CHAPTER 21 | THE MOON AND MERCURY: COMPARING AIRLESS WORLDS 463
- The trenches where Earth’s seafl oor slips downward are 1 km or less
wide. Could Earth-based telescopes resolve such features on the
moon? Why can you be sure that such features are not present on the
moon? - The Apollo command module orbited the moon about 100 km above
the surface. What was its orbital period? (Hint: See Chapter 5.) - From a distance of 100 km above the surface of the moon, what is the
angular diameter of an astronaut in a spacesuit? Could someone have
seen the astronauts from the command module? (Hint: Use the small-
angle formula, Chapter 3.) - If you transmitted radio signals to Mercury when it was closest to
Earth and waited to hear the radar echo, how long would you wait? - Suppose you sent a spacecraft to land on Mercury, and it transmitted
radio signals to Earth at a wavelength of 10 cm. If you saw Mercury
at its greatest angular distance west of the sun, to what wavelength
would you have to tune your radio telescope to detect the signals?
(Hints: See Celestial Profi le 4 and the section on the Doppler shift in
Chapter 7.) - What would the wavelength of maximum output be for infrared
radiation from the surface of Mercury? How would that differ for the
moon? - Calculate the escape velocity from Mercury. How does that compare
with the escape velocity from the moon and from Earth?
Learning to Look
- Examine the mountains at the Apollo 17 landing site (Figure 21-4).
What processes shape mountains on Earth that have not affected
mountains on the moon? - The rock shown in Figure 21-7 was thrown from beyond the horizon
and landed where the astronauts found it. It must have formed a
small impact crater, but the astronauts didn’t fi nd traces from that
impact. Why not? (Hint: When did the rock fall?) - In this photo, astronaut Alan Bean works
at the Apollo 12 lander. Describe the
surface you see. What kind of terrain did
they land on for this, the second human
moon landing? - Why are so many lunar samples breccias?
- What do the vesicular basalts tell you about the evolution of the lunar
surface? - What evidence would you expect to fi nd on the moon if it had been
subjected to plate tectonics? Do you fi nd such evidence? - How does the large-impact hypothesis explain the moon’s lack of
iron? Of volatiles? - How does the tidal coupling between Mercury and the sun differ from
that between the moon and Earth? - What is the difference between the intercrater plains and the smooth
plains in terms of time of formation? - What evidence can you cite that Mercury has a partially molten,
metallic core? - How were the histories of the moon and Mercury similar? How were
they different? - What is the main reason that the surface evolution of both the moon
and Mercury has essentially come to a stop, but the Earth’s surface
evolution continues? - How Do We Know? How is a hypothesis or theory like a container
in which you can carry an assortment of ideas, observations, facts,
and measurements?
Discussion Questions
- Old science-fi ction paintings and drawings of colonies on the moon
often showed very steep, jagged mountains. Why did the artists
assume that the mountains would be more jagged than mountains on
Earth? Why are lunar mountains actually less jagged than mountains
on Earth? - From your knowledge of comparative planetology, propose a
description of the view that astronauts would have if they landed on
the surface of Mercury.
Problems
- Calculate the escape velocity of the moon from its mass and diameter.
(Hint: See Chapter 5.) - Why do small planets cool faster than large planets? Compare surface
area to volume. Why is that comparison important? - The smallest detail visible through Earth-based telescopes is about 1
arc second in diameter. What linear size is this on the moon? (Hint:
Use the small-angle formula, Chapter 3.)
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