Chapter 10 The Outer Planets
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5. Titan’s features are similar in shape to those found on the Earth. More or less, material in
the atmosphere appears to “rain” out of clouds and drain across the surface, reshaping rocks
and forming rivers and lakes. Winds push the dunes around. The “volcanoes” erupt gases and
material which resurfaces the moon. However, all of this occurs at much lower temperatures
than for the terrestrial analogs. The rocks are made of very cold ice. At such temperatures, ice
is as hard as steel or rock on Earth. The liquid in the lakes is made of hydrocarbons—
ethane—not water (oxygen and hydrogen). Most different of all, the volcanoes do not erupt
hot, molten rock, but are eject more or less cold molten hydrocarbons and water (though they
are hotter than the surrounding surface and atmosphere). The “volcanoes” are more like
Earth’s geysers than its volcanoes.
6. The Earth’s sky is blue because small particles in the atmosphere scatter short wavelengths
of light (blue light) more effectively than they scatter longer wavelengths (red light). The
effect is most pronounced at sunset: the Sun appears reddish, because blue light is scattered
out of the line of sight, but the opposite side of the sky looks blue (scattered sunlight). The
situation is similar on Uranus, but the scattering particles are methane crystals, not dust, and
that causes an important difference. As seen from outside, ice crystals of methane in the
atmosphere scatter and reflect blue sunlight, making the atmosphere appear blue. However,
the ice crystals also strongly absorb the red photons, unlike the scattering particles in Earth’s
atmosphere. Whereas the Sun appears redder from the Earth’s surface because blue light is
scattered out of the line of sight, from deep in Uranus’ atmosphere, the Sun should actually
appear bluer than it would otherwise because the red photons have been absorbed. However, it
seems unlikely that one would be able to see the Sun itself through the methane-ice clouds
and haze. The scattering of blue light by the ice crystals suggests that at least in the upper
atmosphere, the sky would appear blue from inside as well—but again, hazy and cloud-filled.
7. Uranus’s moons orbit its equator. Though tilted the system is extremely regular. The
moons are made of more or less the same kinds of material as other gas giant moons (and gas
giants, minus the hydrogen and helium)—frozen volatiles and rock. Together these details
imply that, similar to how the Earth’s moon was formed, material that splashed out of Uranus
during a major collision coalesced into the orbiting moons. If the moons had formed prior to
the collision, tidal forces and the angular momentum of the Uranus-moon systems would have
opposed the forces in the collision trying to “tip” the planet (think of tilting a gyroscope).
Some of the moons might have been ejected and others could have irregular orbits. [However,
if Uranus was tipped slowly, through gravitational interaction with Saturn, the same tidal
forces might have helped also slowly tip the moons’ orbits.]
8. Uranus and Neptune are less massive than Jupiter and Saturn, and have lower escape
velocities. Even though these planets are colder, the lighter gases can still escape their weaker
gravitational fields.
9. Wind systems on the gas giants are driven by heat escaping from the interior; the bands
result from the planets’ rapid rotation and the Coriolis effect. Neptune, being more massive,
should have more internal heat than Uranus, but it is also farther from the Sun, so it is colder
at the surface. This large temperature difference drives the circulation more strongly than it