CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 541
Visual-wavelength image
from Voyager 2Neptune in 19891996 20021998Visual-wavelength
images from
Hubble Space TelescopeGreat Dark Spot■ Figure 24-13
Because Neptune’s equator is inclined almost 29 degrees in its orbit, it
experiences seasons. Because of the planet’s orbital period, each season
lasts about 40 years. Since Voyager 2 visited in 1989, spring has come to
the southern hemisphere, and the weather has clearly changed, which is
surprising because sunlight on Neptune is 900 times dimmer than on Earth.
(NASA, L. Sromovsky, and P. Fry, University of Wisconsin-Madison.)■ Figure 24-14
(a) This visual-wavelength image of Neptune was made through fi lters that
make belts of methane stand out. (NASA and Erich Karkoschka) (b) The infrared
image at right was recorded using one of the Keck 10-meter telescopes and
adaptive optics. The location of methane cloud belts around the planet are
evident. (U. C. Berkeley/W. M. Keck Observatory)
Visual InfraredTh e same observations that helped defi ne the interior of
Uranus can be applied to Neptune. Models suggest that the inte-
rior contains a small heavy-element core, surrounded by a deep
mantle of slushy or solid water mixed with heavier material hav-
ing a chemical composition resembling rock. Neptune’s magnetic
fi eld is a bit less than half as strong as Earth’s and is tipped 47°
from the axis of rotation. It is also off set 55 percent of the way to
the surface (Figure 24-7). As in the case of Uranus, Neptune’s
fi eld is probably generated by the dynamo eff ect acting in the
conducting fl uid mantle rather than in the planet’s core.
Neptune has more internal heat than Uranus, and part of
that heat may be generated by radioactive decay in the minerals
in its interior. Some of the energy also may be released by denser
material falling inward, including, as in the case of Uranus, dia-
mond crystals formed by the disruption of methane.
Neptune is a tantalizing world just big enough to be imaged
by the Hubble Space Telescope but far enough away to make it
diffi cult to study. Th e data from Voyager 2 revealed one detail
that had been nearly undetectable from Earth—rings.The Rings of Neptune
Astronomers on Earth saw hints of rings earlier when Neptune
occulted stars, but the rings were not fi rmly recognized until
Voyager 2 fl ew past the planet in 1989.
Look again at The Rings of Uranus and Neptune
on pages 534–535 and compare the rings of Neptune with those
of Uranus. Notice two additional points:
Neptune’s rings, named after the astronomers involved in
the discovery of the planet, are similar to those of Uranus
but contain more small dust particles that forward-scatter
light.
Also notice a new way that rings can interact with moons:
One of Neptune’s moons produces short arcs in the outermost
ring, and a similar arc has been found in Saturn’s rings.
Like the rings of Uranus, Saturn, and Jupiter, the particles
observed now in Neptune’s rings can’t be primordial. Th at is,
they can’t have lasted in their present form since the formation of
Neptune. Evidently, impacts on moons occasionally scatter
debris through the satellite system, and some of it falls into the
places where the orbits of ring particles are most stable among
the orbits of the moons.