Geology Reference
In-Depth Information
10.5 Summary
The giant planets Uranus and Neptune have extensive
atmospheres surrounding liquid and frozen water, along
with ammonia, methane, and other volatiles, all surround-
ing large rocky cores. Both planets have ring systems and
myriad satellites. Unlike the giant planets Jupiter and
Saturn and their systems of moons and rings that have
been explored by orbiters (Galileo and Cassini, respec-
tively), spacecraft data for the Uranus and Neptune
systems come only from the brief
flybys of the Voyager
spacecraft. Thus, our knowledge of this part of the Solar
System is much more limited.
Of the
five large moons of Uranus, Miranda, Umbriel,
Titania, and Oberon are in synchronous rotation and appear
to be in
uenced by tidal heating through their interactions
with each other and with Uranus. The innermost moons,
such as Miranda, show evidence of the greatest amount of
internal heating, while the outermost, Oberon, preserves the
greatest amount of impact cratering. All but Oberon display
darker trailing hemispheres in comparison with the leading
hemispheres. This is attributed to their immersion in the
Uranus magnetosphere, in which radiation and implanta-
tion of ions occur preferentially on the trailing hemispheres
because the speeds of the satellite orbits are less than the
speeds within the magnetosphere.
About half of Neptune
'
s known moons are in retrograde
orbits, suggesting that they are captured objects. The
largest of these, Triton, is about the size of Earth
'
s Moon
and exhibits active geysers, thought to be driven by a
“
Figure 10.15. An image of Proteus, the 416 km in diameter moon of
Neptune, showing an impact scar named Pharos that is about 230 km
across (NASA Voyager PIA00062).
Although Triton
'
is tenuous atmosphere is extremely thin
(exerting a surface pressure of less than 0.01mbar), it is
suf
cient to generate winds. Analysis of Titan
'
s cloud
patterns and the dark plumes left by the geysers suggest
that the prevailing winds at 1
-
3 km above the surface are
toward the east, while higher-altitude (~8 km) winds are
toward the west.
Proteus, Neptune
'
s second largest moon at about
416 km in diameter, was discovered in Voyager images
(
Fig. 10.15
) and is one of the darkest objects seen in the
Solar System. It resides just outside Neptune
'
s ring system
and orbits very rapidly, taking only 1.1 Earth days to make
one complete orbit. Despite its relatively large size, the
overall shape of Proteus is irregular and the surface is
rugged with topographic relief of 20 km. Geophysical
models suggest that it is about the largest satellite that
can retain an irregular shape without collapsing gravita-
tionally into a sphere. Crater-like features are seen, includ-
ing one structure more than 200 km across.
Nereid is the third largest satellite of the Neptune
system and has a diameter of about 340 km. It, too, was
imaged by Voyager but not in resolution suf
cient to
characterize its surface. Nereid is in a highly inclined
orbit and takes 359 days for one complete orbit around
Neptune.
effect in which solar energy penetrates ice
on the surface and heats volatiles; pressure of the
expanded gasses eventually exceeds the strength of the
ice, venting explosively to the surface, along with
entrained dark (carbonaceous) material. Above the sur-
face the geyser plumes are caught by winds in Triton
greenhouse
”
s
tenuous atmosphere and are carried down wind, leaving
dark streak deposits on the surface. Cryovolcanism is
suggested on Triton, as evidenced by caldera-like depres-
sions with smooth
oors.
Many planetary scientists consider Triton to be an ana-
log for what will be seen when New Horizons reaches
Pluto and returns images of this dwarf planet and its large
moon, Charon. All three objects occupy the same general
zone in the Solar System (in fact, Pluto
'
s orbit carries it
inside the orbit of Neptune at times), and all three have
similar densities and surface spectral properties.
'
