Questions
Review Questions
Is it correct to say that the Moon orbits Earth? If not, what is a more correct description?If the Moon always keeps the same face toward Earth, how is it possible for Earth observers to see more than half of the Moon’s surface?
Why does the sky look black on the Moon even during daytime?
Why is it impossible for liquid water to exist on the surface of the Moon?
Describe two reasons why astronauts needed to wear spacesuits on the lunar surface.
Describe the kinds of features that can be seen on the Moon with a small telescope.
Are impact craters on the Moon the same size as the meteoroids that made the impact? Explain your answer.
Describe the differences between the maria and the lunar highlands. Which kind of terrain covers more of the Moon’s surface? Which kind of terrain is more heavily cratered? Which kind of terrain was formed later in the Moon’s history? How do we know?
Describe the differences between the near and far sides of the Moon. What is thought to be the likely explanation for these differences?
What does it mean to say the Moon is a “one-plate world”? What is the evidence for this statement?
Why was it necessary to send unmanned spacecraft to land on the Moon before sending humans there?What is the evidence that water-ice exists at the lunar poles? Is this evidence definitive?
Why was it useful for the Apollo astronauts to bring magnetometers and seismometers to the Moon?
Could you use a magnetic compass to navigate on the Moon? Why or why not?
Does the absence of a global magnetic field around the Moon prove there is no liquid iron core?
Explain why moonquakes occur more frequently when the Moon is at perigee than at other locations along its orbit.
Why is Earth geologically active while the Moon is not?
What is the regolith? What causes its powdery character?
Why are there no sedimentary rocks on the Moon?
On the basis of Moon rocks brought back by the astronauts, explain why the maria are dark-colored but the lunar highlands are light-colored.
Briefly describe the main differences and similarities between Moon rocks and Earth rocks.
Rocks found on the Moon are between 3.1 and 4.47 billion years old. By contrast, the majority of Earth’s surface is made of oceanic crust that is less than 200 million years old, and the very oldest Earth rocks are about 4 billion years old. If Earth and the Moon are essentially the same age, why is there such a disparity in the ages of rocks on the two worlds?
If Earth’s tidal bulge pointed directly toward the Moon, would the Moon still be receding from Earth? Explain your answer.Why do most scientists favor the collisional ejection theory of the Moon’s formation?
Some people who supported the fission theory proposed that the Pacific Ocean basin is the scar left when the Moon pulled away from Earth. Explain why this idea is probably wrong.
Advanced Questions
Questions preceded by an asterisk (*) involve topics discussed in Box 10-1.
Problem-solving tips and tools
Recall that the average density of an object is its mass divided by its volume. The volume of a sphere is 4πr3/3, where r is the sphere’s radius. The surface area of a sphere of radius r is 4πr2, while the surface area of a circle of radius r is πr2. Recall also that the acceleration of gravity on Earth’s surface is 9.8 m/s2. You may find it useful to know that a 1-pound (1-lb) weight presses down on Earth’s surface with a force of 4.448 newtons. You might want to review Newton’s law of universal gravitation in Section 4-6. The time to travel a certain distance is equal to the distance traveled divided by the speed of motion. Consult Table 10-1 and the Appendices for any additional data.
Suppose two worlds (say, a planet and its satellite) have masses m1 and m2, and the center-to-center distance between the worlds is r. The distance dcm from the center of world 1 to the center of mass of the system of two worlds is given by the formula
(a) Suppose world 1 is Earth and world 2 is the Moon. If Earth and the Moon are at their average center-to-center distance, find the distance from the center of Earth to the center of mass of the Earth-Moon system. (b) Is the Earth-Moon system’s center of mass within Earth? How far below Earth’s surface is it located? (c) Find the distance from the center of the Sun (mass 1.989 × 1030 kg) to the center of mass of the Sun-Earth system. How does this compare to the diameter of the Sun? Is it a good approximation to say that Earth orbits around the center of the Sun?If you view the Moon through a telescope, you will find that details of its craters and mountains are more visible when the Moon is near first quarter phase or third quarter phase than when it is at full phase. Explain why.
In a whimsical moment during the Apollo 14 mission, astronaut Alan Shepard hit two golf balls over the lunar surface. Give two reasons why they traveled much farther than golf balls do on Earth.
Temperature variations between day and night are much more severe on the Moon than on Earth. Explain why.
How much would an 80-kg person weigh on the Moon? How much does that person weigh on Earth?
Using the diameter and mass of the Moon given in Table 10-1, verify that the Moon’s average density is about 3344 kg/m3. Explain why this average density implies that the Moon’s interior contains much less iron than the interior of Earth.
*In Box 10-1 we calculated the tidal force that Earth exerts on two 1-kg rocks located on the near and far sides of the Moon. We assumed that the Earth-Moon distance was equal to its average value. Repeat this calculation (a) for the Moon at perigee and (b) for the Moon at apogee. (c) What is the ratio of the tidal force on the rocks at perigee to the tidal force at apogee?
The youngest lunar anorthosites are 4.0 billion years old, and the youngest mare basalts are 3.1 billion years old. Would you expect to find any impact breccias on the Moon that formed less than 3.1 billion years ago? Explain your answer.
In the maria, the lunar regolith is about 2 to 8 meters deep. In the lunar highlands, by contrast, it may be more than 15 meters deep. Explain how the different ages of the maria and highlands can account for these differences.
The mare basalts are volcanic rock. Is it likely that active volcanoes exist anywhere on the Moon today? Explain.
Calculate the round-trip travel time for a pulse of laser light that is fired from a point on Earth nearest the Moon, hits a reflector at the point on the Moon nearest Earth, and returns to its point of origin. Assume that Earth and the Moon are at their average separation from each other.
Before the Apollo missions to the Moon, there were two diametrically opposite schools of thought about the history of lunar geology. The “cold moon” theory held that all lunar surface features were the result of impacts. The most violent impacts melted the surface rock, which then solidified to form the maria. The opposite “hot moon” theory held that all lunar features, including maria, mountains, and craters, were the result of volcanic activity. Explain how lunar rock samples show that neither of these theories is entirely correct.
*When the Moon originally coalesced, it may have been only one-tenth as far from Earth as it is now. (a) When the Moon first coalesced, was Earth’s tidal force strong enough to lift rocks off the lunar surface? Explain. (b) Compared with the net tidal force that Earth exerts on the Moon today, how many times larger was the net tidal force on the newly coalesced Moon? (This strong tidal force kept the one axis of the Moon oriented toward Earth, and the Moon kept that orientation after it solidified.)
Discussion Questions
Comment on the idea that without the presence of the Moon in our sky, astronomy would have developed far more slowly.
No Apollo mission landed on the far side of the Moon. Why do you suppose this was? What would have been the scientific benefits of a mission to the far side?
NASA is planning a new series of manned missions to the Moon. Compare the advantages and disadvantages of exploring the Moon with astronauts as opposed to using mobile, unmanned instrument packages.
Describe how you would empirically test the idea that human behavior is related to the phases of the Moon. What problems are inherent in such testing?
How would our theories of the Moon’s history have been affected if astronauts had discovered sedimentary rock on the Moon?
Imagine that you are planning a lunar landing mission. What type of landing site would you select? Where might you land to search for evidence of recent volcanic activity?
Web/eBook Question
In 2005 the SMART-1 spacecraft detected calcium on the lunar surface. Search the World Wide Web for information about the SMART-1 mission and this discovery. How was the presence of calcium detected? What does this tell astronomers about the origin of the Moon?