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?