11) Venus has a thicker atmosphere than Earth.
12) In the inner solar system, the largest surface features (such as volcanoes or valleys) are found
on the largest planets.
13) The high level of oxygen in Earth’s atmosphere can be explained by chemical processes.
14) The rise of life on the planet Earth some 4 billion years ago was made possible, in part, by
the plentiful oxygen in Earth’s early atmosphere.
15) The process in which one plate slides under another is called subduction and occurs where
seafloor crust descends into the mantle below continental crust.
16) Earth is the only terrestrial planet to have experienced tectonic stresses and volcanic activity.
17) Earth outgassed as much carbon dioxide as Venus, but on Earth most of it is locked up in the
oceans and rocks.
18) The current atmospheric concentration of carbon dioxide on Earth is at its highest in over a
million years.
19) Earth is habitable in part due to the greenhouse effect.
20) Current CO2 levels are higher than they have been at any other time during the history of the
human species.
21) We are confident that rising CO2 levels are caused by the burning of fossil fuels because
atmospheric CO2 now shows the same carbon isotope ratio as fossil fuels.
22) Historically, there is no link between increased CO2 levels and higher average temperatures.
18
7.3 Process of Science Questions
1) The Gaia Hypothesis: The Gaia hypothesis, due to James Lovelock, maintains that the
Earth—its atmosphere, the oceans, the land, and life—represents a complex interacting system
that will always seek to maintain a life-supporting environment. Discuss the operation of the
Earth’s carbon dioxide cycle and how it might relate to the Gaia hypothesis. The Gaia hypothesis
is still quite controversial. Perform some research (either in your library or on reputable internet
reference sites) and summarize in a few paragraphs the arguments for and against Gaia.
2) Evidence for Life: Imagine that recent advances in technology have allowed the discovery of
several very low mass (much less than the mass of Jupiter) planets around nearby, Sun-like stars.
Assume all that is known about these planets is their mass, their size, and the characteristics of
their orbits (i.e., semi-major axis and eccentricity). You are asked to speculate on whether or not
life (similar to that on Earth) might exist on these planets. How would you answer this question?
You are then asked what new observations we might make to help settle this question. How
would you respond?
3) Public Policy Informed by Science: Research the Montreal Protocol (1987) that led to a world-
wide ban in the use of chlorofluorocarbon compounds (CFCs). What was the scientific case for
the ban? Why do you think such coordinated, world-wide action was possible in this case, yet a
global consensus has not emerged on how to respond to the challenge of climate change?
4) Comparative planetology: Describe at least one way in which comparisons of Earth with the
Moon, Mercury, Venus, and / or Mars have helped us to better understand our own planet.
an understanding of the importance of tectonics.
7.4 Short Answer Questions
1) How does the size of a planet determine its internal temperature?
2) Describe the three sources of internal heat of the terrestrial planets.
3) Explain how we can estimate the geological age of a planetary surface from its number of
impact craters.
4) What is the main visual difference between the lunar maria and the rest of the lunar surface?
What are the implications of this observation?
5) Why do we think Mercury contracted after it formed?
6) Briefly explain why Mercury, Venus, and the Moon do not have significant erosion. Relate
erosional activity to planetary size and distance from the Sun.
7) Suppose Mars had turned out to be significantly smaller than its current size, say about the
size of our Moon. How would this have affected the number of geological features due to each of
the four major geological processes?
8) Summarize some of the evidence suggesting that Mars once had flowing water.
9) Briefly describe how the greenhouse effect makes a planetary surface warmer than it would be
otherwise.
10) What is a magnetosphere and how are magnetospheres related to planetary atmospheres?
11) Briefly describe how the solar wind affects magnetospheres and how auroras are produced.
12) Earth and Venus both presumably had similar gases outgassed from their volcanoes. Briefly
explain how their atmospheres ended up so different.
13) Why did Earth retain most of its water while Venus and Mars lost theirs?
14) Why does Earth have so much more oxygen (O2) than Venus or Mars?
15) Why does Earth have an ultraviolet-absorbing stratosphere, while Venus and Mars do not?
16) What is the “runaway greenhouse effect”?
17) Why does Earth have so little carbon dioxide in its atmosphere, when Earth should have
outgassed about as much of it as Venus?
18) Briefly describe at least three likely consequences of continued human burning of fossil fuels
adding to Earth’s greenhouse effect.
19) What are some of the things that make Earth unique among the terrestrial planets?
7.5 Mastering Astronomy Reading Quiz
1) Suppose we use a baseball to represent Earth. On this scale, the other terrestrial worlds
(Mercury, Venus, the Moon, and Mars) would range in size approximately from that of
________.
A) a golf ball to a baseball
B) a dust speck to a golf ball
C) a dust speck to a basketball
D) a golf ball to a beach ball
2) From center to surface, which of the following correctly lists the interior layers of a terrestrial
world?
A) core, mantle, lithosphere, crust
B) mantle, core, crust, lithosphere
C) core, mantle, crust, lithosphere
D) mantle, core, lithosphere, crust
3) What do we mean when we say that the terrestrial worlds underwent differentiation?
A) When their interiors were molten, denser materials sank toward their centers and lighter
materials rose toward their surfaces.
B) The five terrestrial worlds all started similarly but ended up looking quite different.
C) Their surfaces show a variety of different geological features resulting from different
geological processes.
D) They lost interior heat to outer space.
4) A terrestrial world’s lithosphere is ________.
A) a layer of relatively strong, rigid rock, encompassing the crust and part of the mantle
B) a thin layer of rock that lies between the mantle and crust
C) the interior region in which the planet’s magnetic field is generated
D) a layer of hot, molten rock encompassing the core and part of the mantle
5) The two most important processes in heating the interiors of the terrestrial worlds are
________.
A) (1) heat generated by accretion during the process of formation; (2) heat released by
radioactive decay
B) (1) heat generated by accretion during the process of formation; (2) heat that came from the
gravitational potential energy of incoming planetesimals
C) (1) heat from radioactive decay; (2) heat from thermal radiation
D) (1) volcanism; (2) heat from radioactive decay
6) Which of the following is an example of convection?
A) warm air expanding and rising while cooler air contracts and falls
B) different kinds of material separating by density, like oil and water
C) rocks sinking in water
D) ice floating on a frozen lake
7) How does Earth’s magnetosphere affect our atmosphere?
A) It deflects solar wind particles that would otherwise strip away the atmosphere.
B) It scatters the shorter wavelengths of light from the sun and causes the sky to be blue.
C) It produces chemical reactions that cause ozone to form in the upper atmosphere.
D) all of the above
8) What is a magnetosphere?
A) a region of space around a planet in which the planet’s magnetic field can affect charged
particles
B) the layer of a planet in which its magnetic field is generated
C) the uppermost layer of any planetary atmosphere
D) the region in a planet’s atmosphere in which auroras occur
9) The processes responsible for all surface geology on the terrestrial planets are ________.
A) impact cratering, volcanisms, tectonics, and erosion
B) convection, conduction, and radiation
C) accretion, differentiation, and radioactive decay
D) eruptions, lava flows, and outgassing
10) In the context of planetary geology, what do we mean by outgassing?
A) the release by volcanism of gases that had been trapped in a planetary interior
B) the loss of atmospheric gas to outer space
C) another name for a volcanic eruption
D) the evaporation of water that adds water vapor (a gas) to an atmosphere
11) Which of the following is not an example of tectonics?
A) the gradual disappearance of a crater rim as a result of wind and rain
B) the formation of a cliff when the lithosphere shrinks
C) the slow movement of Earth’s lithospheric plates
D) the stretching of the crust by underlying mantle convection
12) Which of the following is the most basic definition of a greenhouse gas?
A) a gas that absorbs infrared light
B) a gas that absorbs visible light
C) a gas that keeps warm air from rising, and therefore warms the surface
D) a gas that reflects infrared light
13) Suppose we represent Earth with a basketball. On this scale, most of the air in Earth’s
atmosphere would fit in a layer that is ________.
A) about the thickness of a sheet of paper
B) about an inch thick
C) about 6 inches thick
D) about a half-inch thick
14) Which of the following best describes the lunar maria?
A) relatively smooth, flat plains on the Moon
B) densely cratered regions on the Moon
C) mountainous regions on the Moon
D) frozen oceans of liquid water on the Moon
15) Why does the Moon have a layer of “powdery soil” on its surface?
A) It is the result of countless tiny impacts by small particles striking the Moon.
B) Recent, large impacts shattered lunar rock to make this material.
C) It is made by the same processes that make powdery soil on Earth.
D) It exists because the Moon accreted from powdery material after a giant impact blasted the
Earth.
16) What observational evidence supports the idea that Mercury shrank after it formed?
A) the presence of many long, tall cliffs
B) the presence of many impact craters
C) the characteristics of the Caloris Basin
D) Mercury’s unusually high density
17) The northern hemisphere of Mars has fewer craters than the southern hemisphere. This
indicates that
A) geologic processes have resurfaced much of the northern hemisphere.
B) the northern hemisphere did not experience as many impacts.
C) volcanoes were more common in the southern hemisphere.
D) oceans of water once covered the southern hemisphere.
18) Which of the following does not provide evidence that Mars once had abundant liquid water
on its surface?
A) the presence of canali, discovered in the late 1800s by Giovanni Schiaparelli and mapped by
Percival Lowell
B) the presence of features that look like dried up river beds
C) the presence of minerals on the surface that formed in water
D) the appearance of old craters rims that indicate they were eroded by flowing water
19) Based on the surface features of Venus, the event that most significantly altered its surface is
A) a global “repaving” that erased essentially all the surface features that had existed earlier.
B) the eruption of a giant volcano that formed one of Venus’s “continents.”
C) the impact of an unusually large asteroid that left a deep scar on one side of the planet.
D) the onset of mantle convection, which caused Venus’s lithosphere to split into plates like
those on Earth.
20) What do we mean by a runaway greenhouse effect?
A) a greenhouse effect that keeps getting stronger until all of a planet’s greenhouse gases are in
its atmosphere
B) a greenhouse effect that starts on a planet but later disappears as gases are lost to space
C) a greenhouse effect that heats a planet so much that its surface rock melts
D) a process that heats a planet like a greenhouse effect, but that involves a completely different
mechanism of heating that doesn’t actually involve greenhouse gases
21) On average, how fast do the tectonic plates move on Earth’s surface?
A) a few centimeters per year
B) a few millimeters per century
C) about 1 mile per hour
D) a few kilometers per year
22) How does seafloor crust differ from continental crust?
A) Seafloor crust is thinner, younger, and higher in density.
B) Seafloor crust is thicker, older, and higher in density.
C) Seafloor crust is thicker, younger, and lower in density.
D) Seafloor crust is thinner, older, and lower in density.
23) In the context of plate tectonics, what is a subduction zone?
A) a place where a seafloor plate is sliding under a continental plate
B) a place where two plates are slipping sideways against one another
C) a place where two plates are pulling apart
D) a place where two continental plates are colliding
24) Considering Earth’s climate, what is the importance of the carbon dioxide (CO2) cycle?
A) It helps regulates the carbon dioxide concentration in our atmosphere, keeping temperatures
moderate.
B) It helps produce ozone, which blocks ultraviolet radiation from the Sun.
C) It is responsible for the levels of nitrogen and oxygen found in our atmosphere.
D) It will prevent us from suffering any consequences from global warming.
25) Earth has been gradually warming over the past few decades. Based on a great deal of
evidence, scientists believe that this warming is caused by ________.
A) human activities that are increasing the concentration of greenhouse gases in Earth’s
atmosphere
B) the increase in forest fires during recent years
C) the human release of chemicals called CFCs into the stratosphere
D) the fact that our politicians spout a lot of hot air
26) Based on all we know about the terrestrial worlds, what single factor appears to play the
most important role in a terrestrial planet’s geological destiny?
A) its size
B) its composition
C) its distance from the Sun
D) whether or not it has liquid water
7.6 Mastering Astronomy Concept Quiz
1) The cores of the terrestrial worlds are made mostly of metal because ________.
A) metals sunk to the centers a long time ago when the interiors were molten throughout
B) the terrestrial worlds as a whole are made mostly of metal
C) the core contained lots of radioactive elements that decayed into metals
D) over billions of years, convection gradually brought dense metals downward to the core
2) Which internal heat source still actively generates heat within the terrestrial worlds today?
A) heat from radioactive decay
B) heat of accretion
C) heat from differentiation
D) heat from convection
3) The reason that small planets tend to lose interior heat faster than larger planets is essentially
the same as the reason that ________.
A) a large baked potato takes longer to cool than a small baked potato
B) lower density bubbles form and rise upward in boiling water
C) Earth contains more metal than the Moon
D) thunderstorms tend to form on hot summer days
4) Suppose we had a device that allowed us to see Earth’s interior. If we looked at a typical
region of the mantle, what would we see happening?
A) Not much—on human time scales, the mantle looks like solid rock.
B) hot molten rock rising upward throughout the mantle and cool, solid rock falling downward
C) a rapid, up and down churning of the material in the mantle
D) dense metals falling downward while low-density rock rises upward
5) Recent evidence suggests that Mars once had a global magnetic field. Assuming this is true,
which of the following could explain why Mars today lacks a global magnetic field like that of
Earth?
A) Mars’s interior has cooled so much its molten core layer no longer undergoes convection.
B) Mars rotates much slower than the Earth.
C) The Martian core is made of rock, while Earth’s core is made of metal.
D) Mars is too far from the Sun to have a global magnetic field.
6) Which of the following most likely explains why Venus does not have a global magnetic field
like Earth?
A) Its rotation is too slow.
B) It does not have a metallic core.
C) Unlike Earth, Venus does not have a liquid outer core.
D) It has too thick of an atmosphere.
7) What would happen to Earth’s surface temperature if you painted the entire surface of the
Earth black?
A) Its average temperature would go up.
B) Its average temperature would go down.
C) Its temperature would not change.
D) It is impossible to say how the temperature would change.
8) Which of the following general statements about Earth’s atmosphere is not true?
A) The nitrogen and oxygen in Earth’s atmosphere keep the surface pleasantly warm.
B) Gas high in the atmosphere absorbs dangerous X-rays from the Sun.
C) Without the relatively rare gas called ozone, Earth’s surface would be bathed in dangerous
ultraviolet light from the Sun.
D) The oxygen in our atmosphere was released by living organisms.
9) Why is the sky blue (on Earth)?
A) because molecules scatter blue light more effectively than red light
B) because the Sun emits mostly blue light
C) because molecules scatter red light more effectively than blue light
D) because deep space is blue in color
E) No one knows—this is one of the great mysteries of science.
10) Which of the following best describes how the greenhouse effect works?
A) A planet’s surface absorbs visible sunlight and returns this absorbed energy to space as
infrared light. Greenhouse gases slow the escape of this infrared radiation, which thereby heats
the lower atmosphere.
B) Greenhouse gases absorb X-rays and ultraviolet light from the Sun, and this absorbed
radiation then heats the atmosphere and the surface.
C) Greenhouse gases absorb infrared light coming from the Sun, and this absorbed sunlight heats
the lower atmosphere and the surface.
D) The greenhouse effect is caused primarily by ozone, which absorbs ultraviolet light and
thereby makes the atmosphere much hotter than it would be otherwise.
11) Why does the burning of fossil fuels increase the greenhouse effect on Earth?
A) Burning releases carbon dioxide into the atmosphere.
B) Burning fuel warms the planet.
C) Burning depletes the amount of ozone, thereby warming the planet.
D) Burning produces infrared light, which is then trapped by existing greenhouse gases.
12) Most of the Moon’s surface is densely covered with craters, but we find relatively few craters
within the lunar maria. What can we conclude?
A) The maria formed after the heavy bombardment ended.
B) The maria formed within the past 1 billion years.
C) The regions of the maria were hit by fewer impacts than the densely cratered regions.
D) Erosion affects the maria more than it affects other regions of the Moon.
13) Which of the following best describes the geological histories of the Moon and Mercury?
A) Early in their histories, they suffered many impacts and experienced some volcanism and
tectonics, but they now have little geological activity at all.
B) Impact cratering and erosion are the only major geological processes that has affected their
surfaces.
C) All four geological processes were important in their early histories, but only impact cratering
still reshapes their surfaces today.
D) Impact cratering shaped these worlds early in their histories. Then, during the past few
million years, they were reshaped by episodes of volcanism and tectonics.
14) What makes us think that Mars must once have had an atmosphere that was warmer and
thicker?
A) The atmosphere is too cold and thin for liquid water today, yet we see evidence that water
flowed on the surface in the past.
B) We think it for purely theoretical reasons, based on calculations showing that the Sun has
brightened with time.
C) The presence of inactive volcanoes on Mars tells us that there must once have been a lot of
outgassing, and hence a thicker atmosphere.
D) The fact that parts of Mars have a lot of craters tells us that Mars must once have been much
warmer.
15) All of the following statements about Mars are true. Which one might have led to a
significant loss of atmospheric gas to space?
A) Mars lost any global magnetic field that it may once have had.
B) The axis tilt of Mars is thought to change significantly over time.
C) Oxygen reacts chemically with surface rock on Mars.
D) Mars probably once had a much higher density of greenhouse gases in its atmosphere than it
does today.
16) Why is Mars red?
A) Chemical reactions between surface rock and atmospheric oxygen literally rusted the surface.
B) Martian volcanoes released a much redder lava than volcanoes on Earth.
C) The red color of Mars is a result of the scattering of light in the Martian sky.
D) The red color is caused by water ice chemically bound in surface rock.
17) All of the statements below are true. Which one gives the primary reason why the surface of
Venus today is some 450°C hotter than the surface of Earth?
A) Venus has a much stronger greenhouse effect than Earth.
B) Venus is only about 73% as far from the Sun as Earth.
C) Venus has a much higher reflectivity than Earth.
D) Venus has a higher atmospheric pressure than Earth.
18) Many scientists suspect that Venus has a stronger and thicker lithosphere than Earth. If this is
true, which of the following could explain it?
A) the high surface temperature that has “baked out” all the liquid water from Venus’s crust and
mantle
B) the smaller size of Venus, which has allowed it to lose much more internal heat than Earth
C) the slow rotation of Venus
D) the apparent lack of plate tectonics on Venus
19) Which of the following best explain what we think happened to outgassed water vapor on
Venus?
A) Ultraviolet light split the water molecules, and the hydrogen then escaped to space.
B) Water was removed from the atmosphere by chemical reactions with surface rock.
C) It is frozen as water ice in craters near the poles.
D) It turned into carbon dioxide by reacting with nitrogen in Venus’s atmosphere.
20) Which of the following is the main reason why Venus has so little wind erosion?
A) its slow rotation
B) its small size
C) its thick atmosphere
D) its relatively close distance from the Sun
21) All the following statements about Venus are true. Which one offers evidence of a global
repaving about a billion years ago?
A) Venus has relatively few impact craters and these craters are distributed fairly evenly over the
entire planet.
B) Venus has many circular features, called coronae, which appear to be tectonic in origin.
C) Venus’s largest features are three elevated regions that look somewhat like continents.
D) Venus appears to lack any water that could lubricate the flow of rock in its crust and mantle.
22) Why are there fewer large impact craters on the Earth’s seafloor than on the continents?
A) Seafloor crust is younger than continental crust, so it has had less time in which to suffer
impacts.
B) The oceans slow large impactors and prevent them from making craters.
C) Erosion erases impact craters must faster on the ocean bottom than on land.
D) Most impacts occur on the land.
23) Why does Venus have such a large difference in temperature between its “no atmosphere”
temperature and its actual temperature?
A) It has a large amount of carbon dioxide in its atmosphere.
B) It has a high level of volcanic activity.
C) It is so close to the Sun.
D) It has a slow rotation.
24) Which two factors are most important to the existence of plate tectonics on Earth?
A) mantle convection and a thin lithosphere
B) oxygen in the atmosphere and mantle convection
C) the existence of life and oxygen in the atmosphere
D) Earth’s liquid outer core and solid inner core
25) The atmospheres of Venus and Mars are primarily composed of carbon dioxide. Why is the
surface of Venus so much hotter than the surface of Mars?
A) Venus’ atmosphere is thousands of times thicker than Mars’ atmosphere.
B) The clouds of Venus are composed of sulphuric acid, which traps more heat.
C) Venus is much closer to the sun than Mars.
D) Carbon dioxide on Venus behaves differently than it does on Mars.
26) Which two factors are critical to the existence of the carbon dioxide (CO2) cycle on Earth?
A) plate tectonics and liquid water oceans
B) life and atmospheric oxygen
C) life and active volcanism
D) active volcanism and active tectonics
27) Suppose Earth were to cool down a little. How would the carbon dioxide cycle tend to
restore temperatures to normal?
A) Cooler temperatures lead to slower formation of carbonate minerals in the ocean, so carbon
dioxide released by volcanism builds up in the atmosphere and strengthens the greenhouse effect.
B) Cooler temperatures cause volcanoes to become more active, so they release more carbon
dioxide into the atmosphere.
C) Cooler temperatures cause more rain, which would remove more carbon dioxide from the
atmosphere.
D) Cooler temperatures mean more ice and more erosion, which would cause the planet to warm
up.
28) Which of the following is not an expected consequence of global warming?
A) the entire Earth will warm up by the same amount
B) an increase in the severity of winter blizzards
C) an increase in the number and intensity of hurricanes
D) melting of polar ice and glaciers
29) The choices below describe four hypothetical planets. Which one would you expect to have
the hottest interior? (Assume the planets orbit a star just like the Sun and that they are all the
same age as the planets in our solar system.)
A) Size: twice as big as Earth. Distance from Sun: same as Mercury. Rotation rate: once every 6
months.
B) Size: same as the Moon. Distance from Sun: same as Mars. Rotation rate: once every 10 days.
C) Size: same as Mars. Distance from Sun: same as Earth. Rotation rate: once every 18 hours.
D) Size: same as Venus. Distance from Sun: same as Mars. Rotation rate: once every 25 hours.
30) The choices below describe four hypothetical planets. Which one’s surface would you expect
to be most crowded with impact craters? (Assume the planets orbit a star just like the Sun and
that they are all the same age as the planets in our solar system.)
A) Size: same as the Moon. Distance from Sun: same as Mars. Rotation rate: once every 10 days.
B) Size: twice as big as Earth. Distance from Sun: same as Mercury. Rotation rate: once every 6
months.
C) Size: same as Mars. Distance from Sun: same as Earth. Rotation rate: once every 18 hours.
D) Size: same as Venus. Distance from Sun: same as Mars. Rotation rate: once every 25 hours.
31) The choices below describe four hypothetical planets. Which one would you expect to have
the most features of erosion? (Assume the planets orbit a star just like the Sun and that they are
all the same age as the planets in our solar system.)
A) Size: same as Venus. Distance from Sun: same as Mars. Rotation rate: once every 25 hours.
B) Size: same as the Moon. Distance from Sun: same as Mars. Rotation rate: once every 10 days.
C) Size: same as Mars. Distance from Sun: same as Earth. Rotation rate: once every 18 hours.
D) Size: same size as Earth. Distance from Sun: same as Mercury. Rotation rate: once every 6
months.
32) What is the main reason that Mars, compared to Earth, has become so geologically inactive?
A) its size
B) its distance from the Sun
C) its composition
D) its tilt
E) its rotation rate