Neptune also has rings. Earth-based observations showed only
faint arcs instead of complete rings, but Voyager 2's images showed
them to be complete rings with bright clumps.
The inner ring, LeVerrier, is uniform
but the outer ring, Adams, contains three visible arcs.
The existence of arcs is very difficult to
understand because the laws of motion would predict that arcs spread out into
a uniform ring over very short timescales. The gravitational effects of Galatea,
a moon just inward from the ring, is believed to confine the arcs.
Voyager 2 acquired this image as it encountered Neptune in Aug. 1989.
Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown. Even though Neptune's rings are dim and very thin, Voyager still found a surprise in one of them. One of the rings appears to have a curious twisted structure (left). This ring is twisted, much like one of Saturn's many rings. Scientists are still trying to get an exact explanation about what causes the twists.
The Moons of Neptune
Neptune has 8 known moons; 7 small ones (all smaller than about 200 km
is radius) and Triton, nearly half as big as Earth's Moon!
Triton was
discovered in 1846 only a few weeks after the discovery of Neptune itself.
Triton's orbit is retrograde. It is the only large moon to orbit "backwards".
Triton could not have condensed from
the primordial Solar Nebula in this configuration; it must have
formed elsewhere and later been
captured by Neptune (perhaps involving a collision with another
now shattered Neptunian moon). A capture scenario could account
not only for Triton's orbit but also for the unusual orbit of
Neptune's other moon, Nereid, which has the most highly
eccentric of any planet or satellite in the solar system.
There are very few craters visible on Triton; indicating
the surface is relatively
young. Almost the entire southern hemisphere is covered with an
"ice cap" of frozen nitrogen and methane (above).
The most interesting (and totally unexpected) features of this
moon are the ice volcanos (right). The eruptive
material is probably liquid nitrogen, dust, or methane
compounds from beneath the surface. One of Voyager's
images shows an actual plume rising 8 km above the
surface and extending 140 km "downwind" (right). Triton,
Io and Venus are the only bodies in the solar
system besides Earth that are known to be volcanically
active at the present time (though Mars clearly was in the past).
Because of its retrograde orbit, tidal interactions between Neptune and Triton remove energy from Triton thus lowering its orbit. At some very distant future time it will either break up (perhaps forming a ring) or crash into Neptune.