Questions

Review Questions

  1. Why is it impossible to see Mercury or Venus in the sky at midnight?

  2. In his 1964 science fiction story “The Coldest Place,” author Larry Niven described the “dark side” of Mercury as the coldest place in the solar system. What assumption did he make about the rotation of Mercury? Did this assumption turn out to be correct?

  3. What is 3-to-2 spin-orbit coupling? How is the rotation period of an object exhibiting 3-to-2 spin-orbit coupling related to its orbital period? What aspects of Mercury’s orbit cause it to exhibit 3-to-2 spin-orbit coupling? What telescopic observations proved this?

  4. Why was it so difficult to determine the rate and direction of Venus’s rotation? How were these finally determined? What is one proposed explanation for the slow, retrograde rotation of Venus?

  5. Explain why Mercury does not have a substantial atmosphere.

  6. What kind of surface features are found on Mercury? How do they compare to surface features on the Moon? Why are they probably much older than most surface features on Earth?

  7. How do we know that the scarps on Mercury are younger than the lava flows? How can you tell that the scarp in Figure 11-7 is younger than the vertically distorted crater at the center of the figure?

  8. If Mercury is the closest planet to the Sun and has such a high average surface temperature, how is it possible that ice might exist on its surface?

  9. Why do astronomers think that Mercury has a very large iron core?

  10. Why is it surprising that Mercury has a global magnetic field? Why does the 58.646-day rotation period of Mercury imply that the planet can have only a weak magnetic field?

  11. Is there a consistent explanation for why Mercury’s magnetic field is off-center? If Earth’s magnetic field was off-center by the same percentage, how many kilometers would that be?

  12. Why are high concentrations of volatile elements a mystery on Mercury?

  13. Why was it difficult to determine Venus’s surface temperature from Earth? How was this finally determined?

  14. The Mariner 2 spacecraft did not enter Venus’s atmosphere, but it was nonetheless able to determine that the atmosphere contains little water vapor. How was this done?

  15. How did winds trick early observers of Mars into thinking that Mars had vegetation?

  16. Do Venus and Mars have continents like those on Earth?

  17. What is the Martian crustal dichotomy? What is the evidence that the southern highlands are older than the northern lowlands?

  18. What is the evidence that the surface of Venus is only about 500 million years old?

  1. What is flake tectonics? Why does Venus exhibit flake tectonics rather than plate tectonics?

  2. What geologic features (or lack thereof) on Mars have convinced scientists that extensive plate tectonics did not significantly shape the present Martian surface? Does Mars have any plates now?

  3. What magnetic features indicate that Mars might have experienced plate tectonics in the past?

  4. Compare the volcanoes of Venus, Earth, and Mars. Cite evidence that hot-spot volcanism is or was active on all three worlds.

  1. Describe the evidence that there has been recent volcanic activity (a) on Venus and (b) on Mars.

  2. Suppose all of Venus’s volcanic activity suddenly stopped. (a) How would this affect Venus’s clouds? (b) How would this affect the overall Venusian environment?

  3. Why are the patterns of convection in the Venusian atmosphere so different from those in our atmosphere?

  4. Why is it impossible for liquid water to exist on Mars today? If liquid water existed on Mars in the past, what must have been different then?

  5. Why does the atmospheric pressure on Mars vary with the seasons? What is the relationship between this pressure variation and Martian dust storms?

  1. Why is it reasonable to assume that the primordial atmospheres of Earth, Venus, and Mars were roughly the same?

  2. Carbon dioxide accounts for about 95% of the present-day atmospheres of both Mars and Venus. Why, then, is there a strong greenhouse effect on Venus but only a weak greenhouse effect on Mars?

  3. (a) What is a runaway greenhouse effect? (b) What is a runaway icehouse effect?

  1. Explain two ways that heat within Mars helped to maintain a greenhouse effect.

  2. What is a dust devil? Why would you feel much less breeze from a Martian dust devil than from a dust devil on Earth?

  3. (a) Why is Mars red? (b) Why is the Martian sky the color of butterscotch?

  4. Were the Viking Landers able to determine whether life currently exists on Mars or whether it once existed there? Why or why not?

  5. (a) The Spirit rover found the minerals olivine and pyroxene at its landing site on Mars. Explain how this shows that there has been no liquid water at that site for billions of years. (b) What evidence did the Opportunity rover find at its landing site to suggest that liquid water had once been present there?

  6. How was the Mars Odyssey spacecraft able to detect water beneath the Martian surface without landing on the planet?

  7. How do craters reveal evidence for water-ice beneath the Martian surface?

  8. A full moon on Earth is bright enough to cast shadows. As seen from the Martian surface, would you expect a full Phobos or full Deimos to cast shadows? Why or why not?

Advanced Questions

Problem-solving tips and tools

We discussed the relationship between angular distance and linear distance in Box 1-1 and the idea of angular resolution in Section 6-3. You may need to refresh your memory about Kepler’s third law, described in Section 4-4. You may also need to review the form of Kepler’s third law that explicitly includes mass (see Section 4-7 and Box 4-4). Box 4-1 describes the relationship between a planet’s sidereal orbital period and its synodic period (the time from one inferior conjunction to the next). You should recall that Wien’s law (Section 5-4) relates the temperature of a blackbody to λmax, its wavelength of maximum emission. Section 5-9 and Box 5-6 explain the Doppler effect and how to do calculations using it. The linear speed of a point on a planet’s equator is the planet’s circumference divided by its rotation period; recall that the circumference of a circle of radius r is 2πr. The volume of a sphere of radius r is 4πr3/3. The speed of light is given in Appendix 7.

  1. Figure 11-1 shows Mercury with a greatest eastern elongation of 18° and a greatest western elongation of 28°. On November 25, 2006, Mercury was at a greatest western elongation of 20°. Was Mercury at perihelion, aphelion, or some other point on its orbit? Explain your answer.

  2. Venus takes 440 days to move from greatest western elongation to greatest eastern elongation, but it needs only 144 days to go from greatest eastern elongation to greatest western elongation. With the aid of a diagram like Figure 11-1, explain why.

  3. Venus’s sidereal rotation period is 243.01 days and its orbital period is 224.70 days. Use these data to prove that a solar day on Venus lasts 116.8 days. (Hint: Develop a formula relating Venus’s solar day to its sidereal rotation period and orbital period similar to the first formula in Box 4-1.)

  4. In Section 11-2 we described the relationship between the length of Venus’s synodic period and the length of an apparent solar day on Venus. Using this relationship and a diagram, explain why at each inferior conjunction the same side of Venus is turned toward Earth.

  5. This time-lapse photograph was taken on May 7, 2003, during a solar transit of Mercury. Over a period of 5 hours and 19 minutes, Mercury appeared to move across the face of the Sun. Such solar transits of Mercury occur 13 or 14 times each century; they do not happen each time that Mercury is at inferior conjunction. Explain why not. (Hint: For a solar transit to occur, the Sun, Mercury, and Earth must be in a nearly perfect alignment. Does the orbit of Mercury lie in the plane of the ecliptic?)

    R I V U X G
    (Dominique Dierick)
  6. Find the largest angular size that Mercury can have as seen from Earth. In order for Mercury to have this apparent size, at what point in its orbit must it be?

  7. (a) Suppose you have a telescope with an angular resolution of 1 arcsec. What is the size (in kilometers) of the smallest feature you could have seen on the Martian surface during the opposition of 2005, when Mars was 0.464 AU from Earth? (b) Suppose you had access to the Hubble Space Telescope (HST), which has an angular resolution of 0.1 arcsec. What is the size (in kilometers) of the smallest feature you could have seen on Mars with the HST during the 2005 opposition?

  8. For a planet to appear to the naked eye as a disk rather than as a point of light, its angular size would have to be 1 arcmin, or 60 arcsec. (This is the same as the angular separation between lines in the bottom row of an optometrist’s eye chart.) (a) How close would you have to be to Mars in order to see it as a disk with the naked eye? Does Mars ever get this close to Earth? (b) Would Earth ever be visible as a disk to an astronaut on Mars? Would she be able to see Earth and the Moon separately, or would they always appear as a single object? Explain your answers.

  9. Imagine that you are part of the scientific team monitoring a spacecraft that has landed on Mars. At 5:00 p.m. in your control room on Earth, the spacecraft reports that the Sun is highest in the sky as seen from its location on Mars. When the Sun is next at its highest point as seen from the spacecraft, what time will it be in the control room on Earth?

  10. (a) Mercury has a 58.646-day rotation period. What is the speed at which a point on the planet’s equator moves due to this rotation? (Hint: Remember that speed is distance divided by time. What distance does a point on Mercury’s equator travel as the planet makes one rotation?) (b) Use your answer to (a) to answer the following: As a result of rotation, what difference in wavelength is observed for a radio wave of wavelength 12.5 cm (such as is actually used in radar studies of Mercury) emitted from either the approaching or receding edge of the planet?

  11. The orbital period of Mariner 10 is twice that of Mercury. Use this fact to calculate the length of the semimajor axis of the spacecraft’s orbit.

  12. Consider the idea that Mercury has a solid iron-bearing mantle that is permanently magnetized like a giant bar magnet. Using the fact that iron demagnetizes at temperatures above 770°C, present an argument against this explanation of Mercury’s magnetic field.

  13. (a) At what wavelength does Venus’s surface emit the most radiation? (b) Do astronomers have telescopes that can detect this radiation? (c) Why can’t we use such telescopes to view the planet’s surface?

  14. The Mariner 2 spacecraft detected more microwave radiation when its instruments looked at the center of Venus’s disk than when it looked at the edge, or limb, of the planet. (This effect is called limb darkening.) Explain how these observations show that the microwaves are emitted by the planet’s surface rather than its atmosphere.

  15. In the classic Ray Bradbury science fiction story “All Summer in a Day,” human colonists on Venus are subjected to continuous rainfall except for one day every few years when the clouds part and the Sun comes out for an hour or so. Discuss how our understanding of Venus’s atmosphere has evolved since this story was first published in 1954.

  16. Suppose that Venus had no atmosphere at all. How would the albedo of Venus then compare with that of Mercury or the Moon? Explain your answer.

  17. A hypothetical planet has an atmosphere that is opaque to visible light but transparent to infrared radiation. How would this affect the planet’s surface temperature? Contrast and compare this hypothetical planet’s atmosphere with the greenhouse effect in Venus’s atmosphere.

  18. Earth’s northern hemisphere is 39% land and 61% water, while its southern hemisphere is only 19% land and 81% water. Thus, the southern hemisphere could also be called the “water hemisphere.” The Moon also has two distinct hemispheres, the near side (which has a number of maria) and the far side (which has almost none). How are these hemispheric differences on Earth and on the Moon similar to the Martian crustal dichotomy? How are they different?

  19. For a group of properly attired astronauts equipped with oxygen tanks, a climb to the summit of Olympus Mons would actually be a relatively easy (albeit long) hike rather than a true mountain climb. Give two reasons why.

  20. On Mars, the difference in elevation between the highest point (the summit of Olympus Mons) and the lowest point (the bottom of the Hellas Planitia basin) is 30 km. On Earth, the corresponding elevation difference (from the peak of Mount Everest to the bottom of the deepest ocean) is only 20 km. Discuss why the maximum elevation difference is so much greater on Mars.

  21. The Mars Global Surveyor (MGS) spacecraft was in a nearly circular orbit around Mars with an orbital period of 117 minutes. (a) Using the data in Table 11-3, find the radius of the orbit. (b) What is the average altitude of MGS above the Martian surface? (c) The orbit of MGS passed over the north and south poles of Mars. Explain how this makes it possible for the spacecraft to observe the entire surface of the planet.

  22. The elevations in Figure 11-17 were measured using an instrument called MOLA (Mars Orbiter Laser Altimeter) on board Mars Global Surveyor. MOLA fired a laser beam downward, then measured how long it took for the beam to return to the spacecraft after reflecting off the surface. Suppose MOLA measured this round-trip time for the laser beam to reflect off the summit of Olympus Mons, as well as the round-trip time to reflect off the bottom of Hellas Planitia (see Question 58). Which round-trip time is longer? How much longer is it? Do you need to know the distance from Mars Global Surveyor to the surface to answer this question?

  23. (a) The Grand Canyon in Arizona was formed over 15 to 20 million years by the flowing waters of the Colorado River, as well as by rain and wind. Contrast this formation scenario to that of Valles Marineris on Mars. (b) Valles Marineris is sometimes called the “Grand Canyon of Mars.” Is this an appropriate description? Why or why not?

  24. Water has a density of 1000 kg/m3, so a column of water n meters tall and 1 meter square at its base has a mass of n × 1000 kg. On either Earth or Venus, which have nearly the same surface gravity, a mass of 1 kg weighs about 9.8 newtons (2.2 lb). Calculate how deep you would have to descend into Earth’s oceans for the pressure to equal the atmospheric pressure on Venus’s surface, 90 atm or 9 × 106 newtons per square meter. Give your answer in meters.

  25. Marine organisms produce sulfur-bearing compounds, some of which escape from the oceans into Earth’s atmosphere. (These compounds are largely responsible for the characteristic smell of the sea.) Even more sulfurous gases are injected into our atmosphere by the burning of sulfur-rich fossil fuels, such as coal, in electric power plants. Both of these processes add more sulfur compounds to the atmosphere than do volcanic eruptions. On lifeless Venus, by contrast, volcanoes are the only source for sulfurous atmospheric gases. Why, then, are sulfur compounds so much rarer in our atmosphere than in the Venusian atmosphere?

  26. The classic 1950 science fiction movie Rocketship X-M shows astronauts on the Martian surface with oxygen masks for breathing but wearing ordinary clothing. Would this be a sensible choice of apparel for a walk on Mars? Why or why not?

  27. Suppose that the only information you had about Mars was the images of the surface in Figure 11-28. Describe at least two ways that you could tell from these images that Mars has an atmosphere.

  28. Although the Viking Lander 1 and Viking Lander 2 landing sites are 6500 km apart and have different geologic histories, the chemical compositions of the dust at both sites are nearly identical. (a) What does this suggest about the ability of the Martian winds to transport dust particles? (b) Would you expect that larger particles such as pebbles would also have identical chemical compositions at the two Viking Lander sites? Why or why not?

  29. Why do you suppose that Phobos and Deimos are not round like our Moon?

  30. The orbit of Phobos has a semimajor axis of 9378 km. Use this information and the orbital period given in the text to calculate the mass of Mars. How does your answer compare with the mass of Mars given in Table 11-3?

  31. Calculate the angular sizes of Phobos and Deimos as they pass overhead, as seen by an observer standing on the Martian equator. How do these sizes compare with that of the Moon seen from Earth’s surface? Would Phobos and Deimos appear as impressive in the Martian sky as the Moon does in our sky?

  32. You are to put a spacecraft into a synchronous circular orbit around the Martian equator, so that its orbital period is equal to the planet’s rotation period. Such a spacecraft would always be over the same part of the Martian surface. (a) Find the radius of the orbit and the altitude of the spacecraft above the Martian surface. (b) Suppose Mars had a third moon that was in a synchronous orbit. Would tidal forces make this moon tend to move toward Mars, away from Mars, or neither? Explain your answer.

Discussion Questions

  1. Before about 350 b.c.e., the ancient Greeks did not realize that Mercury seen in the morning sky (which they called Apollo) and seen in the evening sky (which they called Hermes) were actually the same planet. Discuss why you think it took some time to realize this.

  2. If you were planning a new mission to Mercury, what features and observations would be of particular interest to you?

  3. What evidence do we have that the surface features on Mercury were not formed during recent geological history?

  4. Describe the apparent motion of the Sun during a “day” on Venus relative to (a) the horizon and (b) the background stars. (Assume that you can see through the cloud cover.)

  5. If you could examine rock samples from the surface of Venus, would you expect them to be the same as rock samples from Earth? Would you expect to find igneous, sedimentary, and metamorphic rocks like those found on Earth (see Section 9-3)? Explain your answers.

  6. In 1978 the Pioneer Venus Orbiter spacecraft arrived at Venus. It carried an ultraviolet spectrometer to measure the chemical composition of the Venusian atmosphere. This instrument recorded unexpectedly high levels of sulfur dioxide and sulfuric acid, which steadily declined over the next several years. Discuss how this observation suggests that volcanic eruptions occurred on Venus not long before Pioneer Venus Orbiter arrived there.

Web/eBook Questions

  1. Elongations of Mercury. Access the animation “Elongations of Mercury” in Chapter 11 of the Universe Web site or eBook. (a) View the animation and notice the dates of the greatest eastern and greatest western elongations. Which time interval is greater: from a greatest eastern elongation to a greatest western elongation, or vice versa? (b) Based on what you observe in the animation, draw a diagram to explain your answer to the question in (a).

  2. Just as Mercury can pass in front of the Sun as seen from Earth (see Question 43), so can Venus. Transits of Venus are quite rare. The dates of the only transits in the twenty-first century are June 8, 2004, and June 6, 2012; the next ones will occur in 2117 and 2125. A number of European astronomers traveled to Asia and the Pacific islands to observe the transits of Venus in 1761 and 1769. Search the World Wide Web for information about these expeditions. Why were these events of such interest to astronomers? How definitive were the results of these observations?

  3. Search the World Wide Web for information about possible manned missions to Mars. How long might such a mission take? How expensive would such a project be? What would be the advantages of a manned mission compared to an unmanned one?

  4. Conjunctions of Mars. Access and view the animation “The Orbits of Earth and Mars” in Chapter 11 of the Universe Web site or eBook. (a) The animation highlights three dates when Mars is in opposition, so that Earth lies directly between Mars and the Sun. By using the “Stop” and “Play” buttons in the animation, find two times during the animation when Mars is in conjunction, so that the Sun lies directly between Mars and Earth (see Figure 4-6). For each conjunction, make a drawing showing the positions of the Sun, Earth, and Mars, and record the month and year when the conjunction occurs. (b) When Mars is in conjunction, at approximately what time of day does it rise as seen from Earth? At what time of day does it set? Is Mars suitably placed for telescopic observation when in conjunction?