A trillion and five moons

By Phil Plait | September 13, 2011 9:45 am

On July 29, 2011, the unending dance of Saturn’s moons lined them up perfectly for a stunning view by the Cassini spacecraft: five moons arrayed for your pleasure:

[Click to embiggen.]

From left to right that’s Janus, Pandora (in the rings), Enceladus, Mimas, and Rhea. Perspective plays a role here; Rhea is three times bigger than Enceladus, but was much closer to Cassini when this picture was taken, so it looks even bigger.

But the moons themselves are so different from each other! Janus is a lump, too small to have enough gravity to crush itself into a sphere. Enceladus is mostly ice, so it appears very bright in this image compared to its rocky siblings. You can just barely see part of the monster crater Herschel peeking out of the dark side of Mimas, while Rhea is peppered with smaller craters. And Pandora orbits inside Saturn’s rings themselves, its meager gravity enough to entrain the particles in the thin F ring and keep it in place.

And, of course, the rings themselves, composed of countless tiny ice crystals. Over millions of years, collisions have ground them into pieces ranging in size from barely big enough to see to perhaps 10 meters across, the volume of a roomy two-car garage.

Amazing. And this vista was taken just a couple of weeks after Cassini’s seventh anniversary in orbit around Saturn. Even after all that time, and tens of thousands of images, it still has the capability to take our breath away.

Related posts:

Cassini’s Pentaverate
Cassini’s slant on the rings
A little weekend Saturn awesomeoness
The real Pandora, and two mooning brothers

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (19)

Links to this Post

  1. En la luna - Esceptica | September 14, 2011
  2. Running rings around the competition « unbound page | September 18, 2011
  1. Messier Tidy Upper

    Thankyou BA & thankyou Cassini – great image and blog item. :-)

    How large does a ring particle get before it counts as a moon or moonlet?

    When do we stop counting – as Asimov once wrote and the BA seems to be alluding to in his title here, Saturn has trillions of moons including every particle in the rings!

  2. Becca Stareyes

    BA, to correct your statement, Enceladus actually has a pretty high density relative to its sibling-moons — its brightness is more due to the fact its South Pole is actively spewing out frost, which coats the moon. Granted, even with its density, Enceladus is mostly ice, but it does have less ice than its sibling moons, just not on its surface.

    Also MTU @ 1, I don’t think there’s a formal definition, but Pan, Daphnis and Aegaeon are on the small end — Pan and Daphnis are in the A ring itself, but have mostly cleared out nearby space (they’re in gaps in the outer A ring). Aegaeon is probably generating the G ring, but there may be other chunks of ice in there. I think currently the informal definition is ‘anything we can see on its own and track’, but some of the larger ‘propeller moonlets’ — unresolved 100 m (or so) chunks of ice in the A Ring that make propeller-shaped disturbances — have become things we can predict well enough to identify from orbit to orbit. We haven’t seen the moonlets themselves (only the disturbances in the ring material), but that’s more a function of our resolution, not anything special about the moonlets (they’re uncommon enough that you can pick one out from its orbit and size, but still small enough that they don’t clear a full gap in the rings).

  3. This was a great way to start out my day. Images like this are inspiring while also humbling, showing us just how small we are in the grand scheme of it all. This is definitely a desktop wallpaper.

  4. What a beautiful picture! Assuming that all the moons are in the ring plane (big assumption, I know), I hypothesize that the apparent distance above or below the plane is inversely proportional to the distance from the Cassini camera. That makes it Enceladus, Janus, Mimas, Rhea, Pandora in order of increasing distance, I think.

    The camera is obviously not exactly in the ring plane, so it makes this a bit tricky to judge.



    How large does a ring particle get before it counts as a moon or moonlet?

    Becca Stareyes:

    I think currently the informal definition is ‘anything we can see on its own and track’, […].

    Actually, anything larger than Rosie O’Donnell’s ass!

  6. Mark

    I can’t wait until we’re seeing images like this of the Jovian system. Amazing!

  7. QuietDesperation

    Eh… (tilts head) need to move Enceladus a skosh to the right. (stares) Tilt the ring plane more.

    Perfect! Get the camera! Now we make… (d r a m a t i c p a u s e) the magics!

  8. Anchor

    Becca @2 is right – they’re ALL mostly ice. It’s misleading to characterize the others as “rocky”: their relatively older surfaces are just dingier than Enceladus’ fresh coating.

  9. RSS Feeds have changed. I now only seeing a small excerpt… Anything you could do to get them back to the full article would be GREATLY appreciated.

    Love your stuff! Keep up the good work!

  10. Paul

    Given that the rings are relatively thin, would it be possible to swing Cassini very close to them in order to observe moonlets up close? Perhaps in the late stages of Cassini’s mission?

  11. Neil NZ

    Is Cassini likely to be funded beyond 2017? If not, what will happen to it when the funds dry up?

  12. Crux Australis

    At first, I counted only 4 moons; then I realized that big one on the right isn’t Saturn!

  13. Jim Craig

    Crux Australis, you and me both! Glad to see I wasn’t the only one. :)

  14. Messier Tidy Upper

    @2. Becca Stareyes : Thanks. :-)

    @9. Crux Australis & Jim Craig : Aptly enough Rhea the big moon in question is also thought to have rings itself :






    @5. Mark : “I can’t wait until we’re seeing images like this of the Jovian system. Amazing!”

    Did you miss the images from when they had the Galileo spaceprobe orbiting Jove then? Some of those were marvellous too. 😉

  15. Gus

    How long did the “window” last for Cassini to take this picture?

  16. Keith Bowden

    It never ceases to amaze me that no matter the resolution, or how close Cassini gets, the rings always seem both impossibly solid (though somewhat transparent) and impossibly thin. There never seems to be any area that’s thinner/thicker than the surrounding areas of rings, nor do they ever seem to resolve into individual fragments, a la the CGI in the opening credits on Star Trek: The Next Generation. (Or have I missed these photos?)

    Beautiful stuff.

  17. Rolf Wucherer

    Beautiful! Some day Cassini will run out of juice and, I guess, go into permanent orbit around Saturn. I suggest we make it an honorary moon! It, too, is not large enough to crush itself into a sphere, but we may be able to pinpoint its orbit. We could call it… Cassini!


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