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The Phoebe ring's huge extent dwarfs the main rings. Inset: 24 micron
Spitzer image of part of the ring.

On 6 October 2009, the discovery of a tenuous disk of material in the
plane of and just interior to the orbit of Phoebe was announced. The disk
was aligned edge-on to Earth at the time of discovery. This disk can be
loosely described as another ring. Although very large (the apparent size
of two full moons as seen from Earth), the ring is virtually invisible -
it was discovered using NASA's infra-red Spitzer Space Telescope.[62] The
ring was seen over the entire range of the observations, which extended
from 128 to 207 times the radius of Saturn,[63] with calculations
indicating that it may extend outward up to 300 Saturn radii and inward to
the orbit of Iapetus at 59 Saturn radii;[64] Phoebe orbits the planet at
an average distance of 215 radii. The ring is about 20 times as thick as
the diameter of the planet.[65] Since the ring's particles are presumed to
have originated from impacts (micrometeoroid and larger) on Phoebe, they
should share its retrograde orbit,[64] which is opposite to the orbital
motion of the next inner moon, Iapetus. This ring lies in the plane of
Saturn's orbit, or roughly the ecliptic, and thus is tilted 27 degrees
from Saturn's equatorial plane and the other rings. Phoebe is inclined by
5° with respect to Saturn's orbit plane (often written as 175°, due to
Phoebe's retrograde orbital motion), and its resulting vertical excursions
above and below the ring plane agree closely with the ring's observed
thickness of 40 Saturn radii.

The existence of the ring was proposed in the 1970s by Steven Soter.[64]
The discovery was made by Anne J. Verbiscer and Michael F. Skrutskie (of
the University of Virginia) and Douglas P. Hamilton (of the University of
Maryland, College Park).[63][66] Verbiscer, Skrutskie and Hamilton had
been graduate students together at Cornell University.[67]

Ring material migrates inward due to reemission of solar radiation,[63]
and would thus strike the leading hemisphere of Iapetus. Infall of this
material causes a slight darkening and reddening of the leading hemisphere
of Iapetus (similar to what is seen on the Uranian moons Oberon and
Titania) but does not directly create the dramatic two-tone coloration of
that moon.[68] Rather, the infalling material initiates a positive
feedback thermal self-segregation process of ice sublimation from warmer
regions, followed by vapor condensation onto cooler regions. This leaves a
dark residue of "lag" material covering most of the equatorial region of
Iapetus's leading hemisphere, which contrasts with the bright ice deposits
covering the polar regions and most of the trailing
hemisphere.[69][70][71]