It’s been a good couple of weeks for Solar System news, it seems. After the recent excitement about traces of water on the Moon, this week a team has published (Verbiscer et al.) their discovery of a new ring around Saturn.
Here’s an artist’s impression of the ring, shown to scale with Saturn and its more familiar ring system: as you can see, they’re pretty dwarfed by the newcomer.
Artist's impression of the Phoebe Ring with Saturn to scale. Credit: NASA/JPL-Caltech/Keck.
The position of the ring is in line with the orbit of Phoebe, one of Saturn’s moons, so it looks as though the ring is made up of dust particles thrown up by comet impacts on Phoebe’s surface. Corroborating this view is the fact that the height of the ring when viewed side on - about 250,000 km - matches the range of heights which Phoebe reaches during the course of its orbit.
The ring was discovered in infrared images taken by the Spitzer space telescope. In Figure 1 of the paper you can make out the ring as the bright horizontal band across the middle of, most prominently, the image labelled ‘MIPSON’ in red. The reason it’s taken so long to discover (it’s almost exactly 400 years since Galileo spotted the first rings round Saturn) is that the particles which make it up are very finely distributed: probably less than twenty grains per cubic kilometre of space. The total mass of the ring is more than 300 billion kg which gives an idea of how vast a volume the thing takes up (though it’s quite low compared to the denser inner rings).
The large width of the ring is due mainly to radiation pressure from the Sun. The light impacting against the grains gives them kicks which slightly alter the shape of their orbits. Many grains each receiving slightly different perturbations leads to the ring spreading out along the direction from which the sunlight in coming.
There’s a long-standing mystery which this new ring could solve, too. One of Saturn’s other moons, Iapetus, has long been known to have a two-tone colouration: one side of it is much darker than the other. A popular suggestion to explain this has been that the dark material is made of up particles which have fallen into the moon’s orbit from somewhere farther out and slowly coated one half of it. (It would have to be the dark side that was formed this way and not the light because the dark hemisphere is the ‘front’ edge as Iapetus moves through its orbit, and so would pick up more particles as it ploughs through them.)
Saturn's moon Iapetus, with the edge of its 'dark side' visible. Credit: NASA
Verbiscer et al. suggest this new ring as a source for such particles. They cite some recent studies which found similarities in the appearance and spectra of Iapetus’s dark patch with the surface of Phoebe, which certainly makes the explanation an attractive one. Appropriately enough these observations were carried out using the Cassini spacecraft, named after seventeenth-century astronomer Giovanni Domenico Cassini. The discovery of Iapetus was one of Cassini’s many contributions to astronomy, and he was the first to suggest that its apparent changes in brightness were due to its having a dark and a light side.
The Nature website also has a news article on the discovery.
Reference
Verbiscer AJ, Skrutskie MF, & Hamilton DP (2009). Saturn’s largest ring. Nature PMID: 19812546
