Chapter 7 Other Worlds: An Introduction to the Solar System

7.7 Questions and Exercises

Review Questions

1: Venus rotates backward and Uranus and Pluto spin about an axis tipped nearly on its side. Based on what you learned about the motion of small bodies in the solar system and the surfaces of the planets, what might be the cause of these strange rotations?

2: What is the difference between a differentiated body and an undifferentiated body, and how might that influence a body’s ability to retain heat for the age of the solar system?

3: What does a planet need in order to retain an atmosphere? How does an atmosphere affect the surface of a planet and the ability of life to exist?

4: Which type of planets have the most moons? Where did these moons likely originate?

5: What is the difference between a meteor and a meteorite?

6: Explain our ideas about why the terrestrial planets are rocky and have less gas than the giant planets.

7: Do all planetary systems look the same as our own?

8: What is comparative planetology and why is it useful to astronomers?

9: What changed in our understanding of the Moon and Moon-Earth system as a result of humans landing on the Moon’s surface?

10: If Earth was to be hit by an extraterrestrial object, where in the solar system could it come from and how would we know its source region?

11: List some reasons that the study of the planets has progressed more in the past few decades than any other branch of astronomy.

12: Imagine you are a travel agent in the next century. An eccentric billionaire asks you to arrange a “Guinness Book of Solar System Records” kind of tour. Where would you direct him to find the following (use this chapter and Appendix F and Appendix G):

  1. the least-dense planet
  2. the densest planet
  3. the largest moon in the solar system
  4. excluding the jovian planets, the planet where you would weigh the most on its surface (Hint: Weight is directly proportional to surface gravity.)
  5. the smallest planet
  6. the planet that takes the longest time to rotate
  7. the planet that takes the shortest time to rotate
  8. the planet with a diameter closest to Earth’s
  9. the moon with the thickest atmosphere
  10. the densest moon
  11. the most massive moon

13: What characteristics do the worlds in our solar system have in common that lead astronomers to believe that they all formed from the same “mother cloud” (solar nebula)?

14: How do terrestrial and giant planets differ? List as many ways as you can think of.

15: Why are there so many craters on the Moon and so few on Earth?

16: How do asteroids and comets differ?

17: How and why is Earth’s Moon different from the larger moons of the giant planets?

18: Where would you look for some “original” planetesimals left over from the formation of our solar system?

19: Describe how we use radioactive elements and their decay products to find the age of a rock sample. Is this necessarily the age of the entire world from which the sample comes? Explain.

20: What was the solar nebula like? Why did the Sun form at its center?

Thought Questions

21: What can we learn about the formation of our solar system by studying other stars? Explain.

22: Earlier in this chapter, we modeled the solar system with Earth at a distance of about one city block from the Sun. If you were to make a model of the distances in the solar system to match your height, with the Sun at the top of your head and Pluto at your feet, which planet would be near your waist? How far down would the zone of the terrestrial planets reach?

23: Seasons are a result of the inclination of a planet’s axial tilt being inclined from the normal of the planet’s orbital plane. For example, Earth has an axis tilt of 23.4° (Appendix F). Using information about just the inclination alone, which planets might you expect to have seasonal cycles similar to Earth, although different in duration because orbital periods around the Sun are different?

24: Again using Appendix F, which planet(s) might you expect not to have significant seasonal activity? Why?

25: Again using Appendix F, which planets might you expect to have extreme seasons? Why?

26: Using some of the astronomical resources in your college library or the Internet, find five names of features on each of three other worlds that are named after real people. In a sentence or two, describe each of these people and what contributions they made to the progress of science or human thought.

27: Explain why the planet Venus is differentiated, but asteroid Fraknoi, a very boring and small member of the asteroid belt, is not.

28: Would you expect as many impact craters per unit area on the surface of Venus as on the surface of Mars? Why or why not?

29: Interview a sample of 20 people who are not taking an astronomy class and ask them if they can name a living astronomer. What percentage of those interviewed were able to name one? Typically, the two living astronomers the public knows these days are Stephen Hawking and Neil deGrasse Tyson. Why are they better known than most astronomers? How would your result have differed if you had asked the same people to name a movie star or a professional basketball player?

30: Using Appendix G, complete the following table that describes the characteristics of the Galilean moons of Jupiter, starting from Jupiter and moving outward in distance.

Table A
Moon Semimajor Axis (km3) Diameter Density (g/cm3)

This system has often been described as a mini solar system. Why might this be so? If Jupiter were to represent the Sun and the Galilean moons represented planets, which moons could be considered more terrestrial in nature and which ones more like gas/ice giants? Why? (Hint: Use the values in your table to help explain your categorization.)

Figuring for Yourself

31: Calculate the density of Jupiter. Show your work. Is it more or less dense than Earth? Why?

32: Calculate the density of Saturn. Show your work. How does it compare with the density of water? Explain how this can be.

33: What is the density of Jupiter’s moon Europa (see Appendix G for data on moons)? Show your work.

34: Look at Appendix F and Appendix G and indicate the moon with a diameter that is the largest fraction of the diameter of the planet or dwarf planet it orbits.

35: Barnard’s Star, the second closest star to us, is about 56 trillion (5.6 × 1012) km away. Calculate how far it would be using the scale model of the solar system given in Overview of Our Planetary System.

36: A radioactive nucleus has a half-life of 5 × 108 years. Assuming that a sample of rock (say, in an asteroid) solidified right after the solar system formed, approximately what fraction of the radioactive element should be left in the rock today?


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