First observations of the Phoebe ring in optical light

Abstract

The Phoebe ring, Saturn’s largest and faintest ring, lies far beyond the planet’s well-known main rings. It is primarily sourced by collisions with Saturn’s largest irregular satellite Phoebe, perhaps through stochastic macroscopic collisions, or through more steady micrometeoroid bombardment. The ring was discovered with the Spitzer Space Telescope at 24μm and has a normal optical depth of ∼2×10−8 (Verbiscer, A.J., Skrutskie, M.F., Hamilton, D.P. [2009]. Nature 461, 1098–1100). We report the first observations of sunlight scattered off the Phoebe ring using the Cassini spacecraft’s ISS camera at optical wavelengths. We find that material between ≈130 and 210 saturnian radii (RS) from the planet produces an I/F of 1.7±0.1×10-11 per RS of the line-of-sight distance through the disk. Combining our measurements with the Spitzer infrared data, we can place constraints on the ring-particles’ light-scattering properties. Depending on the particles’ assumed phase function, the derived single-scattering albedo can match either photometric models of Phoebe’s dark regolith or brighter sub-surface material excavated by macroscopic impacts on Phoebe.