524 PART 4^ |^ THE SOLAR SYSTEM
the atmosphere, falls as rain, and drains over the surface, washing dark,
organic material into drainage channels and on into the lowlands.▶ (^) The methane in Titan’s atmosphere can be destroyed by sunlight, so it
must be continuously replenished. It probably vents from the icy crust
and may form methane volcanoes. Lakes of liquid methane have been
found in the moon’s polar regions.
▶ (^) Some of the larger moons, such as Tethys, have old, dark, cratered
surfaces with cracks and smoothed areas that suggest past activity.
Enceladus, a rather small moon, is the most refl ective object in the
solar system and has large smooth areas, so it must have been very
active. It orbits in a resonance with the moon Dione, and that may
cause tidal heating inside Enceladus.
▶ (^) Water vapor has been found above the south pole of Enceladus where
water vents into space and forms ice crystals, which resupply Saturn’s
E ring.
▶ (^) The moon Iapetus has a bright icy surface on its trailing side, but its
leading side is coated with very dark material, possibly debris from
Phoebe. In addition, Iapetus has a long equatorial ridge higher than
Mount Everest on Earth. The ridge and the moon’s equatorial bulge may
have formed when the moon was young, molten, and spinning rapidly.
▶ (^) The origin and evolution of Saturn’s moons is not as clear as for
Jupiter’s Galilean moons. Orbital interactions and impacts have been
important to these moons.
Review Questions
- Why is Jupiter more oblate than Earth? Do you expect all Jovian
planets to be oblate? Why or why not? - How do the interiors of Jupiter and Saturn differ? How does this affect
their magnetic fi elds? - What is the difference between a belt and a zone?
- How can you be certain that Jupiter’s ring does not date from the
formation of the planet? - If Jupiter had a satellite the size of our own moon orbiting outside
the orbit of Callisto, what would you predict for its density and surface
features? - Why are there no craters on Io and few on Europa? Why should you
expect Io to suffer more impacts per square kilometer than Callisto? - Why are the belts and zones on Saturn less distinct than those on
Jupiter? - If Saturn had no moons, what do you suppose its rings would look
like? - Where did the particles in Saturn’s rings come from?
- How can Titan keep an atmosphere when it is smaller than airless
Ganymede? - If you piloted a spacecraft to visit Saturn’s moons and wanted to land
on a geologically old surface, what features would you look for? What
features would you avoid? - Why does the leading side of some satellites differ from the trailing
side? - How Do We Know? Why would you expect research in archaeology to
be less well funded than research in chemistry?
Discussion Questions
- Some astronomers argue that Jupiter and Saturn are unusual, while
other astronomers argue that all solar systems should contain one or
two such giant planets. What do you think? Support your argument
with evidence. - Why don’t the Terrestrial planets have rings?
Problems
- What is the maximum angular diameter of Jupiter as seen from Earth?
What is the minimum? Repeat this calculation for Saturn and Titan.
(Hint: Use the small-angle formula, Chapter 3.) - The highest-speed winds on Jupiter are in the equatorial jet stream,
which has a velocity of 150 m/s. How long does it take for these winds
to circle Jupiter? - What are the orbital velocity and period of a ring particle at the outer
edge of Jupiter’s ring? At the outer edge of Saturn’s A ring? (Hints: The
radius of the edge of the A ring is 136,500 km. Also, see Chapter 5.) - What is the angular diameter of Jupiter as seen from the surface of
Callisto? (Hint: Use the small-angle formula, Chapter 3.) - What is the escape velocity from the surface of Ganymede if its mass is
1.5 1023 kg and its radius is 2.6 103 km? (Hint: See Chapter 5.) - If you were to record the spectrum of Saturn and its rings, you would
fi nd light from one edge of the rings redshifted and light from the
other edge blueshifted. If you observed at a wavelength of 500 nm,
what difference in wavelength should you expect between the two
edges of the rings? (Hints: See Problem 3 and Chapter 7.) - What is the difference in orbital velocity between particles at the
outer edge of Saturn’s B ring and particles at the inner edge of the B
ring? (Hint: The outer edge of the B ring has a radius of 117,500 km,
and the inner edge has a radius of 92,000 km.) - What is the difference in orbital velocity between the two coorbital
satellites if the semimajor axes of their orbits are 151,400 km
and 151,500 km? The mass of Saturn is 5.7 1026 kg. (Hint: See
Chapter 5.)
Learning to Look
- This image shows a segment of the
surface of Jupiter’s moon Callisto.
Why is the surface dark? Why are some
craters dark and some white? What does
this image tell you about the history of
Callisto? - The Cassini spacecraft recorded this
image of Saturn’s A ring and the Encke
Gap. What do you see in this photo that
tells you about processes that confi ned
and shape planetary rings?
NASA/JPLNASA/JPL/Space Science Institute