Wednesday, October 07, 2009

One Ring To Bind Them


All over the news right now is what is probably going to be #1 in a long list of Things I Wish Had Been Discovered Before I Started Writing The Quiet War And Gardens Of The Sun.

Discovered by the Spitzer space telescope and visible only in infrared light is a vast ring tilted at twenty-six degrees to the Saturn's equatorial plane - the plane in which the more familiar ring system and the inner moons orbit. It's very very big, this ring, circling Saturn at a distance of 13 million kilometres: if you look at Saturn from Earth, imagine that it's sitting inside a ring that is twice the apparent diameter of the full Moon. Factor in the relative distances of the Moon and Saturn, (384,000 kilometres v. the minimal distance, at opposition, of roughly 1300,000,000 kilometers), and you'll realise that this is a very big structure indeed: the biggest known ring in the Solar System

It's very big, but it's also very tenuous. More tenuous than even the vacuum inside electronic vacuum tubes: there are on average twenty grains of icy dust in each cubic kilometre of this giant ring. For good reason, its discoverers are calling it the Ghost Ring. It's no coincidence that it shares the same orbital inclination and distance as Saturn's moon Phoebe; it's almost certainly composed of material knocked off that small and eccentric moon by meteoritic impacts. Phoebe is an odd little moon; not only is its orbit steeply inclined, it's also retrograde - it travels in the opposite direction to the inner moons. Images taken by the Cassini orbiter as it passed Phoebe on its approach to Saturn show an irregular and heavily cratered body, with slivers of bright ice showing through a dark outer crust of carbonaceous material. It's similar in composition to Kuiper Belt Objects, in fact, and was probably captured by Saturn when something perturbed its orbit and it wandered in towards the Sun.


There's speculation that the material in the Ghost Ring has contributed to the distinctive colouration of the outermost of the large moons, Iapetus. Famously, Iapetus is bright on one side and dark on the other, a property spotted by the man who discovered it, Giovanni Domenico Cassini, in the seventeenth century. Now, it seems that the dark sooty material from the ring has been swept up by Iapetus's leading hemisphere over a couple of billion years.


But is the Ghost Ring really the largest ring in the Solar System? Saturn has many other small irregular moons in wide eccentric orbits. Most belong to collisional families and are probably fragments from a larger body that was shattered by some impact after it was captured by Saturn's gravity: the Inuit Group; the Norse Group; and the Gallic Group. Phoebe belongs to the Norse Group. The moons in the Gallic Group orbit even further out. Could they, too, have generated a ghost ring?

8 Comments:

Anonymous Chris Brown said...

I think this is crazy people should really find something to do with there time...

October 07, 2009 7:48 PM  
Blogger PeteY said...

This is pretty cool. I wonder, though, about the possibility of another, still bigger ring - wouldn't the Spitzer scan have spotted it? Maybe not, if it's tenuous. But also, how come this stuff is falling off Phoebe? With Enceladus, there are the Fountains to scoosh water off the moon, but there's no indication Phoebe is active.

October 08, 2009 12:12 AM  
Blogger PeteY said...

Oops, sorry, meteoritic impacts. Should check before I post. Still, though, not everything that gets whacked with meteors leaves a significant dust trail. Most moons don't have rings, do they?

October 08, 2009 12:22 AM  
Anonymous constant gina said...

There is so much more in the universe I would love to find out more about..!!!

October 08, 2009 4:47 PM  
Blogger Oliver said...

Paul, I can't believe that you fell for the "we never saw it before" line. It wasn't there before! It's clearly the result of Saturn's ill judged decision to take sides in the war that started with the preemptive strike of our valiant Venusian allies on the marshalling yards of the Jovian fleet; as a result the contracting giga-vortiplon has been unleashed...
Now it is time for us to do our part by revealing and destroying the insidious volatiles with which the outer system has been polluting our precious co-planetary companion, Luna. Let the bombardment of Cabeus commence!

October 09, 2009 9:01 AM  
Blogger Paul McAuley said...

Pete, How it works, I think, is that Phoebe is a very small moon, so most of the debris from impacts escapes its gravity well and goes into orbit around Saturn. In the case of other, bigger moons, most of the debris falls back to the surface (although Rhea seems to have rings orbiting it, rather than orbiting Saturn). I imagine that the Spitzer team are looking for other ghost rings - this one is very hard to see, and wasn't in any way predicted to be there. Wouldn't be surprised if one or more of the smaller moons of Uranus or Neptune are corbiting with ghopst rings of their own debris. Kuiper Belt Objects may have rings too - Pluto almost certainly does.

Oliver - Is the ring going to turn into cosmic string and contract and slice Saturn in two like a cheesecutter? And now we've hit Cabeus, what kind of counterstrike can we expect? What have we started?

October 09, 2009 1:55 PM  
Blogger Oliver said...

From what I saw this morning the Cabeus assault was swatted aside; we are meddling with powers beyond our control...

October 09, 2009 2:42 PM  
Blogger Paul McAuley said...

It was kind of suspicious, wasn't it, that the LCROSS target was switched at more or less the last minute... What didn't they want to hit?

October 09, 2009 4:14 PM  

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