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Chapter 14 - Satellites
CHAPTER 14
SATELLITES
CHAPTER OUTLINE AND LECTURE NOTES
1. Kinds of Satellites
My primary reason for covering all satellites (except the Moon) in a single chapter was to
facilitate comparisons among similar bodies even if they orbit different planets.
2. General Properties of Satellites
The surfaces of many satellites, even some fairly small ones, show evidence of cracking, geyserlike activity and other kinds of internal activity, which must have been driven by internal energy
sources. The only three possible sources of internal energy seem to be gravitational potential
energy, radioactivity, and tidal heating. For some satellites, tidal heating is not currently
important but may have been in the past when the satellites’ orbits were different.
3. The Satellites of Mars
Even before Galileo and NEAR-Shoemaker obtained images of asteroids, Phobos (Figure 14.3
and 14.4) and Deimos (Figure 14.5) gave us a pretty good idea of what asteroids look like.
4. The Galilean Satellites
Io (Figure 14.7) and Europa (Figures 14.13) seem to be the only two satellites for which we are
clearly able to account for the reason for the activity shown at the surface. In these two cases,
the explanation is tidal heating. The Galileo images of Jupiter’s satellites can be viewed at
http://galileo.jpl.nasa.gov.
5. The Icy Moons of Saturn
The icy satellites of Saturn are so small that they should quickly have lost the energy produced
by the infall of the material from which they formed. Tidal heating is likely to be responsible
for the fact that some of the icy satellites of Saturn show evidence of internal activity while
others do not, but this explanation requires that the distances of the satellites have increased
since Saturn formed and that pairs of satellites have moved in and out of resonance with each
other.
6. Titan
Our knowledge of Titan changed dramatically when Cassini began to obtain radar images
(Figure 14.35) of the surface and especially when Huygens descended to the surface in January
2005. The Cassini and Huygens images present clear evidence for a system remarkably similar
to the Earth’s hydrological system but based on methane rather than water. You can learn the
latest from Titan at the Cassini website http://saturn.jpl.nasa.gov.
7. Satellites of Uranus
The satellites of Uranus show more evidence of internal activity than do those of Saturn. Again,
orbital resonances are the likely explanation.
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Chapter 14 - Satellites
8. Satellites of Neptune
The fact that we have any Voyager images of Neptune and its satellite Triton (Figure 14.45) is a
testimonial to the effort and ingenuity of engineers at the Jet Propulsion Laboratory. The
imaging system on the Voyager was designed primarily for observations at Jupiter. At Neptune,
the light level is only a few percent that at Jupiter. This required longer exposures, which
would have been smeared because the spacecraft moved during the exposure. The JPL
engineers solved the problem by commanding Voyager to rotate at just the rate needed to
compensate for the motion of the spacecraft. Even so, the longer exposure times meant that we
have many fewer images of Neptune and Triton than we do of Jupiter and the Galilean
satellites.
KEY TERMS
collision fragment — A satellite that probably is a fragment of a larger satellite broken apart by a
collision with a meteoroid.
corona — A type of surface feature of Uranus’s satellite Miranda. Coronae consist of parallel ridges
and troughs that produce a striped appearance.
plume — A rising column of gas over a hot region in the interior or atmosphere of a body.
regular satellite — Regularly spaced satellites with nearly circular orbits that form miniature “solar
systems” about their parent planets.
tidal heating — The frictional heating of the interior of a satellite as it is flexed and released by a
variable tidal force due to its parent planet.
volatile — An element or compound that vaporizes at low temperature. Water and carbon dioxide
are examples of volatiles.
ANSWERS TO QUESTIONS AND PROBLEMS
Conceptual Questions
1. A regular satellite
2. Its orbit may be very eccentric and highly inclined to the equator of the planet it orbits. Its orbit
may also be retrograde.
3. Masses were determined by how much the gravities of the satellites affected the motion of the
spacecraft.
4. Radioactive materials are concentrated in rocky material rather than icy material.
5. If the satellite is large and orbits close to a massive planet.
6. Io has the density of rock, so it must be mostly rock and little ice. Callisto has the density of
ice, so it must be mostly ice with little rock.
7. There are no impact craters on Io.
8. Tidal heating
9. Ice in the surface layer of Europa may flow to reduce the heights of crater walls and fill in
craters. Also, there may be geyser-like flows of water that cover old craters and then freeze.
10. Ganymede once may have had significant tidal heating in addition to radioactive heating. At
that time tidal flexing may have cracked Ganymede’s surface and allowed water to flood
portions of the surface.
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Chapter 14 - Satellites
11. They have surface cracks and smooth regions that appear to have been flooded by watery
material from within.
12. The diagram should show that as Iapetus is about to pass behind Saturn we see the bright
trailing side and when it emerges from behind Saturn we see the dark leading side.
13. Both atmospheres are made mostly of nitrogen. Titan’s atmosphere is colder and has a higher
pressure.
14. Solar radiation is absorbed by gases in Titan’s mesosphere.
15. The orange color is due to molecules formed in reactions that are triggered by solar ultraviolet
radiation.
16. There may be rain and bodies of liquid ethane. Dark tar-like hydrocarbons may cover the
surface. The solid surface may be ice rather than rock.
17. Coronae resemble the ridge and trough systems in the younger, brighter regions of Ganymede.
The rolling, cratered terrain resembles regions of Umbriel and several other satellites.
18. The tidal bulge due to Triton trails Triton’s orbital position because Triton orbits in the opposite
direction to the rotation of Neptune. Thus the tidal bulge pulls Triton back, rather than forward,
in its orbit.
19. They are plumes of dark material ejected in geysers and blown downwind by the thin
atmosphere of Triton.
Problems
1. 1900 km/m3
2. Every five orbital periods of the inner satellite
3. We are seeking the synodic period of revolution of Phobos, given by 1/S = 1/P(Phobos) –
1/P(Mars), where P(Phobos) is 7.7 hours and P(Mars) is the rotation period of Mars = 24.6
hours. Inserting these values gives S = 11.2 hours.
4. Kepler’s 3rd law is P2 = d3, so we can show consistency with Kepler’s 3rd law by showing that
P2/d3 is the same for the four satellites.
Satellite P2/ d3
(days2/(million km3)
Io
41.8
Europa
41.8
Ganymede
41.8
Callisto
41.7
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