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16. Describe the basic internal structure of Mercury.
17. How was the rotation rate of Mercury determined?
18. What is the relationship between Mercury’s rotational period and orbital period?
19. The features of Mercury are named in honor of famous people in which fields of endeavor?
20. What do our current ideas about the origins of the Moon and Mercury have in common? How do they
differ?
Thought Questions
21. One of the primary scientific objectives of the Apollo program was the return of lunar material. Why was
this so important? What can be learned from samples? Are they still of value now?
22. Apollo astronaut David Scott dropped a hammer and a feather together on the Moon, and both reached
the ground at the same time. What are the two distinct advantages that this experiment on the Moon had
over the same kind of experiment as performed by Galileo on Earth?
23. Galileo thought the lunar maria might be seas of water. If you had no better telescope than the one he
had, could you demonstrate that they are not composed of water?
24. Why did it take so long for geologists to recognize that the lunar craters had an impact origin rather than
a volcanic one?
25. How might a crater made by the impact of a comet with the Moon differ from a crater made by the impact
of an asteroid?
26. Why are the lunar mountains smoothly rounded rather than having sharp, pointed peaks (as they were
almost always depicted in science-fiction illustrations and films before the first lunar landings)?
27. The lunar highlands have about ten times more craters in a given area than do the maria. Does this mean
that the highlands are 10 times older? Explain your reasoning.
28. At the end of the section on the lunar surface, your authors say that lunar night and day each last about
two Earth weeks. After looking over the information in Earth, Moon, and Sky and this chapter about the
motions of the Moon, can you explain why? (It helps to draw a diagram for yourself.)
29. Give several reasons Mercury would be a particularly unpleasant place to build an astronomical
observatory.
30. If, in the remote future, we establish a base on Mercury, keeping track of time will be a challenge. Discuss
how to define a year on Mercury, and the two ways to define a day. Can you come up with ways that
humans raised on Earth might deal with time cycles on Mercury?
31. The Moon has too little iron, Mercury too much. How can both of these anomalies be the result of giant
impacts? Explain how the same process can yield such apparently contradictory results.
Figuring for Yourself
32. In the future, astronomers discover a solid moon around a planet orbiting one of the nearest stars. This
moon has a diameter of 1948 km and a mass of 1.6 × 1022 kg. What is its density?
33. The Moon was once closer to Earth than it is now. When it was at half its present distance, how long was
its period of revolution? (See Orbits and Gravity for the formula to use.)
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34. Astronomers believe that the deposit of lava in the giant mare basins did not happen in one flow but in
many different eruptions spanning some time. Indeed, in any one mare, we find a variety of rock ages,
typically spanning about 100 million years. The individual lava flows as seen in Hadley Rille by the Apollo
15 astronauts were about 4 m thick. Estimate the average time interval between the beginnings of
successive lava flows if the total depth of the lava in the mare is 2 km.
35. The Moon requires about 1 month (0.08 year) to orbit Earth. Its distance from us is about 400,000 km
(0.0027 AU). Use Kepler’s third law, as modified by Newton, to calculate the mass of Earth relative to the
Sun.
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Figure 10.1 Spirit Rover on Mars. This May 2004 image shows the tracks made by the Mars Exploration Spirit rover on the
surface of the red planet. Spirit was active on Mars between 2004 and 2010, twenty times longer than its planners had expected. It
“drove” over 7.73 kilometers in the process of examining the martian landscape. (credit: modification of work by NASA/JPL/Cornell)
Chapter Outline
10.1 The Nearest Planets: An Overview
10.2 The Geology of Venus
10.3 The Massive Atmosphere of Venus
10.4 The Geology of Mars
10.5 Water and Life on Mars
10.6 Divergent Planetary Evolution
Thinking Ahead
The Moon and Mercury are geologically dead. In contrast, the larger terrestrial planets—Earth, Venus, and
Mars—are more active and interesting worlds. We have already discussed Earth, and we now turn to Venus
and Mars. These are the nearest planets and the most accessible to spacecraft. Not surprisingly, the greatest
effort in planetary exploration has been devoted to these fascinating worlds. In the chapter, we discuss some
of the results of more than four decades of scientific exploration of Mars and Venus. Mars is exceptionally
interesting, with evidence that points to habitable conditions in the past. Even today, we are discovering things
about Mars that make it the most likely place where humans might set up a habitat in the future. However, our
robot explorers have clearly shown that neither Venus nor Mars has conditions similar to Earth. How did it
happen that these three neighboring terrestrial planets have diverged so dramatically in their evolution?
10.1 The Nearest Planets: An Overview
Learning Objectives
By the end of this section, you will be able to:
Explain why it’s difficult to learn about Venus from Earth-based observation alone
Describe the history of our interest in Mars before the Space Age
Compare the basic physical properties of Earth, Mars, and Venus, including their orbits
Earthlike Planets: Venus and Mars
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As you might expect from close neighbors, Mars and Venus are among the brightest objects in the night sky.
The average distance of Mars from the Sun is 227 million kilometers (1.52 AU), or about half again as far from
the Sun as Earth. Venus’ orbit is very nearly circular, at a distance of 108 million kilometers (0.72 AU) from the
Sun. Like Mercury, Venus sometimes appears as an “evening star” and sometimes as a “morning star.” Venus
approaches Earth more closely than does any other planet: at its nearest, it is only 40 million kilometers from
us. The closest Mars ever gets to Earth is about 56 million kilometers.
Appearance
Venus appears very bright, and even a small telescope reveals that it goes through phases like the Moon.
Galileo discovered that Venus displays a full range of phases, and he used this as an argument to show that
Venus must circle the Sun and not Earth. The planet’s actual surface is not visible because it is shrouded by
dense clouds that reflect about 70% of the sunlight that falls on them, frustrating efforts to study the
underlying surface, even with cameras in orbit around the planet (Figure 10.2).
Figure 10.2 Venus as Photographed by the Pioneer Venus Orbiter. This ultraviolet image shows an upper-atmosphere cloud
structure that would be invisible at visible wavelengths. Note that there is not even a glimpse of the planet’s surface. (credit:
modification of work by NASA)
In contrast, Mars is more tantalizing as seen through a telescope (Figure 10.3). The planet is distinctly red, due
(as we now know) to the presence of iron oxides in its soil. This color may account for its association with war
(and blood) in the legends of early cultures. The best resolution obtainable from telescopes on the ground is
about 100 kilometers, or about the same as what we can see on the Moon with the unaided eye. At this
resolution, no hint of topographic structure can be detected: no mountains, no valleys, not even impact
craters. On the other hand, bright polar ice caps can be seen easily, together with dusky surface markings that
sometimes change in outline and intensity from season to season.
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	Chapter 10 Earthlike Planets:Venus and Mars
	Thinking Ahead
	10.1 The Nearest Planets: An Overview