Observations from the Spitzer Space Telescope have revealed the largest-known planetary ring in the Solar System. The newly discovered ring is associated with Saturn’s distant moon Phoebe, which orbits the giant planet at a radius of about 13 million kilometers, or about 200 times the radius of Saturn. Until now, the largest-known planetary rings were Jupiter’s gossamer rings and Saturn’s E ring - sheets of dust that extend to about 5-10 times the radius of their respective planets. Reporting the discovery in this week’s Nature, UVa’s Anne Verbiscer, Mike Skrutskie and their colleague Douglas Hamilton at the University of Maryland, College Park, also present numerical simulations that show how repeated impacts on Phoebe can keep the ring supplied with dust.

This faint but enormous ring may also explain a longstanding mystery: the two-tone coloration of another Saturnian moon, Iapetus. This moon’s leading hemisphere is significantly darker than its trailing hemisphere, leading to suggestions that the front face might be coated with dust spiralling in from Saturn’s darker outer moons, including Phoebe. Verbiscer and colleagues calculate that, over the history of the Solar System, material from the ring could have supplied Iapetus’s front face with a blanket of dark dust meters thick.