Exercises: Cratered Worlds

Collaborative Group Activities

  1. We mentioned that no nation on Earth now has the capability to send a human being to the Moon, even though the United States once sent 12 astronauts to land there. What does your group think about this? Should we continue the exploration of space with human beings? Should we put habitats on the Moon? Should we go to Mars? Does humanity have a “destiny in space?” Whatever your answer to these questions, make a list of the arguments and facts that support your position.
  2. When they hear about the giant impact hypothesis for the origin of the Moon, many students are intrigued and wonder why we can’t cite more evidence for it. In your group, make a list of reasons we cannot find any traces on Earth of the great impact that formed the Moon?
  3. We discussed that the ice (mixed into the soil) that is found on the Moon was most likely delivered by comets. Have your group make a list of all the reasons the Moon would not have any ice of its own left over from its early days.
  4. Can your group make a list of all the things that would be different if Earth had no Moon? Don’t restrict your answer to astronomy and geology. Think about our calendars and moonlit romantic strolls, for example. (You may want to review Earth, Moon, and Sky.)
  5. If, one day, humanity decides to establish a colony on the Moon, where should we put it? Make a list of the advantages and disadvantages of locating such a human habitat on the near side, the far side, or at the poles. What site would be best for doing visible-light and radio astronomy from observatories on the Moon?
  6. A member of the class (but luckily, not a member of your group) suggests that he has always dreamed of building a vacation home on the planet Mercury. Can your group make a list of all reasons such a house would be hard to build and keep in good repair?
  7. As you’ve read in this chapter, craters on the Moon are (mostly) named after scientists. (See the official list.) The craters on Mercury, on the other hand, are named for writers, artists, composers, and others in the humanities. (See the official list.) Living persons are not eligible. Can each person in your group think of a scientist or someone in the arts whom they especially respect? Now check to see if they are listed. Are there scientists or people in the arts who should have their names on the Moon or Mercury and do not?
  8. Imagine that a distant relative, hearing you are taking an astronomy course, calls you up and tells you that NASA faked the Moon landings. His most significant argument is that all the photos of the Moon show black skies, but none of them have any stars showing. This proves that the photos were taken against a black backdrop in a studio and not on the Moon. Based on your reading in this chapter, what arguments can your group come up with to rebut this idea?

Review questions

  1. What is the composition of the Moon, and how does it compare to the composition of Earth? Of Mercury?
  2. Why does the Moon not have an atmosphere?
  3. What are the principal features of the Moon observable with the unaided eye?
  4. Frozen water exists on the lunar surface primarily in which location? Why?
  5. Outline the main events in the Moon’s geological history.
  6. What are the maria composed of? Is this material found elsewhere in the solar system?
  7. The mountains on the Moon were formed by what process?
  8. With no wind or water erosion of rocks, what is the mechanism for the creation of the lunar “soil?”
  9. What differences did Grove K. Gilbert note between volcanic craters on Earth and lunar craters?
  10. Explain how high-speed impacts form circular craters. How can this explanation account for the various characteristic features of impact craters?
  11. Explain the evidence for a period of heavy bombardment on the Moon about 4 billion years ago.
  12. How did our exploration of the Moon differ from that of Mercury (and the other planets)?
  13. Summarize the four main hypotheses for the origin of the Moon.
  14. What are the difficulties with the capture hypothesis of the Moon’s origin?
  15. What is the main consequence of Mercury’s orbit being so highly eccentric?
  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

  1. 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?
  2. 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?
  3. 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?
  4. Why did it take so long for geologists to recognize that the lunar craters had an impact origin rather than a volcanic one?
  5. 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?
  6. 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)?
  7. 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.
  8. 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.)
  9. Give several reasons Mercury would be a particularly unpleasant place to build an astronomical observatory.
  10. 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?
  11. 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

  1. 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?
  2. 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 Chapter for the formula to use.)
  3. 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.
  4. 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.