Key Terms for Review

anorthosite

capture theory

cocreation theory

collision-ejection theory

continental drift

convection

core (of Earth)

crust

dynamo theory

ejecta blanket

fission theory

highlands

impact breccias

ionosphere (thermosphere)

mantle

mare (plural maria)

mare basalt

mascons

mesosphere

neap tide

northern lights (aurora borealis)

ozone layer

planetary differentiation

plate tectonics

regolith

rille

scarps

seafloor spreading

seismic waves

seismograph

southern lights (aurora australis)

spring tide

stratosphere

synchronous rotation

troposphere

Van Allen radiation belts

Review Questions

Question 6.1

The Moon’s surface is best described as:

  1. fine-grained powder
  2. solid rock
  3. rocky rubble
  4. liquid water oceans and dry land
  5. molten rock

Question 6.2

What type of chemical or molecule is most common in Earth’s atmosphere?

  1. carbon dioxide
  2. oxygen
  3. water
  4. nitrogen
  5. hydrogen

Question 6.3

Why does Earth’s albedo change daily? Seasonally?

Question 6.4

Why is Earth’s surface not riddled with craters as is that of the Moon?

Question 6.5

List the layers of Earth’s atmosphere.

Question 6.6

Describe the process of plate tectonics. Give specific examples of geographic features created by plate tectonics.

Question 6.7

How do we know about Earth’s interior, given that the deepest wells and mines extend only a few kilometers into its crust?

Question 6.8

What occurs on Earth when two tectonic plates of roughly equal density collide head-on?

Question 6.9

What occurs when a higher-density tectonic plate collides head-on with a lower-density plate?

188

Question 6.10

Describe the interior structure of Earth.

Question 6.11

Why is the center of Earth not molten?

Question 6.12

Describe Earth’s magnetosphere.

Question 6.13

What are the Van Allen radiation belts?

Question 6.14

Why do charged particles from the Sun (the solar wind) get trapped in the Van Allen radiation belts?

Question 6.15

What kind of features can you see on the Moon with a small telescope?

Question 6.16

Explain why the maria appear darker than the lunar highlands.

Question 6.17

Why are there so few craters on the maria?

Question 6.18

Where on the Moon did the largest impact craters occur?

Question 6.19

Briefly describe the main differences and similarities between Moon rocks and Earth rocks.

Question 6.20

How do we know that the maria were formed after the lunar highlands?

Question 6.21

What is a tidal force? How do tidal forces produce tides in Earth’s oceans?

Question 6.22

What is the difference between spring tides and neap tides? During which phase(s) of the Moon do each occur?

Question 6.23

Why do most scientists support the collision-ejection theory for the Moon’s formation?

Question 6.24

Why are nearly all of the craters on the Moon circular, even though many impacts there were not head-on?

Question 6.25

Why hasn’t the water ice near the Moon’s poles been evaporated by sunlight?

Advanced Questions

Question 6.26

Explain how the outward flow of energy from Earth’s interior drives the process of plate tectonics.

Question 6.27

Why do some geologists believe that Pangaea was the most recent in a succession of supercontinents?

Question 6.28

Why are active volcanoes, such as Mount St. Helens, usually located in mountain ranges along the boundaries of tectonic plates?

Question 6.29

Why is more lunar detail visible through a telescope when the Moon is near quarter phase than when it is at full phase?

Question 6.30

Why are the Moon rocks retrieved by astronauts so much older than typical Earth rocks, even though both worlds formed at nearly the same time?

Question 6.31

Some people who supported the fission theory proposed that the Pacific Ocean basin is the scar left when the Moon pulled away from Earth. Explain why this idea is wrong.

Question 6.32

Apollo astronauts left seismometers on the Moon that radioed seismic data back to Earth. The data showed that moonquakes occur more frequently when the Moon is at perigee (closest to Earth) than at other locations along its orbit. Give an explanation for this finding.

Question 6.33

Why do you think that no Apollo missions landed on the far side of the Moon?

Question 6.34

How might studying albedo help astronomers locate habitable worlds orbiting other stars?

Discussion Questions

Question 6.35

If Earth did not have a magnetic field, do you think auroras would be more common or less common than they are today? Explain.

Question 6.36

Comment on the idea that without the Moon’s presence, life would have developed far more slowly.

Question 6.37

Identify and compare the advantages and disadvantages of lunar exploration by astronauts as opposed to mobile, unmanned instrument packages and robots.

Question 6.38

When was the last earthquake near your hometown? How far is your hometown from a plate boundary? What kinds of topography (for example, mountains, plains, seashore) dominate the geography of your hometown area? Does that topography and the frequency of earthquakes seem to be consistent with your hometown’s proximity to a plate boundary?

Question 6.39

The ice on the Moon is believed to be mixed with rock just under the Moon’s surface. How might that water be economically extracted and purified?

What If…

Question 6.40

Earth had two moons? Assume one is at our Moon’s distance and the other is at half that distance. Describe the motion of the two moons in the sky and how they might appear to us. What would be different here on Earth?

Question 6.41

The Moon orbited Earth in the opposite direction from Earth’s rotation, rather than in the same direction? What would be different about Earth and life on it? Assume that today such a counter-revolving Moon would be at the same distance as our Moon.

Question 6.42

Our Moon were one-tenth of its actual distance from Earth? What would be different about Earth and life on it? Hint: The heights of tides vary as 1/r3, where r is the distance between the centers of Earth and the Moon.

Question 6.43

The Moon, located in its current orbit, were as massive as Earth? What would be different about Earth, life on Earth, and the Moon?

Question 6.44

Earth’s interior were entirely solid, rather than partly molten? What would be different about Earth and life on it?

189

Question 6.45

Earth were now in synchronous rotation with the Moon? That is, suppose that Earth rotated at the same rate that the Moon orbits Earth. What would be different about Earth and life on it?

Web Questions

Question 6.46

Use the Web to learn more about plate tectonics. What global changes might accompany the formation and breakup of a supercontinent? How might these changes affect the evolution of life? What life-forms dominated Earth when Pangaea existed some 200 million years ago, and also when fragments of the preceding supercontinent were as dispersed as today’s continents?

Question 6.47

Search the Web for information about “Pangaea Ultima,” a supercontinent that may form in the distant future. When is it expected to form? How will it compare to the Pangaea of 200 million years ago (see Figure 6-7)?

Question 6.48

Use the Web to determine the status of the Antarctic and Arctic ozone holes. How has the situation changed over the past few years? Explain why most scientists who study this issue blame chemicals called CFCs for the existence of the ozone holes.

Question 6.49

In 1989, representatives of many countries signed a treaty called the Montreal Protocol to protect the ozone layer. Use the Web to learn the current status of this treaty. How many nations have signed it? Has the treaty been amended? If so, how? List when various substances that destroy the ozone layer are being phased out.

Question 6.50

Search the Web for current information about upcoming lunar science missions. When is each scheduled to be launched? What new investigations are planned for each mission? What existing scientific issues may these missions resolve?

Question 6.51

Search the Web for current information about tourist trips into space. When are these scheduled? How long are the trips? What will they cost? What preparation is involved?

Question 6.52

Refinements to the modeled mass and impact angle of the planetesimal that struck Earth and created the Moon have recently been made. Search the Web for this information and explain the justification for these new parameters.

Question 6.53

Some astronomers have observed changes in brightness and color on the Moon, called lunar transient phenomena. Search the Web and explain what these events are and what their origins are believed to be.

Got It?

Question 6.54

Can the Earth’s ozone layer, which has been depleted over decades, be naturally replenished? If so, how?

Question 6.55

Since infrared radiation (heat) from the Sun does not enter a greenhouse, how do greenhouses get warm?

Question 6.56

How can we tell by looking at the Moon that it rotates?

Question 6.57

How were the craters on the Moon formed?

Question 6.58

When was the last year that anyone landed on the Moon?

Observing Projects

Observing Tips and Tools: You can learn a lot by observing the Moon through binoculars. Note that the Moon will appear right side up through binoculars or a terrestrial telescope such as a spotting scope, but will appear inverted through an astronomical telescope. You will need to take this into account if you use a map of the Moon to help with your observations. Inexpensive Moon maps are available, such as “Moon Map” published by Sky & Telescope magazine. To determine the lunar phase (when you cannot examine the Moon directly), you can find it on most calendars or by checking the weather page in your newspaper. You can also consult current issues of Sky & Telescope and Astronomy magazines, look on the Web, or use the Starry Night™ program.

Question 6.59

Use Starry Night™ to view the Earth from space. Open the view named Favourites > Explorations > Earth’s Surface. The view shows the rotating Earth from a point in space about 12,000 km above the surface. The clouds and atmosphere have been removed from the image and the side of Earth facing away from the Sun is artificially brightened so that you can use the location scroller and Zoom controls to inspect the entire surface of our home planet. (a) Can you see any evidence of plate tectonics on Earth? (b) Can you see any evidence of life or of man-made structures or objects?

Question 6.60

Observe the Moon through a telescope or binoculars for a few nights over a period of two weeks between new Moon and full Moon. Make sketches of various surface features, such as craters, mountain ranges, and maria. (a) How does the appearance of these features change with the Moon’s phase? (b) Which features are most easily seen at a low angle of illumination (i.e., near to the terminator)? (c) Which features show up best at full Moon, when the Moon’s surface is directly illuminated by the Sun (i.e., far from the terminator)? (d) Compare the texture of the lunar surface within the dark maria with that of the lunar highlands.

Question 6.61

If you live near the ocean, observe the tides to see how the times of high and low tides are correlated with the position of the Moon in the sky.

190

Question 6.62

Use the Starry Night™ program to view the Moon as it appears from Earth and compare this with the view of Earth as it would appear from the Moon. Open the view named Favourites > Explorations > Moon From Earth. With the Time Flow Rate set at 1 day, Step time forward while observing the Moon in the view. Use the hand tool to adjust the screen as necessary to follow the Moon in the sky from day to day. When the Moon sets below the horizon, select File > Revert to return to the original view. Right-click (ctrl-click on a Mac) over the image of the Moon and select Magnify from the contextual menu to center a magnified image of the Moon in the view. Next, type ctrl-H (type B on a Mac) to remove the horizon from the view so that you may follow the appearance of the Moon over time without interference from the horizon. (a) Step time forward and observe the features on the Moon. Do you see a different view of the Moon from night to night or do you always see the same features on the Moon? Explain why the appearance of the Moon shows this behavior as time changes. Select Favourites > Explorations > Tranquility Base to change your viewing location to the Apollo 11 landing site on the Moon, with the Earth visible in the lunar sky. With the Time Flow Rate at 1 hours, click the Play button in the toolbar. (b) Describe and explain your observations of the position of Earth in the lunar sky. Why does the lunar horizon not interfere with the visibility of Earth from this lunar viewing location?

Question 6.63

Use Starry Night™ to observe the apparent size of the Moon as seen from the Earth. Select Favourites > Explorations > Moon Size. The view shows the full Moon as it would appear from the North Pole of Earth. Surrounding the Moon is a yellow circle that is precisely 30 arcminutes in diameter. As you can see, at the time and date of this view, the Moon does not quite fill this circle. To prevent the phases of the Moon from interfering with your observations of the size of the Moon in the sky, the dark hemisphere of the Moon is artificially brightened in this simulation. With the Time Flow Rate at 1 days, step time forward and observe the size of the Moon relative to the 30-arcminute reference circle. (a) Does the apparent size of the Moon remain constant? If not, explain what this tells us about the shape of the Moon’s orbit around Earth. (b) Approximately how many days does it take for the Moon to go through a complete cycle of variation of its apparent size? (Hint: Count the number of one-day time steps between two successive occasions when the Moon appears to fit precisely inside the 30′ reference circle.) (c) As you make your observations, do the lunar surface features appear to remain stationary in the view? (They would remain stationary, if the Moon’s synchronous rotation were the only motion that the Moon had, relative to Earth.) If not, explain. (Hint: A Web search on the word “libration” [not libation!] may help.)

191

WHAT IF…: The Moon Didn’t Exist

Throughout history, people have woven myths about the Moon and its effects on everything from child-birth to stock market activity. Countless romances have begun under a full Moon, and entire nations have dedicated themselves to reaching it.

What if the Moon never existed? Would life have even developed on Earth? If so, how would it be different? Would a self-aware species like ourselves have evolved?

Delayed Origins

It would have been much more difficult for life to start evolving on a Moonless Earth. In the first place, the minerals from which life developed in our Earth’s young oceans were swept down there, in large part, by gigantic tides, a thousand times higher than the tides of today, created by the Moon when it was some 10 times closer to Earth than at present. The newly formed Earth, spinning more than 4 times faster than at present, caused these tides to move miles inland and back out to sea every 1½ hours or so.

A Moonless Earth would still have tides, caused by the Sun. However, these tides would never be more than one-third as high as Earth’s present tides. Minerals would wash into the oceans incredibly slowly from the flow of rivers. As a result, it would likely take much longer, hundreds of millions or even billions of years longer, for enough minerals to be dissolved for life to firmly establish itself.

Harsh Conditions

Animal life’s transition from oceans to land would also be much harder because of continuous winds, between 50 and 150 mi/h, created by the planet’s rapid rotation. The resulting waves (wind causes waves) would be enormous and perpetual. Fish with legs near shores would almost certainly be pounded to a pulp rather than being able to sedately move onto land and then back into the water, as apparently happened on Earth.

But we see on Earth how life develops in the most incredible forms and places. It seems plausible, then, that sea life would find a way to make the transfer to dry land and continue to evolve there.

Allowing then for diverse terrestrial life on the Moonless Earth, what would be different about that life compared to life on our Earth? For one thing, creatures evolving there would have to withstand the perpetual pummeling from winds and the debris they carry. Turtle-like shells are one solution.

Obviously, there would be no eclipses or moonlight—all clear nights would be equally dark and star-filled. Therefore, nocturnal animals would be less successful at hunting, foraging, and traveling. Instead, these animals might evolve more enhanced senses to compensate for their inability to see visible light at night.

Rush Hour

Clearly, naked apes do not seem a likely bet on a Moonless Earth, or birds battling the ever-present winds. But given enough time, complex, even self-aware life likely would evolve. After all, we evolved because of the challenges faced by our ancestors, and Earth without a Moon would clearly provide challenges of its own.

The physiology of life on Earth evolved based on a 24-hour day. This is most evident in our biological clocks, or circadian rhythms, which are the internal mechanisms that regulate sleeping, waking, eating, and other cyclic activities. Faced with a 6-hour day, these circadian rhythms would be hopelessly out of sync with the natural world. To function in such a world, all of its creatures would have to evolve biological clocks based on 6-hour cycles, which certainly could have occurred. Considering all that we have to do now, imagine what life would be like with only 6 hours in each day!

Wobbling Earth

Finally, rapidly rotating terrestrial planets without large moons are unstable, and calculations show that they dramatically change the direction of their rotation axes. If Earth had no Moon, we would have seasons that vary dramatically over the millennia, from those like we have today to times of no seasons to times when the Sun passes over every place on Earth, including the poles! Our Moon stabilizes Earth’s rotation, preventing such phenomena. It would be incomparably harder for complex life to persist on a Moonless Earth than it is on our world.

192