lived than the others? Especially considering how large the ring is (8000km wide and having the second-largest radius), what’s the timescale for the moon’s mass to be exhausted and the ring to begin to dissipate? Or will such a thing not happen?

  20. Gary:
    The Moon is indeed far more massive and therefore tends to hold on to its dust. The smaller the body, the easier it is to eject the dust into planetary orbit and to form a ring. That said, even more massive moons can produce dust. I believe Hyperion is the source of dust we’ve seen on other moons near its orbit. (Although I believe I was told that pre-Cassini, so newer measurements may have refuted that by now.)

    Flooey: A good question. Bear in mind that the G ring is INCREDIBLY tenuous, so there’s not much mass there. But how much? Well…. (buckle your seat belts, there be math ahead)
    Let’s say that the orbital radius of the center of the G ring is 166,000 km (pretty accurate) and its width is 7,200 km. That’s a surface area of about 7.5×10^19 cm^2. (Yes, I’m an astrophysicist. I use cm.) However, the optical depth of the G ring is 1×10^-6, so only one millionth of that area actually has particles in it. So the total surface area of the particles is about 7.5×10^13 cm^2. Now, if we assume all particles are 1 micron in size and that they’re spherical, they have cross-sections of 1.3×10^-7 cm^2. So that’s around 6.0×10^20 particles or so. Assume that they’re all pure crystalline water (density: 1 g/cm^3), then that’s a mass of about 2.5×10^9 grams, or a ball about 8.4 meters in radius.
    If you want a timescale for depletion, then consider this: assume that the moonlet is spherical and has a radius of 250 meters (as per the estimate in our press release). That’s a mass of up to about 6.5×10^13 g. (I say “up to” because the density could be maybe 1/3 of that of water given what we found in the ring moons like Pan.) So the ring mass is around 2.5×10^9 g, the moon’s mass is 10,000 times larger. The only remaining question is how long does a given particle last in the ring… from what I’ve heard for other regions, 50–100 years seems about right. Call it 100 years, so it would take about a million years for this moonlet to ablate away at the present rate. (However: if the moonlet is ablating away, the rate at which it does so will probably drop as r^2, the cross-sectional area for impacts.) That seems like a short timescale, but not entirely crazy.

  21. (Ooo, I just realized I snuck in an extra factor of 4 into the particle cross-sectional area computation. It doesn’t substantially change the results, I shouldn’t though.)

  22. Copernic

    @Weiss
    Regarding the other forces at play, that would seem to initiate an even wider distribution of dust particles making them even more un-ring-like. That would be like explaining why a puddle of spilled water is perfectly round because of brownian motion and my blowing across the table (the metaphor stops there, by the way). Only Keplerian forces planet rings, but wouldn’t they also prevent dust eminating from a single point source from migrating into a ring stretching around a gas giant. As a piece of ice/dust, if you want to move ahead along the orbit, and take your place as a member of a magestic ring of saturn, it requires you pick up speed, and higher velocities push you into outer orbits, taking you out of the ring club, right?
    Remember, I’m no astrophysicist, I’m just asking.

    I need a tetraflop computer so’s I can model ring formation from a disintegrating moon.

  23. Copernic

    (should read “Only Keplerian forces explain planet rings”)
    ((I also cannot attest to the accuracy of this statement only that it is what I intended to say)).
    Thx

  24. @Copernic

    No. First off, the key point here is that once you’re on a SLIGHTLY different orbit, you will eventually spread out around the planet from the moon you came from. A 1 meter shift, over enough time, will do the trick. Second, if you’re feeling the magnetic field, things get a bit more interesting. You’re not on a Keplerian orbit anymore. In the extreme, you co-rotate with the magnetic field and that also holds you at your current radius. Dust particles are a somewhat annoying intermediate case between charged particles (like electrons) and large bodies (like ring particles and moons).

    Also, Colin Mitchell (of CICLOPS) suggests that I redo the live expectancy calculation for 10 micron particles rather than 1 micron. (I’m used to thinking of 1 micron thanks to the E ring, but the two rings are sufficiently different that I believe that Dr. Mitchell makes a good point.) So: cross-section is 3.1×10^-6 cm^2, meaning we need 2.4×10^19 particles. That’s a mass of 1.0×10^11, or a ball of radius 29 m. However, such particles should last 10,000 — 100,000 years. The moonlet could then last 7 to 70 million years, if I’m doing my math right.

  25. Copernic

    John Weiss, so what you are saying is that we could dump a Gorgonian Freighter’s worth of dust in a stable orbit and given enough time, and enough material there would be enough left to form a ring around the planet.

    Like the cars on my kids electronic race set, those cars travelling around the outer lanes, while slightly faster have to go farther and may find themselves drifting behind and on the opposite side of the track from our car in the middle lane (the moon), also cars travelling on the inner lane pull ahead and also find themselves at the opposite side of the track as well (or anywhere else along the ring’s orbit). And of course, the plastic track itself is not only a Keplerian orbit, but also a magnetic field, keeping some of the cars(dust) from careening off the track or into the inner grass (Saturn).

    Am I close?

  26. I’d say you’ve got it, Copernic.

  27. Copernic

    Thanks. And so then some follow-on questions.
    – Is it assumed that magnetic fields drive and distinguish ring formation and sustainment? Like putting up walls on a racetrack?

    – Can the mass, width, orbital radius, mass of the planet, strength of the magnetic field, etc tell us the age of the ring?

    – How would a moon accreted from the surrounding dust in the ring look different than a ring borne from a shedding moon?

  28. matt h

    ‘m not sure I can add much to JW’s explanations, but here it goes.

    First, as JW pointed out, what makes this moonlet different from our moon is that it is really small, so it doesn’t have much gravity. This means that if something smacks into the G-ring object fast enough to launch a cloud of dust, that dust won’t just fall back onto the moon, but instead escape from the moon and go into orbit around Saturn.

    Since the particles were launched from the moonlet at a finite speed, they won’t have exactly the same orbit as the moon. Some particles end up on an orbit that is on average slightly closer to Saturn, and some end up on orbits slightly farther away. The particles closer to saturn move a little faster than those further away, so over time a clump of dust becomes a streak, then a spiral that gets more and more tightly wound until it becomes essentially a complete ring. In this case, things are a bit more complicated, because there are gravitational perturbations from Mimas that keep the particles from spreading all the way around Saturn, and instead they form an “arc” that extends only about a sixth of the way around the planet.

    Now, what happens to dust after it gets into the arc is a bit trickier, and still under study, but I will try to give you some idea of one possibility. Most of the G ring actually lies outside the orbit of this little moon, so more of the dust seems to be drifting outwards than inwards. It turns out this could be explained by interactions between the dust grains and the very tenuous plasma that surrounds Saturn. This plasma made up of charged particles like ions and electrons. Since these particles are charged, they get pulled around by Saturn’s magnetic field. In fact, many of these particles whip around Saturn once every 10-11 hours, which is roughly Saturn’s spin period. By contrast, the dust in the G ring, which is in keplerian motion, only goes around saturn once every 18 hours or so. This means that the ions are moving around saturn faster than the dust grains, which means that when ions and dust grains hit each other, the dust grains tend to speed up a little bit. This slight acceleration causes the dust to move to progressively “higher” orbits, so they drift further from Saturn, forming the rest of the G ring. Note that any particle in the G ring could in principle be accelerated by such interactions with the plasma, but in practice only the tiny dust grains are moved enough by their interactions with the plasma that they can drift any significant distance away from the arc.

  29. – Depends on the ring. The A, B, and C rings are basically unaffected by the magnetic field. (Except for spokes, anyway.) The particles are too large to notice the effects. The D, E, and G rings are more influenced by the magnetic field (and magnetosphere in general), but how much it affects things varies. The F ring I believe is not strongly influenced by the magnetosphere, but there’s definitely dust in there so I can’t say for certain. I don’t know of anyone modeling those effects, but I don’t pay that much attention to the F ring, I confess.

    — Sort of. The dusty rings are probably all being generated even today, so you don’t get an age as much as an estimate of the size of the parent body. With the A, B, and C rings, you can infer some things about the ages from how spread out they are, but there’s some controversy about that age estimate. The spreading is due to particle-particle dynamics (collisions and gravity), not the magnetosphere.

    –I’m not quite sure what you’re asking, here. IF you’re wondering the new moonlet might not be accreting material even as some is being lost, yes, that’s definitely possible. It probably sweeps up some of its lost material, although I’m not sure what sort of efficiency is expected there.

  30. Winter Solstice Man

    We all know the rings were created by the aliens who made the Monolith in 2001: A Space Odyssey. They tore a moon apart in the process.

    It’s all in the novel version. If they were able to do the FX for Saturn’s rings in 1968, that is where the USS Discovery 1 would have gone.

  31. Torbjörn Larsson, OM

    IF you’re wondering the new moonlet might not be accreting material even as some is being lost, yes, that’s definitely possible.

    Thanks, and thanks for the doing the heavy lifting math.

    on orders of meters

    Oh, Emily has posted again, and AFAIU the G ring thickness is more on the order of 500 – 1000 km thick:

    There are two “G arc” flybys planned for early 2010 and late 2015. Cassini mission planner Dave Seal gave me the goods on how close Cassini can get to the G arc and S/2008 S1.

    Yes, the G ring is “dusty and extended”, but the core of the ring, where the arc and this parent body lives, is only about 500 km wide radially and 500 km thick vertically. (The full ring is more like 11,000 km radial extent by 1,000 km vertical extent.)

    But what is 4-5 orders of magnitude between friends? :-(

  32. Torbjörn Larsson, OM

    D’oh, I messed up the HTML. Again:

    There are two “G arc” flybys planned for early 2010 and late 2015. Cassini mission planner Dave Seal gave me the goods on how close Cassini can get to the G arc and S/2008 S1.

    Yes, the G ring is “dusty and extended”, but the core of the ring, where the arc and this parent body lives, is only about 500 km wide radially and 500 km thick vertically. (The full ring is more like 11,000 km radial extent by 1,000 km vertical extent.)

    But what is 4-5 orders of magnitudes between friends? :-(

    Oh, Emily has posted again, and AFAIU the G ring thickness is more on the order of 500 – 1000 km thick:

    There are two “G arc” flybys planned for early 2010 and late 2015. Cassini mission planner Dave Seal gave me the goods on how close Cassini can get to the G arc and S/2008 S1.

    Yes, the G ring is “dusty and extended”, but the core of the ring, where the arc and this parent body lives, is only about 500 km wide radially and 500 km thick vertically. (The full ring is more like 11,000 km radial extent by 1,000 km vertical extent.)

    But what is 4-5 orders of magnitudes between friends? :-(

  33. Torbjorbn:
    Sorry, just noticed this:
    Actually, Emily Lakdawalla over on the Planetary Society blog claims that moonlets associated with rings looks like accretion bodies embedded in a ring with “dust on all sizes”

    I think that you have to differentiate between moonlets. I can’t speak to S/2008 S1, but I wouldn’t necessarily expect it to look like the moonlets imbedded in the outer A ring (Pan and Daphnis) or just past the edge (Atlas, Prometheus, Pandora). It’s possible that they have similar densities and shapes, but I don’t see why they would. With the moonlets in/near the main rings, we found strong evidence of accretion of fluffy mantle material (ring particles) onto a denser core, perhaps a larger chunk of whatever formed the rings. There’s so little material in the G ring (and I don’t know that anyone thinks that there has ever been in the history of the G ring) that I don’t see this process occurring there.

    Oh, and thanks for the clarification, Matt H. He’s the expert on these rings, so y’all should listen to what he says. :-D

  34. Greg in Austin

    L Ron Hubbub said,

    “I don’t see how they could have missed it all these years. It has a giant white square around it!”

    You see, its comments like these that really bug me. How am I supposed to pretend to be working in my cube if I keep laughing out loud suddenly?

    Funniest dang thang I’ve read all week!

    8)

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