Chapter 7 The Outer Planets

214 Chapter 7 The Outer Planets
FIGURE 7-37 Neptune’s Rings Two main
rings are easily seen in this view alongside overexposed
edges of Neptune. Careful examination
also reveals a faint inner ring. A fainter-still sheet of particles, whose
outer edge is located between the two main rings, extends inward
toward the planet. (NASA)
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satellite and planet could form together but rotate in opposite
directions. Indeed, only a few of the small outer satellites
of Jupiter and Saturn have retrograde orbits, and these bodies
are probably captured asteroids. Some scientists have
therefore suggested that Triton may have been captured 3 or
4 billion years ago by Neptune’s gravity.
Upon being captured, Triton was most likely in a highly
elliptical orbit. However, the tidal force the moon creates on
Neptune’s liquid surface would have made Triton’s orbit
more circular. Conversely, the tidal force created on Triton by
Neptune due to the moon’s changing distance to Neptune
would have caused the moon to stretch and flex, providing
enough energy to melt much of the satellite’s interior and
obliterate Triton’s original surface features, including craters.
Triton’s south polar region is shown in Figure 7-38. Note
that very few craters are visible. Calculations based on these
observations indicate that Triton’s present surface is about
100 million years old.
Triton does exhibit some surface features seen on other
icy worlds, such as long cracks resembling those on Europa
and Ganymede. Other features unique to Triton are quite
puzzling. For example, the top half of Figure 7-38 reveals a
wrinkled terrain that resembles the skin of a cantaloupe.
Triton also has a few frozen lakes like the one shown in
Figure 7-39. Some scientists have speculated that these lakelike
features are the calderas of extinct ice volcanoes. A mixture
of methane, ammonia, and water, which can have a
melting point far below that of pure water, could have
formed a kind of cold lava on Triton.
Voyager instruments measured a surface temperature of
36 K (–395ºF), making Triton the coldest world that our
probes have ever visited. Nevertheless, Voyager cameras did
glimpse two towering plumes of gas extending up to 8 km
above the satellite’s surface. These are apparently plumes of
nitrogen gas warmed by interior radioactive decay and
escaping through vents or fissures.
In the same way that our Moon raises tides on Earth,
Triton raises tides on Neptune. Whereas the tides on Earth
cause our Moon to spiral outward, the tides on Neptune
cause Triton (in its retrograde orbit) to spiral inward. Within
the next quarter of a billion years, Triton will reach the
Roche limit, the distance at which a planet creates tides on its
moon’s solid surface high enough to pull its moon apart.
Pieces of Triton will then literally float into space until the
entire moon is demolished! By destroying Triton, Neptune
South polar region
Canteloupe terrain
Dark, wind-blown
deposits
FIGURE 7-38 Triton’s South Polar Cap
Approximately a dozen high-resolution Voyager 2
images were combined to produce this view of
Triton’s southern hemisphere. The pinkish polar cap is probably made of
nitrogen frost. A notable scarcity of craters suggests that Triton’s surface
was either melted or flooded by icy lava after the era of bombardment
that characterized the early history of the solar system. (NASA)
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