Such is the case for Saturn’s bright ring system. The
bright rings appear bright because they reflect about 60
percent of the sunlight falling on them. Scientists suspected
from their brightness alone that they are made of water ice,
and spectral observations
confirmed this suspicion,
clearly showing the distinct
signature of water. A
slight reddish tint to the
rings indicates that they are not made of pure ice but must
be contaminated with other materials, such as silicates.
The icy moons around Saturn or the frozen comets in the
outer Solar System could easily provide this contaminating
material.
Saturn’s bright rings are the brightest in the Solar System.
In stark contrast, the rings of Uranus and Neptune
are among the darkest objects known in the Solar System.
Only 2 percent of the sunlight falling on them is reflected
back into space, making the ring particles blacker than
coal or soot. No silicates or similar rocky materials are this
dark. The rings of Uranus
and Neptune are likely
composed of organic materials
and ices that have
been radiation darkened
by high-energy, charged particles in the planets’ magnetospheres.
(Radiation processing of organic ices, such as
methane, blackens the ice by releasing carbon from the ice’s
molecular structure.) Jupiter’s rings are neither as bright as
Saturn’s nor as dark as those of Uranus and Neptune, suggesting
that they may be rich in dark silicate materials, like
the innermost of Jupiter’s small moons