502 PART 4^ |^ THE SOLAR SYSTEM
abcVisual-wavelength images■ Figure 23-5
(a) The main ring of Jupiter, illuminated from behind, glows brightly in this
visual-wavelength image made by the Galileo spacecraft while it was within
Jupiter’s shadow. (b) Digital enhancement and false color reveal the halo of
ring particles that extends above and below the main ring. The halo is just
visible in panel a. (c) Structure in the ring is probably caused by the gravi-
tational infl uence of Jupiter’s inner moons. (a. and b.: NASA; c. NASA/Johns
Hopkins University Applied Physics Lab/Southwest Research Institute)
enough not to break. However, a moon composed of separate
rocks and particles held together by their mutual gravity could
not survive inside a planet’s Roche limit. Tidal forces would
destroy such a moon. If a planet and its moon have the same
average densities, the Roche limit is at 2.44 times the planet’s
radius. Jupiter’s main ring has an outer radius of 130,000 km
(1.8 Jupiter radii) and lies inside Jupiter’s Roche limit. Th e rings
of Saturn, Uranus, and Neptune also lie within those planets’
respective Roche limits.
Now you can understand the dust in Jupiter’s ring. If a dust
speck gets knocked loose from a larger rock orbiting inside the
Roche limit, the rock’s gravity cannot hold the dust speck. And
the billions of dust specks in the ring can’t pull themselves
together to make a larger body—a moon—because of the tidal
forces inside the Roche limit.
You can also be sure that the ring particles are not old. Th e
pressure of sunlight and Jupiter’s powerful magnetic fi eld alter
the orbits of the particles, and they gradually spiral into the
planet. Images show faint ring material extending down toward
Jupiter’s cloud tops, and this is evidently dust specks spiraling
inward. Dust is also lost from the ring as electromagnetic eff ects
force it out of the plane of the ring to form a low-density halo
above and below the ring (Figure 23-5b). Yet another reason the
ring particles can’t be old is that the intense radiation around
Jupiter can grind dust specks down to nothing in a century or so.
For all these reasons, the rings seen today can’t be made up of
material that has been in the form of small particles for the entire
time since the formation of Jupiter.
Obviously, the rings of Jupiter must be continuously resup-
plied with new material. Dust particles can be chipped off rocks
ranging in size from gravel to boulders within the ring, and small
moons that orbit near the outer edge of the rings lose particles as
they are hit by meteorite impacts. Observations made by the
Galileo spacecraft show that the main ring is densest at its outer
edge, where the small moon Adrastea orbits, and that another small
moon, Metis, orbits inside the ring. Clearly these moons must be
structurally strong to withstand Jupiter’s tidal forces. Images from
the Voyager and Galileo probes also reveal much fainter rings,
called the gossamer rings, extending twice as far from the planet
as the main ring. Th ese gossamer rings are most dense at the orbits
of two small moons, Amalthea and Th ebe, more evidence that ring
particles are being blasted into space by impacts on the moons.
Besides supplying the rings with particles, the moons help
confi ne the ring particles and keep them from spreading out-
ward. You will fi nd that this is an important process in planetary
rings when you study the rings of Saturn later in this chapter.
Your exploration of Jupiter reveals that it is much more than
just a big planet. It is the gravitational and magnetic center of an
entire community of objects. Occasionally the community suf-
fers an intruder.