Question 7.1

Which pair of planets has atmospheres with the most similar chemical compositions?

1. Earth and Venus
2. Earth and Mars
3. Venus and Mars
4. Mercury and Mars
5. Mercury and Venus

Question 7.2

Which planet is least likely to have water ice on or just under its surface?

1. Earth
2. Mercury
3. Mars
4. Venus

Question 7.3

Which object is most similar to Venus in mass and diameter?

1. Earth
2. Mars
3. Mercury
4. our Moon

Question 7.4

Why is Mercury so difficult to observe? When is the best time to see the planet? (Hint: Guided Discovery: The Inner Solar System in Section 7-5 can help.)

Question 7.5

Compare the surfaces of Mercury and our Moon. How are they similar? How are they different?

Question 7.6

Compare the interiors of Mercury and Earth. How are they similar? How are they different?

Question 7.7

On Mercury, when would you expect it to be coldest? Noon, just before sunset, midnight, or just before sunrise? Explain your reasoning.

Question 7.8

Why is Mercury more heavily cratered than Earth?

Question 7.9

Briefly describe a scientific theory explaining why Mercury has such a large iron core.

Question 7.10

Astronomers often refer to Venus as Earth’s twin. What physical properties do the two planets have in common? In what ways are the two planets dissimilar?

Question 7.11

Why is it hotter on Venus than on Mercury?

Question 7.12

What evidence exists for active volcanoes on Venus?

Question 7.13

Describe the Venusian surface. What kinds of geologic features would you see if you could travel around the planet?

Question 7.14

Why do astronomers believe that Venus’s surface was not molded by the kind of continuous tectonic activity that shaped Earth’s surface?

Question 7.15

Why is Mars red?

Question 7.16

What is the greenhouse effect? What role does it play in the atmospheres of Venus and Earth?

Question 7.17

During which configuration of planets is it easiest to observe Mars from Earth? (Hint: Guided Discovery: The Inner Solar System in Section 7-5 can help.)

Question 7.18

Explain why the distribution of craters on Mercury, the Moon, and Mars are different from one another.

Question 7.19

How would you tell which craters on Mars were formed by meteoritic impacts and which by volcanic activity?

Question 7.20

What geologic features indicate that plate tectonic activity once occurred on Mars? What features created by tectonic activity on Earth are not found on Mars?

Question 7.21

Why are volcanoes on Mars taller than those on Earth?

Question 7.22

What evidence do astronomers have that there are active volcanoes on Venus?

Question 7.23

What evidence have astronomers accumulated that liquid water once existed in large quantities on Mars’s surface? What evidence is there that water still exists under its surface?

Question 7.24

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

Question 7.25

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, explain why.

Question 7.26

As seen from Earth, the brightness of Venus changes as it moves along its orbit. Describe the main factors that determine Venus’s variations in brightness as seen from Earth. (Hint: See the discussion of Venus in Section 2-5.)

Question 7.27

How might Venus’s cloud cover change if all of Venus’s volcanic activity suddenly stopped? How might these changes affect the overall Venusian environment?

Question 7.28

Compare Venus’s continents with those on Earth. What do they have in common? How are they different?

Question 7.29

Explain why Mars has the longest synodic period of all of the planets, although its sidereal period is only 687 days.

Question 7.30

With carbon dioxide accounting for about 95% of the atmospheres of both Mars and Venus, why is there little greenhouse effect on Mars today?

Question 7.31

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

Question 7.32

If you were designing a space vehicle to land on Venus, what special features would be necessary? In what ways would this mission and landing craft differ from a spacecraft designed for a similar mission to Mercury?

Question 7.33

Suppose someone told you that the Viking mission failed to detect life on Mars simply because the tests were designed to detect terrestrial life-forms, not Martian life-forms. How would you respond?

Question 7.34

Compare the scientific opportunities for long-term exploration offered by the Moon and Mars. What difficulties would there be in establishing a permanent base or colony on each of these two worlds?

229

Question 7.35

Imagine you are an astronaut living at a base on Mars. Describe your day’s activities, what you see, the weather, the spacesuit you are wearing, and so on.

Question 7.36

Mercury had synchronous rotation? How would the temperatures on such a planet be different than they are today? Where would humans most likely set up camp on such a world?

Question 7.37

Venus could support self-aware life, but it still had permanent cloud cover? In what ways would life there be different than it is on Earth? How would their perceptions of the cosmos be different from ours?

Question 7.38

Mars had the same mass, surface features, and atmosphere as Earth? In what ways would life on Mars be different than it is on Earth?

Question 7.39

Mars rotated once every 20 Earth days rather than once every 1.026 Earth days? What would be different on the alternate Mars?

Question 7.40

The carbon dioxide content of Earth’s atmosphere increased? What would happen to Earth? (This is not a completely hypothetical question, because carbon dioxide levels are increasing today.)

Question 7.41

Search the Web for the latest information about the Messenger mission to Mercury. Exactly when was it launched? What scientific experiments does it carry? What scientific issues are these instruments intended to resolve? How long is it expected to function in orbit around Mercury?

Question 7.42

Elongations of Mercury Access the animation “Elongations of Mercury” in Chapter 7 of the Discovering the Universe Web site. (a) Note the dates of the greatest eastern and western elongations in the animation. 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).

Question 7.43

Search the Web for the latest information about proposed future missions to Venus. What scientific experiments will they carry? What scientific issues are these instruments intended to resolve?

Question 7.44

In 1999, two NASA spacecraft—Mars Climate Orbiter and Mars Polar Orbiter—failed to reach their destinations. Search the Web for information on these missions. What were their scientific goals? How and why did the missions fail? Which current and future missions, if any, are intended to replace these missions?

Question 7.45

Search the Web for information about possible manned missions to Mars. How long would such a mission take? How expensive would it be? What are some advantages and disadvantages of a manned mission compared to an unmanned one?

Question 7.46

Conjunctions of Mars Access and view the animation “The Orbits of Earth and Mars” in Chapter 7 of the Discovering the Universe Web site. (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 2 times during the animation when Mars is in conjunction, so that the Sun lies directly between Mars and Earth (see Figure 2-4). 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 observations when it is in conjunction?

Question 7.47

Mercury can be seen in the eastern sky at dawn for a few days around greatest western elongation. It can also be seen in the western sky at dusk for a few days around greatest eastern elongation. The following table shows the dates of the next few greatest elongations of Mercury. Magazines such as Sky & Telescope and Astronomy will tell you which, if any, of these greatest elongations is going to be especially favorable for viewing the planet. Alternatively, you can use your Starry Night™ planetarium software to examine these elongations. Open Sky Guide > Guided Tours > Our Solar System, stars and galaxies > The Planets > Mercury > A fast little world. This view shows Mercury and its orbital path around the Sun. Note how close Mercury stays to the Sun during its orbital journey. Stop the advance of time. Adjust the Date to each of the listed dates in the table below for Greatest western elongation and click on Sunrise to place the Sun on the horizon. You will note that the favorable elongations are those for which the angle of Mercury’s orbit with the horizon is largest. Now, adjust the Date to each of the listed dates for Greatest eastern elongation and click on Sunset and note the favorable dates for observing Mercury in the evening. Set the Time Flow Rate to 1 minute at any of these times and run time forward to observe Mercury’s descending motion with respect to the horizon. If an elongation is favorable, make plans to observe this innermost planet of the solar system, a planet that is rarely seen by anyone but astronomers. Set aside several evenings (or mornings) around the date of the favorable elongation to reduce the chances of being “clouded out.” Select an observing site that has a clear, unobstructed view of the horizon where the Sun sets (or rises). Make arrangements to have a telescope at your disposal, if possible. Search for the planet on the dates you have selected and make a drawing of its appearance through your telescope.

Question 7.48

Venus can be seen in the eastern sky in the hours before dawn for several months around greatest western elongation. It can also be seen in the western sky in the hours after dusk for several months around greatest eastern elongation. The following table shows you the optimum dates around which to observe Venus at one of these positions. If possible, view the planet through a telescope and make a sketch of the planet’s appearance. From your sketch, can you determine whether Venus is closer to us or farther from us than the Sun?

Question 7.49

Observe Venus through a telescope once a week for a month and make a sketch of the planet’s appearance on each occasion. From your sketches, can you determine whether Venus is approaching us or moving away from us?

Question 7.50

Consult such magazines as Sky & Telescope and Astronomy or use your Starry Night™ program to determine Mars’s location among the constellations. If Mars is suitably placed for observation, arrange to view the planet through a telescope and sketch this planet, noting particularly its phase. What magnifying power seems to give you the best image? Can you distinguish any surface features? Can you see a polar cap or dark markings? If not, can you offer an explanation for Mars’s bland appearance?

Question 7.51

Use Starry Night™ to examine Mercury. Select Favourites > Explorations > Mercury from the menu. Stop the advance of time and use the Zoom controls and the location scroller to examine the surface of the planet Mercury. Estimate the diameter of the largest craters on Mercury’s surface by measuring their size on the screen with a ruler and comparing these diameters to the diameter of Mercury. (a) What are the diameters (in kilometers) of the largest craters on Mercury? (b) Zoom in to examine the surface features of Mercury in more detail and compare these features with those of our Moon. Comment on the similarities and differences between these two planetary objects, neither of which has an atmosphere (i.e., presence of craters, light ray patterns, maria, peaks within craters).

Question 7.52

Use the Starry Night™ program to observe the apparent motion of Mercury on the celestial sphere. Select Favourites > Explorations > Mercury Motion. This view, from the center of a transparent Earth, is locked and centered upon the Sun. The background stars have been removed but the planets are visible, including the inferior planets Mercury and Venus. With the Time Flow Rate set to 1 day, click the Play button and observe the motions of these two inferior planets against the background stars as seen from Earth. (a) Use the time controls to find the first date after January 1, 2015, when Mercury is at its farthest point to the left of the Sun, and the first date after January 1, 2015, when Mercury is at its farthest point to the right of the Sun. What is your interpretation of these two dates and how would you label them? (b) If you wanted to make naked-eye observations of Mercury on these two dates, indicate the best time of day to make your observations for each date.

Question 7.53

Use Starry Night™ to examine Mars. Open Favourites > Explorations > Mars Surface. Use the Zoom controls and the location scroller to explore this planet’s surface features. You will notice that four volcanoes (mons) and the Valles Marineris have been labeled. (a) Which of the volcanoes appears to be the largest? (b) Right-click on Mars (Ctrl-click on a Mac) and select Markers and Outlines… from the contextual menu. In the Mars Markers and Outlines dialog window, click the lower radio button to the left of the List label. Then, in the dropdown box to the right of the List label, choose Type. In the rightmost dropdown box, select Crater as the type of feature and then click the Check all Shown button. Describe the distribution of craters on the Martian surface. (c) What does the distribution of craters in the region around the volcanoes suggest about the time at which these volcanoes formed on Mars?