New evidence from the Cassini Saturn probe indicates that the planet’s moon Rhea may itself have rings!
First off, no pictures of the ring, sorry (that’s an artist’s conception above). If it exists, it’s very tenuous. Basically, Cassini is outfitted with a suite of instruments. One of them is the Magnetospheric Imaging Instrument, which can detect charged particles around it. When Cassini passed by Rhea, the MII detected a sharp drop in electrons, as if it were being shielded by something. From an analysis of the data, it looks like rings fit the bill.
The important aspect of this is that the number of electrons dropped on both sides as it passed the moon. That really says "rings" to the scientists.
I’ll note that this is not confirmed yet, but the folks at Cassini seem pretty confident. It’s not impossible for a moon to have rings; an old impact could have ejected dust and gas into orbit around Rhea. The moon is 1500 km (950) miles across, so it’s fair-sized; smaller than our Moon but the second biggest of Saturn’s. It has enough gravity to hold on to a ring for a while.
The scientists have a podcast they put together to explain all this, which is thoughtful of them. The press release has more info, too.
The solar system is a surprising place. That’s one of the reasons I like it here.
March 6th, 2008 at 1:35 pm
It will be very interesting if there really is a ring, but I’d like to note that “Cassini folks” sort of implies that all instruments are supporting this finding when, in fact, it’s just a subset. The imaging instrument (I think Cornell’s Matt Tiscareno did the analysis) has looked for the putative ring and found nothing to date. I’m not sure if any of the other optical remote sensing instruments have looked as well, but if they have, I’ve not heard of them finding anything. As I recall, we should have seen something if the in situ data (and the conclusions drawn from them) is correct. Still, we’re continuing the search! (Even if there is no ring, this data is odd and suggestive of something interesting, right?)
March 6th, 2008 at 2:10 pm
If Rhea occults a star, we may be able to tell if there are any rings around Rhea as the occultation happens. Thats how a ring systems can be seen around Jupiter, Uranus and Neptune. Let’s hope there is an occultation sometime down the road.
March 6th, 2008 at 2:21 pm
Seven days.
March 6th, 2008 at 2:33 pm
Well that would be cool… First moon with rings.
March 6th, 2008 at 2:35 pm
If a ring exists then presumably Rhea is a much more spherically-symmetric object than our moon – IIRC lunar orbits are unstable because the gravity field is warped by mascons near the lunar surface. On a related subject, I’ve usually heard it stated that moons of moons are unstable and wouldn’t happen, which is presumably not the case if a Rhean ring would be stable.
Presumably such a ring would only be visible when backlit, rather like the rings of Jupiter.
March 6th, 2008 at 2:44 pm
Have you seen the article on the Large Binocular Telescope?
It is located in Arizona. Some nice pictures are included.
Check it out on Yahoo Science module.
March 6th, 2008 at 2:44 pm
Moons are moons aren’t particularly unstable, apart from the small size of the moons’ Hill spheres (which is also dependent on the distance from the planet). After all, all moons orbit bodies which themselves orbit the Sun. That said, most largish moons have Hill spheres that are not terribly large (relative to the body), making it difficult to find stable orbits.
And they may not be easiest to see backlit. It’ll depend on the sizes of the particles making up the ring. I believe forward-scattering — looking when they’re backlit — has been looked at (possibly in the “In Saturn’s Shadow” observation), and nothing was seen there. If the particles are larger, however, they’ll be easier to see in back-scattering.
Occultations could work, although I’m not sure if anyone is looking at that. (Jupiter’s rings, for the record, were not discovered via occultations. They were discovered by Voyager.)
March 6th, 2008 at 2:53 pm
Oh, Phil: It’s MIMI: http://sd-www.jhuapl.edu/CASSINI/
March 6th, 2008 at 3:12 pm
Is there any case where an natural object is known to orbit a moon?
March 6th, 2008 at 5:37 pm
Note that the original paper talks of a dust halo with embedded possible rings! While the JPL PR writers went with the latter, a LANL press release more correctly put the halo in the headline.
March 6th, 2008 at 5:56 pm
News like this is just incredible.
March 6th, 2008 at 7:28 pm
[...] Bad Astronomy berichtet liefert Cassini Anzeichen dafür dass der Saturnmond selber Ringe [...]
March 6th, 2008 at 8:21 pm
I’ve wondered if our LaGrange points have any accumulated debris.
ANy input on that, Phil?
GAry 7
March 6th, 2008 at 8:45 pm
There’s rings around Uranus!?!? Sounds like someone needs to work on their personal hygiene.
March 6th, 2008 at 9:11 pm
What I don’t understand is why only the electrons dropped, why not other solar wind particles such as protons, etc. Is this indicative of the density and size of ring particles? Or is there some other reason, or was it all observed just not reported?
Gary Ansorge:
According to Wikipedia, there’s no large asteroids in the Earth-Sun lagrangian points, but there is a bunch of dust. http://en.wikipedia.org/wiki/Lagrangian_point#Natural_examples
March 6th, 2008 at 11:28 pm
To andy: “I’ve usually heard it stated that moons of moons are unstable” Not necessarily; it basically depends on how far away the secondat moons is from the primary.
I’ve emailed one of the people on the Cassini imaging team and he statred that they were not able to see the putative ring of Rhea.
With respect to making rings aorund a moon: I don’t think it’d be as straighforward as an impact putting debris in orbit around Rhea. What would happen to the debris is (a) if they are enegetic enough they’d escape Rhea’s grav. influence to orbit Saturn itself (b) for medium energy stuff, they’d probably go into high, suborbital trajectories and come back to Rhea (c) the low energy debris would just form an ejecta blanket around the crater. It’s not that easy to make a ring around a moon…
March 7th, 2008 at 12:07 am
Speaking of moon’s, I’ve been wondering about something you might be able to answer: If there were an observer on the far side of Earth’s moon with knowledge of modern physics but ignorant about the structure of our own solar system, could they, without leaving their home or launching spacecraft, deduce the existence of Earth? Because it seems to me like the Moon would be indistinguishable from a planet with a 2,551,000 second long day from that point, but I know that astronomers can be very clever when necessary, so I can’t be sure.
March 7th, 2008 at 3:31 am
Interesting to note that the NASA press release stated Rhea lacks an atmosphere yet Enceladus has one – but Rhea is larger (second only to Titan of Saturn’s many moons) and I’d presume more massive too ..
Why is that – anyone?
Cool discovery though I’m keen to see more findings on this.
If asteroids can have moons – and some (eg. Ida and Sylvia) do then why not a moon with rings although I would’ve thought the primary planets gravity would intervene & prevent it?
Thanks Phil Plait, Dr Bad Astronomer sir, well done as ever!
BTW. Your name ( & webblog) was mentioned and book shown on the big lecture screen at a talk on astronomical hoaxes by Steve Roberts of the Victorian Astronomical Society & Australian Skeptics at the March meeting of the South Australian Astronomical Society the other night. Thought you’d like to know! The video is even available if folks want .. Good talk too.
March 7th, 2008 at 3:34 am
Oh & Saturn’s also got the moons that switch orbits (Janus & Epimetheus?) too right ..?
Plus the odd spongecake texture surface, oddly rotating natural satellite Hyperion correct?
(Off top of head could look it up but …nah!)
March 7th, 2008 at 4:17 am
Spoketh # Lugosion 06 Mar 2008 at 8:45 pm
“There’s rings around Uranus!?!? Sounds like someone needs to work on their personal hygiene.”
No more asinine joke sabout Ouranos please .. I’ve heard ‘em all – & pity the poor colonist who’ll one day (perhaps, if humans are .. well not lucky but rather smart & brave) live in that Uranian system!
March 7th, 2008 at 10:11 am
Colonists on Uranus?
Nice.
I bet they will be Klingons.
March 7th, 2008 at 10:37 am
AR: Yeah, you could tell with sufficiently precise measurements. The Moon doesn’t orbit the Sun, it orbits Earth. It’s distance from the Sun varies with each month (which is different from the annual variation). In principle this could be measured (the effect is ~0.3%), but would probably be pretty rough to actually do. Using a heliostat to measure the apparent diameter of the Sun throughout a year might permit this.
zandperl: There are no solar wind protons at Rhea, Saturn’s magnetosphere keeps them out. There are protons in the magnetosphere, but they have such large gyroradii that detecting something like a ring around Rhea is probably right out.
StevoR: Right all around (Hyperion and Janus and Epimetheus). The co-orbital will swap again in 2009 sometime.
And Enceladus only has an atmosphere at the south pole, as far as I know. In that location, it’s due to the jets in that region (like Io’s patchy atmosphere due to its volcanoes). If you shut off the source, you’d lose the atmosphere fairly quickly I’m pretty sure.
changcho: It depends somewhat on the impact angle. A large, glancing blow could kick debris into orbit. This is, after all, how we think we got our Moon. I’m not sure that that’s any less probable than capturing ring material with no obvious way to dissipate energy. (Apart from three-body captures, of course.)
March 12th, 2008 at 1:31 am
Thanks for that John Weiss.
Didn’t know Enceladus’ atmosphere was only at the south pole … Figured it would’ve spread all around the planet although I knew it was thin. Cool! Leran something new every day ..