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alan
QUOTE
The newly discovered ring spans from 128 to 207 times the radius of Saturn or farther and is 2.4 million kilometres thick. It was found using NASA's Spitzer Space Telescope, which revealed an infrared glow thought to come from sun-warmed dust in a tenuous ring.

The discovery was announced on Tuesday at a meeting of the American Astronomical Society's Division of Planetary Sciences in Fajardo, Puerto Rico. "This is a unique planetary ring system, because it's the largest planetary ring in the solar system," team leader Anne Verbiscer of the University of Virginia in Charlottesville told the meeting.

The source of the ring's material seems to be Saturn's far-flung moon Phoebe, which orbits the planet at an average distance of 215 times the radius of Saturn. When Phoebe is hit by wayward space rocks, the impacts could generate debris that fills the rings.

http://www.newscientist.com/article/dn1792...und-saturn.html

maschnitz
There's a nice illustration of it up already on Wikipedia:
http://en.wikipedia.org/wiki/Rings_of_Saturn#Phoebe_ring
brellis
What a gorgeous discovery!

This is the kind of thing I wondered about when Cassini was first arriving at Saturn: are there some pebbles and ice cubes we can't see from Earth that Cassini might strike, either upon arrival or during one of its orbits?

Granted, the material in the Phoebe Ring must be spread quite thin, and it's quite far from Saturn, but Cassini flew by Phoebe on its way to Saturn in 2004. I wonder if they had any idea then that Phoebe might be feeding a ring of its own!
Julius
Could it be that Phoebe is a captured burnt out comet!?? blink.gif
PhilCo126
Here's another drawing:
stevesliva
Still an artist's impression. But the article said the discovery was made using Spitzer when it still had cryogen, and that the observations were of basically empty space.
imipak
There's a great interview with Dr Verbiscer on BBC Radio 4's "The World at One" current affairs programme:

http://www.bbc.co.uk/programmes/b006qptc

Starts about 26 minutes in. The "Listen Again" links for the 7th October should, I think, work outside the UK.

(Edit - corrected spelling of Dr Verbiscer's name. Twice! D'oh rolleyes.gif )
Rob Pinnegar
QUOTE (Julius @ Oct 6 2009, 10:18 PM) *
Could it be that Phoebe is a captured burnt out comet!?? blink.gif

If I recall correctly, the general feeling is that it is most likely a captured Centaur (an object similar to 2060 Chiron), which somehow ended up in orbit around Saturn.

Centaurs often end up turning into short-period comets, so it's possible that Phoebe *would* have ended up as a burnt-out comet if Saturn hadn't nabbed it first. But it could also have wound up getting thrown into the Sun, or right out of the solar system altogether, probably by Jupiter. (When you're a small Solar System body, messing with Jupiter is a *bad* idea. It never turns out well.)
maschnitz
Hyperion appears to be more like a burnt-out comet - not dense at all (half the density of water), and very porous.

Some of the press coverage was saying that micrometeor impacts cause the ring, not solar radiation. Still, I think of this like a captured comet with its tail wrapped around Saturn.
dilo
This is the last present from cryo-Spitzer... sad.gif
Interestingly, the ring coul be the source of Iapetus dark material!
centsworth_II
QUOTE (dilo @ Oct 7 2009, 12:33 PM) *
Interestingly, the ring could be the source of Iapetus dark material!

Googling a bit, I see that [Phoebe] has long been looked at as a possible source for the dark material on Iapetus, but the spectroscopy did not match up. Maybe the dark Iapetus material has other sources in addition to Phoebe, or maybe the material in the ring is altered somehow before it lands on Iapetus:
"...based on spectral comparison only, Phoebe does not appear to be the elusive source. Because material from Phoebe could conceivably undergo alteration upon transference or arrival to Iapetus, our new data do not preclude the possibility of Phoebe as a source for Iapetus dark material...."

I don't know if the discovery of the ring was a surprise or just a case of looking hard to see something that was suspected to be there. Maybe it was not known if material from Phoebe formed a ring or just a tail. I don't know how strong the suspicions were either way, or if the size of the ring is a total surprise.
ngunn
What's special about Phoebe? If the material is thrown off by impacts then why don't all moons make rings? Why hasn't our moon made a ring around Earth?

Maybe the origin is primordial and Saturn has it's own miniature Kuiper belt. Phoebe would then be just the largest TIO.
volcanopele
I don't think there is anything special about Phoebe. It just happens to be large enough to throw enough material into space with micrometeorite impacts that its co-orbital ring is visible to Spitzer. I would not be surprised if the other small detritus out in the outer Saturn system (and the other outer planet systems) have similar, but much fainter, rings. Keep in mind that the proposed scenario put out be Verbiscer et al. is akin to what is going on at Janus and Epimetheus, Pallene, Methone and Anthe, Aegaeon, Mab, and Adrastea and Metis.

The reason it doesn't happen on the Moon is that the Moon is massive enough that most impact ejecta re-accretes back onto the Moon. For these little guys, like Phoebe, they are too small to hold on to much of the ejecta from micrometeorite impacts, and dust from these impacts goes into orbit around the parent planet.
SFJCody
Maybe Neptune's large irregular Nereid also has a ring associated with it. huh.gif
Gsnorgathon
Would Phoebe's retrograde orbit contribute to the production of ring particles? Would it result in higher impact velocities with whatever bits of stuff happened to be whizzing by?
volcanopele
not necessarily. As I pointed out before, for example, Janus and Epimetheus have a similar dust ring at their orbits made of micrometeorite ejecta and they have a prograde orbits around Saturn.

And adding to my list of "Moons with rings at their orbits", I should have also added Pan and Atlas.
HughFromAlice
News of the new Saturnian ring has travelled fast. Without being chauvinistic, I am glad to say I come from a forward looking scientifically literate country.......... our only quality national daily 'The Australian' has been quick to snaffle up this newsworthy item and give it a prominent place today. Who says we're behind the times down this way??????

Click to view attachment

Round here it's out with the delta Vs and albedo calculations mad.gif and back to the good old trines and sesquiquadrates smile.gif ! Perhaps I'll swallow national pride and keep checking the UMSF pages laugh.gif !
mchan
Unenviable that you have astrologers finding a giant ring around Saturn. smile.gif
djellison
Would it be shorter to write a list of Saturnian moons that DON'T have associated dust rings? And would the same not be true of...er...anywhere with moons?
brellis
QUOTE (mchan @ Oct 9 2009, 06:20 AM) *
Unenviable that you have astrologers finding a giant ring around Saturn. smile.gif


Would they be Australogers? laugh.gif
Gsnorgathon
QUOTE (volcanopele @ Oct 7 2009, 02:27 PM) *
not necessarily. As I pointed out before, for example, Janus and Epimetheus have a similar dust ring at their orbits made of micrometeorite ejecta and they have a prograde orbits around Saturn.

And adding to my list of "Moons with rings at their orbits", I should have also added Pan and Atlas.

I wasn't very clear there... what I meant was, "might Phoebe's retrograde orbit have something to do with how huge the ring is, by contributing to higher-velocity impacts that would produce more ejecta and/or more widely-scattered ejecta, when compared with the rings formed by impacts on other moons?" But I'm having a hard time figuring out how that might work, anyway.

If Phoebe's a captured TNO, maybe it's just got a higher volatiles concentration, and the ring's a toroidal comet tail.
Hungry4info
Back during the Cassini flyby of Phoebe, I recall them saying that Phoebe was confirmed not to be a captured body. Is my memory faulty? (It was a long time ago.
nprev
I don't know how they could have confirmed that, frankly. The retrograde orbit alone is practically a smoking gun; AFAIK, all other known retrograde satellites are thought to be captured.
Anne Verbiscer
QUOTE (Julius @ Oct 7 2009, 12:18 AM) *
Could it be that Phoebe is a captured burnt out comet!?? blink.gif


That is precisely one of the scenarios we considered when we wrote our proposal to search for a ring with Spitzer. smile.gif
nprev
Interesting! Could you please describe some of the others, Anne?
elakdawalla
One idle thought led to another: I wondered if Cassini got whacked by any Phoebe ring particles as it passed through on its way in to SOI. Then I realized there's an instrument on Cassini designed to measure that, the CDA. Of course, that data's old enough that it's in the PDS. If I had time, I'd dig into it and see if I could understand or deal with the CDA data. I don't have time, though, so I just sent an email to the CDA team leader asking about it and hope I'll get a reply. But I thought I'd mention it here, just in case anybody else feels like attempting to dig into the PDS and see what CDA data looks like!

--Emily
stevesliva
QUOTE (stevesliva @ Oct 7 2009, 08:12 AM) *
Still an artist's impression. But the article said the discovery was made using Spitzer when it still had cryogen, and that the observations were of basically empty space.


Finally saw a "real" photo somewhere:
http://apod.nasa.gov/apod/ap091013.html

Original here:
http://www.spitzer.caltech.edu/Media/relea...c2009-19a.shtml
Anne Verbiscer
QUOTE (nprev @ Oct 10 2009, 10:14 PM) *
Interesting! Could you please describe some of the others, Anne?


Sorry to take so long to answer this!

Some of those 'other scenarios' explored the possibility that Phoebe at one time or another exhibited Chiron-like activity. The two objects have similar colors and perihelion distances; however, Phoebe appears to have more water on its surface (seen in near-infrared spectra) than Chiron does. Other possible sources of dust considered were collisional, from both major impacts and micrometeoritic erosion.
Paolo
I resurrect this years old thread to post this: the Cassini ISS instrument has apparently managed to image the Phoebe ring
First observations of the Phoebe ring in optical light
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