The jumble of fragments that make up Saturn’s rings is
understood to be a product of tidal disruption of a moon or
planetesimal, but moons can contribute material to rings
in other ways as well, as seen in the case of Jupiter’s ring
system. Collisions may play a role, but other, less obvious
contributors are also present. The brightest of Jupiter’s
rings is a narrow strand only 6,500 km across, consisting of
material from Metis and Adrastea (see Figure 11.29). These
two moons orbit in Jupiter’s equatorial plane, and the ring
they form is thin. Beyond the main ring, however, are the
very different gossamer rings, so called because they are
extremely tenuous. The gossamer rings are formed of dust
from the moons Amalthea and Thebe. Unlike the main ring,
the gossamer rings are rather thick; the inner gossamer
ring, associated with Amalthea, is actually located within
the outer gossamer ring, which is formed of material from
Thebe. These rings are so thick because the orbits of the
moons that supply the ring material are slightly tilted with
respect to Jupiter’s equatorial plane. The orbital planes of
these moons wobble over the course of only a few months
(recall from Chapter 2 that the wobble of the orbital plane
of Earth’s Moon takes almost 19 years). The gossamer rings,
made up of material from these wandering moons, are
spread as far below and above Jupiter’s equatorial plane as
the orbits of the satellites that form them.
The innermost ring in Jupiter’s system, called the Halo
Ring, consists mostly of material from the main ring. As the
dust particles in the main ring drift slowly inward toward
the planet, they become electrically charged and are pulled
into this rather thick torus, or doughnut-shaped ring, by
electromagnetic forces associated with Jupiter’s powerful
magnetic field.
Moons also can contribute ring material through volcanism.
Volcanoes on Jupiter’s moon Io continually eject sulfur
particles into space, many of which drift inward under
the influence of pressure from sunlight and find their way
into the Jupiter ring. The particles in Saturn’s E Ring are ice
crystals ejected by cryovolcanism from the moon Enceladus,
which is located in the very densest part of the E Ring
(see Figure 11.9 and Math Tools 11.3).