CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 543
Neptune’s magnetic fi eld is peculiar, and its moons and rings also
deserve careful attention.
You can assume that Neptune formed from the solar nebula
much as did Uranus. Like Uranus, Neptune’s hazy atmosphere,
marked by changing cloud patterns, hides a mantle of partially
frozen ices where astronomers suspect the dynamo eff ect generates
its off -center magnetic fi eld. Inside that mantle is a denser core.
Neptune, like Jupiter and Saturn but unlike Uranus, has signifi -
cant amounts of heat fl owing out to space from its interior.
Neptune’s satellite system suggests a peculiar history. Triton,
the largest moon, revolves around Neptune in a retrograde orbit,
while Nereid’s long-period orbit is highly elliptical. Th ese orbital
oddities suggest that the satellite system may have been disturbed
by an encounter with a large planetesimal or during the capture of
Triton. You have seen evidence in other satellite systems of impacts
with large objects, so such an event is not unreasonable.
A number of smaller moons orbit Neptune near its ring
system. Because the rings are bright in forward scattering, you
can conclude that they contain some dust, and the shepherding
of small satellites must confi ne their width and produce the
observed arcs. As with the other Jovian planets, such rings could
not have survived since the origin of the solar system, so the rings
must be occasionally supplied with fresh particles probably gen-
erated by the impacts of meteorites and comets on Neptune’s
moons. Once again, the evidence of major impacts in the solar
system’s history assures you that such impacts do occur.SCIENTIFIC ARGUMENT
Why is Neptune blue but its clouds white?
To solve this problem you must build a scientifi c argument that
follows a process step by step. When you look at something, you
really turn your eyes toward it and receive light from the object.
When you look at Neptune, the light you receive is sunlight that is
refl ected from various layers of Neptune and journeys to your eyes.
Because sunlight contains a distribution of photons of all visible
wavelengths, it looks white to human eyes, but sunlight entering
Neptune’s atmosphere must pass through hydrogen gas that con-
tains a small amount of methane. While hydrogen is transparent,
methane is a good absorber of longer wavelengths, so red photons
are more likely to be absorbed than blue photons. Once the light
is scattered from deeper layers, it must run this methane gauntlet
again to emerge from the atmosphere, and again red photons are
more likely to be absorbed. The light that finally emerges from
Neptune and eventually reaches your eyes is poor in longer wave-
lengths and thus looks blue.
The methane-ice-crystal clouds lie at high altitudes, so sunlight
does not have to penetrate very far into Neptune’s atmosphere to
refl ect off the clouds, and consequently it loses many fewer of its
red photons. The clouds look white.
This discussion shows how a careful, step-by-step analysis of a
natural process can help you better understand how nature works.
For example, build a step-by-step argument to answer the follow-
ing: Where does the energy come from to power Triton’s surface
geysers?Th e surface of Triton contains evidence that the icy moon
has been active recently and may still be active. You would expect a
moon located so close to the Kuiper belt to have lots of craters, but
Triton has few. Th e average age of the surface is no more than
100 million years and may be even less. Some process has erased
older craters. Evidence of geological activity includes long linear
features that appear to be fractures in an icy crust, and some roughly
round basins that appear to have been fl ooded time after time by
liquids from the interior (Figure 24-16b). Triton is much too cold
for the liquid to be molten rock or even liquid water. Rather, the
fl oods must have been composed of water that contained agents
such as ammonia, which would lower the freezing point of the
liquid. It isn’t possible to say at present whether Triton is still active,
but 100 million years isn’t very long in the history of the solar sys-
tem. Triton may still suff er periodic eruptions and fl oods.
Another form of activity on Triton leaves dark smudges visible
in the bright nitrogen ices near its south pole. Th ese appear to be
caused by nitrogen ice beneath the crust. Warmed slightly by the
sun, the nitrogen ice can change from one form of solid nitrogen to
another and release heat that can vaporize some of the nitrogen.
Heat rising from the interior can also vaporize nitrogen. Th is nitro-
gen vapor vents through the crust, forming nitrogen gas geysers up
to 8 km (5 mi) high. Th e venting gas may carry dark material from
below the ice that falls to the surface to form the dark smudges.
Another possibility is that methane is carried along with the nitro-
gen, and sunlight converts some of the methane into dark organic
material, which falls to the surface.
Active worlds must have a source of energy, so you are probably
wondering where Triton gets its energy. Triton is big enough to
retain some thermal energy from low-level radioactivity in its inte-
rior. Th at may be enough to melt some ices and cause fl ooding on
the surface. Th e nitrogen geysers may be powered partly by heat
from the interior and partly by sunlight. In fact, Triton is very effi -
cient at absorbing sunlight. Its thin atmosphere does not dim sun-
light, and its crust is composed of ices that are partially transparent
to light. As the light penetrates into the ice and is absorbed, it
warms the ices. However, the crust is not very transparent to infra-
red radiation, so the heat is trapped in the ice. So, the crust of
Triton appears to be heated, in part, by an icy form of the green-
house eff ect.
Th is low-level heating would not be enough to erase nearly
all of Triton’s craters, so some planetary astronomers wonder if
Triton might have been captured by Neptune relatively recently,
meaning, within the last billion years. Tidal forces from such a
capture would have caused enough tidal heating to melt Triton
and totally resurface it. Enough of that heat may remain to keep
Triton active to this day.
The History of Neptune
Can you tell the story of Neptune? In some ways it seems to be
a simpler, smaller version of Jupiter and Saturn, but, like Uranus,