Figure 1: Artist’s conception of Saturn’s giant Phoebe ring as seen in infrared light. Credit: NASA/JPL/Space Science Institute.
Saturn’s giant Phoebe ring, discovered in 2009, is believed to be formed by particles thrown off from Phoebe, a distant moon of Saturn whose surface is as dark as coal. When it was discovered, the Phoebe ring was observed to lie between 128 and 207 Saturn radii from the planet, with a vertical extent of 40 Saturn radii. A recent study published by Hamilton et al. (2015) using new observations from NASA’s Wide-field Infrared Survey Explorer (WISE) mission found that the Phoebe ring is much larger than previously measured.
WISE acquired an infrared image of the entire Phoebe ring which shows the ring spanning between 100 and 270 Saturn radii from the planet. This is well beyond the orbit of Phoebe which goes around Saturn between 180 and 270 Saturn radii from the planet. The Phoebe ring covers an area of sky ~7,000 times larger than Saturn itself. It is likely that small moons with Phoebe-like orbital inclinations that orbit Saturn more distant than Phoebe itself are additional contributors of ring material. If many of these small moons turn out to be undiscovered 100 m sized or kilometre-sized objects, then they are likely to be an important additional source of ring material.
Figure 2: Measured profile of Saturn’s giant Phoebe ring. Ring flux is clearly detectable to at least 270 Saturn radii, well beyond Phoebe’s maximum distance of 250 Saturn radii from Saturn. All ring profiles agree well inward to about 100 Saturn radii at which point scattered light from Saturn becomes problematic. Hamilton et al. (2015).
Hamilton et al., “Small particles dominate Saturn’s Phoebe ring to surprisingly large distances”, Nature 522, 185-187 (11 June 2015)