Chapter 7 Astronomy Homework

_____ found in the Atlantic Ocean between North America and Europe is composed of dense and relatively young rock.

seafloor crust.

Which of the following Mars surface features provides dramatic evidence that volcanism has played a role in shaping the surface of Mars? (A.) the polar caps. (B.) the southern highlands. (C.) Valles Marineris. (D.) Olympus Mons.

(D.) Olympus Mons. Correct. Olympus Mons is a very large shield volcano. You can also see numerous other volcanoes on Mars, including three large ones on the Tharsis bulge.

Layered _____ exposed by erosion can be seen when looking at the Grand Canyon in the United States.

sedimentary rock.

The extremely deep ocean Marianas Trench is a result of _____.

subduction.

Australia is composed of relatively old and thick _____.

continental crust.

The earthquakes that occur in Southern California generally occur above a _____.

fault.

Old Faithful Geyser at Yellowstone National Park in the United States acquires its energy from a _____.

hot spot.

The slowly increasing distance between South America and Africa is due to _____.

seafloor spreading.

Shown below are the four terrestrial planets of our solar system. Assume that all the planets started out equally hot inside. Rank the planets based on their expected cooling rates, from fastest cooling to slowest cooling.

Mercury, Mars, Venus, Earth. Correct. Smaller planets cool faster than larger planets for the same reason that smaller hot potatoes cool faster than larger hot potatoes: An object's total heat content depends on its volume, while its rate of heat loss depends on its surface area. Smaller objects have greater ratios of surface area to volume (the surface area-to-volume ratio) and therefore cool faster. Note that the differing cooling rates explain why the smallest planet, Mercury, has lost most of its interior heat by now, while the largest terrestrial planets — Venus and Earth — still are quite hot inside.

In Part A, you found that Planet Z should not have polar ice caps or liquid water. What single change to Planet Z's characteristics would allow it to have these things? (A.) a smaller size. (B.) a larger axis tilt. (C.) a greater distance from its star. (D.) a larger size.

(C.) a greater distance from its star. Correct. The planet is too hot for liquid water or ice, so moving it farther from its star would allow it to cool down. If it cooled enough—but not too much—it could have surface liquid water and ice caps.

Shown following are three terrestrial planets of our solar system. Rank the planets based on the amount of time the surface of the planet has had a moderate to high level of volcanic/tectonic activity, from longest to shortest.

Earth, Mars, Mercury. Correct. Earth still has a great deal of tectonic activity today, Mars has much less ongoing tectonic activity, and Mercury probably has very little or no ongoing tectonic activity. Note that we can trace these facts directly back to the cooling rates from Part A: Tectonic activity requires interior heat, so planets that cool faster lose their tectonic activity in a shorter time.

The following images show the four terrestrial planets in our solar system. Rank the planets from left to right based on the amount by which the greenhouse effect increases their surface temperatures, compared to what their temperatures would be without the greenhouse effect, from largest to smallest increase.

Venus, Earth, Mars, Mercury. Correct. A stronger greenhouse effect means a greater temperature increase, which is why the rankings here are the same as the rankings for Parts B and C. The differences are quite extreme: Mercury has no greenhouse effect, so its temperature is determined solely by its distance from the Sun and its reflectivity. The greenhouse effect raises the temperature of Mars by about 6°C from what it would be otherwise; it raises Earth's temperature by about 31°C (which means our planet would be frozen over without the greenhouse effect); and it raises Venus's temperature by about 510°C, explaining the extremely high temperature of Venus.

The following images show the four terrestrial planets in our solar system. Rank these planets from left to right based on the atmospheric pressure at the surface, from highest to lowest. (Not to scale.)

Venus, Earth, Mars, Mercury. Correct. Note that the pressure differences are quite extreme. Mercury has essentially no atmosphere and no pressure. Earth's atmospheric pressure is more than 100 times that of Mars, and Venus's atmospheric pressure is about 90 times that of Earth.

Listed following are characteristics of the atmospheres of Venus, Earth, and Mars. Match each atmospheric characteristic to the appropriate planet.

(1.) Venus: sulfuric acid clouds, almost no surface winds, runaway greenhouse effect. (2.) Earth: atmosphere composed primarily of nitrogen, ultraviolet-absorbing stratosphere. (3.) Mars: extremely low density atmosphere, global dust storms. Correct. Be sure to recognize that Venus has very little surface wind because of its slow rotation rate. Venus suffered a runaway greenhouse effect because of its distance from the Sun; If Earth were placed at the same distance, our planet would suffer the same fate. Earth has an ultraviolet-absorbing stratosphere because of the oxygen in the atmosphere, which at high altitudes forms molecules of ultraviolet-absorbing ozone.

Listed below are geographic features of the terrestrial worlds. In each case, identify the geological process: impact cratering, volcanism, erosion, or tectonics (where tectonics is any large-scale processes affecting the structure of the planetary crust), most responsible for the feature described. Match the geographic feature to the appropriate geologic process.

(1.) Volcanism: big island of Hawaii, smooth surfaces of the lunar maria, Mars's Olympus Mons. (2.) Impact cratering: old surface features of the lunar highlands. (3.) Erosion: Earth's Grand Canyon. (4.) Tectonics: current locations of Earth's continents, Mars's Valles Marineris, Mercury's many long, tall cliffs. Correct. Remember that the four processes are interrelated, so although one may be most important to a particular feature, others often also play a role. For example, some erosion has occurred on the volcanic island of Hawaii, there are impact craters on the slopes of Olympus Mons, and volcanism and tectonics almost always go hand-in-hand.

Make a prediction: If the rise in carbon dioxide concentration continues at its current pace, the concentration in the year 2050 will be about _____ parts per million. (A.) 400. (B.) 430. (C.) 460. (D.) 510.

(C.) 460. Correct. Note that this concentration would be more than 60% higher than the concentration of 280 parts per million at the beginning of the industrial age (around the year 1750). It is also far higher than the carbon dioxide concentration reached naturally during the prior 800,000 years. In fact, the situation could be even worse: Careful study of the graph shows an acceleration of the rate of increase in recent years, which would lead the carbon dioxide concentration to be even higher than 460 parts per million by 2015.

What do we conclude if a planet has few impact craters of any size? (A.) The planet was never bombarded by asteroids or comets. (B.) Its atmosphere stopped impactors of all sizes. (C.) Other geological processes have wiped out craters.

(C.) Other geological processes have wiped out craters.

Which of a planet's fundamental properties has the greatest effect on its level of volcanic and tectonic activity? (A.) size. (B.) distance from the Sun. (C.) rotation rate.

(A.) size.

Which describes our understanding of flowing water on Mars? (A.) It was never important. (B.) It was important once, but no longer. (C.) It is a major process on the Martian surface today.

(B.) It was important once, but no longer.

All of the following statements are true. Which one provides strong observational support for the claim that greenhouse gases make a planet warmer than it would be otherwise? (A.) Earth has a higher average temperature than Mars. (B.) Venus has a higher average temperature than Mercury. (C.) Mercury is much hotter than the Moon. (D.) Earth is the only planet with an ozone layer in its atmosphere.

(B.) Venus has a higher average temperature than Mercury. Correct. The fact that Venus is hotter than Mercury despite being nearly twice as far from the Sun tells us that its thick carbon dioxide atmosphere must warm it significantly—just as we expect from the theory of the greenhouse effect.

In Part A, you found that Planet Z should not have seasons. What single change to Planet Z's characteristics would cause it to have seasons? (A.) a greater distance from its star. (B.) a larger axis tilt. (C.) a smaller size. (D.) a faster rotation rate.

(B.) a larger axis tilt. Correct. Seasons are caused primarily by axis tilt, so a planet without axis tilt is not expected to have seasons (unless it has a highly elliptical orbit).

In general, what kind of terrestrial planet would you expect to have the thickest lithosphere? (A.) a large planet. (B.) a small planet. (C.) a planet located far from the Sun.

(B.) a small planet.

On the graphs shown, you can identify an ice age by looking for __________. (A.) a trough (bottom of a dip) on the carbon dioxide graph. (B.) a trough (bottom of a dip) on the temperature graph. (C.) a place on the temperature graph where the temperature curve falls steeply. (D.) a peak on the temperature graph.

(B.) a trough (bottom of a dip) on the temperature graph. Correct. For example, the graph shows that the most recent ice age ended only about 10,000 years ago, and there are numerous other ice ages shown on the graph.

When you zoom in on the section labeled "Southern Highlands," which geologic processes are most clearly evident? (A.) volcanism and erosion. (B.) impact cratering and erosion. (C.) impact cratering and volcanism. (D.) tectonics and erosion. (E.) volcanism and tectonics.

(B.) impact cratering and erosion. Correct. The most obvious features of the southern highlands are the many impact craters, but a close examination shows that many of them have been "smoothed out," indicating erosion that has occurred over time.

How many of the five terrestrial worlds are considered "geologically dead"? (A.) none. (B.) two. (C.) four.

(B.) two.

The energy that warms Earth's surface comes primarily in the form of __________. (A.) heat from Earth's interior. (B.) visible light from the Sun. (C.) infrared light from the Sun. (D.) ultraviolet light from the Sun. (E.) heat from the Sun.

(B.) visible light from the Sun. Correct. In the figure, this fact is illustrated by the yellow squiggly line at the far left.

Which of the following is a strong greenhouse gas? (A.) nitrogen. (B.) water vapor. (C.) oxygen.

(B.) water vapor.

Although the data show only a correlation between the carbon dioxide concentration and the global average temperature, scientists have other reasons to think that a rise in the carbon dioxide concentration actually causes a rise in the global average temperature. All of the following statements are true. Which statements lend support to the idea that carbon dioxide is a cause of planetary warming? (A.) Models of Earth's climate that include recent increases in the carbon dioxide concentration match observed temperature increases better than those that do not include it. (B.) Isotope ratios in atmospheric carbon dioxide show that much of the carbon dioxide in Earth's atmosphere today comes from the burning of fossil fuels. (C.) Models of the greenhouse effect successfully predict the temperatures of Venus and Mars from their atmospheric carbon dioxide amounts. (D.) We understand the physical mechanism of the greenhouse effect, through which carbon dioxide can increase a planet's temperature.

(A.) Models of Earth's climate that include recent increases in the carbon dioxide concentration match observed temperature increases better than those that do not include it. (C.) Models of the greenhouse effect successfully predict the temperatures of Venus and Mars from their atmospheric carbon dioxide amounts. (D.) We understand the physical mechanism of the greenhouse effect, through which carbon dioxide can increase a planet's temperature. Correct. Together, the success of the models and our clear understanding of the mechanism of the greenhouse effect leave little room for doubt that carbon dioxide is indeed a cause of higher temperatures on a planet.

Based on the evidence that atmospheric carbon dioxide is a cause of planetary warming, what aspect of the graphs should most concern us? (A.) The carbon dioxide concentration today is significantly higher than at any time in the past 800,000 years and is rapidly rising. (B.) Earth's past carbon dioxide concentration rises and falls naturally. (C.) Earth's past temperature rises and falls naturally.

(A.) The carbon dioxide concentration today is significantly higher than at any time in the past 800,000 years and is rapidly rising. Correct. Therefore, if past trends continue, we would expect Earth's temperature to rise substantially as a result of this increase in the carbon dioxide concentration.

According to scientists, the naturally occurring greenhouse effect makes Earth about 31∘C warmer than it would be if there were no greenhouse gases in our atmosphere. How do scientists "know" what Earth's temperature would be without greenhouse gases? (A.) They calculate this temperature from Earth's reflectivity and distance from the Sun. (B.) Ancient fossils allow them to infer Earth's temperature at a time before our atmosphere contained greenhouse gases. (C.) They estimate it by averaging guesses made by many individual scientists. (D.) They assume that this temperature would be about the same as the temperature of Mars, which has much less of an atmosphere than Earth.

(A.) They calculate this temperature from Earth's reflectivity and distance from the Sun. Correct. Aside from the greenhouse effect, the only factors that affect a planet's average temperature are its reflectivity and distance from the Sun. Since both distance and reflectivity have been measured, the expected temperature can be calculated easily and precisely. (Note that this assumes that the Sun's total emission of energy remains steady; measurements and theory both indicate that it varies very little over time scales less than a few million years.)

Seismic studies on Earth reveal a "lost continent" that held great human cities just a few thousand years ago but is now buried beneath the Atlantic seafloor. (A.) This is not plausible. Plates move only a few centimeters per year, so a continent could not be subducted in a few thousand years. Neither could erosional processes bury a continent on the time scale of human civilization. (B.) This would be considered reasonable. Erosion has been strong in a few thousand years that "lost continent" buried beneath the Atlantic seafloor. (C.) This is plausible. Plates move a few kilometers per year, so a continent can be subducted in a few thousand years. (D.) This would be surprising. There is no place for a "lost continent" of the Earth's surface.

(A.) This is not plausible. Plates move only a few centimeters per year, so a continent could not be subducted in a few thousand years. Neither could erosional processes bury a continent on the time scale of human civilization.

Based solely on an understanding of the greenhouse effect (as displayed in the figure), which one of the following statements is true? (A.) We should expect an increase in the greenhouse gas concentration to lead to global warming. (B.) We do not yet understand the greenhouse effect well enough to make predictions about how it affects our planet. (C.) Humans are causing global warming. (D.) Global warming poses a grave threat to our future.

(A.) We should expect an increase in the greenhouse gas concentration to lead to global warming. Correct. The evidence discussed in this tutorial makes it clear that greenhouse gases make a planet's surface warmer than it would otherwise be, so we should expect a rise in the greenhouse gas concentration to make Earth warmer. It is possible that there can be mitigating factors through feedbacks, but the basic link between greenhouse gas concentration and global warming is very strong.

Greenhouse gases in the atmosphere, such as carbon dioxide and water vapor, make Earth warmer than it would be otherwise because these gases __________. (A.) absorb infrared light emitted by the surface. (B.) absorb visible light coming from the Sun. (C.) form clouds that emit thermal radiation. (D.) reflect visible light coming from the Sun.

(A.) absorb infrared light emitted by the surface. Correct. Although the absorbed infrared light is quickly reemitted, it is reemitted in a random direction. As a result, greenhouse gases tend to slow the escape of infrared light from Earth to space, so that there is more heat (which means more energy) in the atmosphere than there would be if the infrared light escaped directly to space.

Earth's temperature remains fairly steady, which means that Earth must return nearly the same amount of energy to space that it receives from the Sun. In what forms does Earth return most of this energy to space? (A.) infrared light emitted by the surface and atmosphere. (B.) visible light reflected by the surface. (C.) visible light emitted by the surface and atmosphere. (D.) ultraviolet light reflected by the surface. (E.) visible light reflected by clouds.

(A.) infrared light emitted by the surface and atmosphere. (B.) visible light reflected by the surface. (E.) visible light reflected by clouds. Correct. The total amount of energy returned to space in these three forms of radiation is nearly equal to the amount of energy that reaches Earth in the form of sunlight. (The values are not exactly equal; the fact that Earth's average temperature has been warming in recent decades means that Earth is absorbing more energy than it is returning to space. The difference is very small as a percentage of the total energy, but substantial enough to be causing all the effects we see from global warming.)

Assuming that features you see on Mars are similar to features found on Earth, what would a casual inspection of the interactive photo of Mars lead you to suspect about water on Mars? (A.) Abundant surface water is found in large, brownish pools inside craters. (B.) There are numerous small streams flowing with water. (C.) Surface water only exists as frozen ice. (D.) No surface water currently exists in any form.

(C.) Surface water only exists as frozen ice. Correct. There is nothing on the brownish surface to suggest liquid water, and close-up photos confirm that there is no liquid water on Mars today. However, the prominent polar caps look much like Earth's polar caps, and would therefore make you suspect that they are made of water ice. In fact, they contain both frozen carbon dioxide and frozen water.

Notice that the peaks and troughs on the temperature graph occur at the about the same times as peaks and troughs on the carbon dioxide graph. What can we infer from this fact alone? (A.) Higher carbon dioxide concentrations cause higher global average temperatures. (B.) The carbon dioxide concentration is inversely related to the global average temperature. (C.) There is a correlation between the carbon dioxide concentration and the global average temperature. (D.) Higher global average temperatures cause higher carbon dioxide concentrations.

(C.) There is a correlation between the carbon dioxide concentration and the global average temperature. Correct. A correlation means that two things go up and down together. In this case, there is a correlation between the temperature and the carbon dioxide concentration because both were generally high at the same times in the past and low at the same times in the past.

Based on Planet Z's size, orbital distance, and rotation rate, which of the following properties is it likely to have? (A.) erosion due to liquid water. (B.) seasons. (C.) active volcanoes. (D.) strong winds and violent storms. (E.) active tectonics. (F.) polar ice caps. (G.) a surface crowded with impact craters. (H.) an atmosphere produced by outgassing.

(C.) active volcanoes. (E.) active tectonics. (H.) an atmosphere produced by outgassing. Correct. Continue on to explore why the planet has these characteristics.

What, if anything, do you think we should be doing to alleviate the threat of global warming? (A.) increase the amount of carbon dioxide in our atmosphere. (B.) burn fossil fuels. (C.) replace fossil fuels with alternative energy sources. (D.) reduce the emissions of greenhouse gases. (E.) improve energy efficiency.

(C.) replace fossil fuels with alternative energy sources. (D.) reduce the emissions of greenhouse gases. (E.) improve energy efficiency.

In Part A, you found that Planet Z should not have strong winds and violent storms. What single change to Planet Z's characteristics would cause it to have strong winds and violent storms? (A.) a smaller size. (B.) a larger axis tilt. (C.) a greater distance from its star. (D.) a faster rotation rate.

(D.) a faster rotation rate. Correct. The basic requirements for strong winds and violent storms are an atmosphere and relatively rapid rotation. An atmosphere is necessary to have wind of any type, while rotation is necessary to create the forces (in particular, the Coriolis force) that tend to drive winds on a planet's surface. Storms will be even stronger if there is also evaporation of surface water.

You have found that Planet Z should have active tectonics and volcanism and an atmosphere produced by volcanic outgassing. What single factor explains why the planet should have these characteristics? Planet Z: (A.) lacks axis tilt. (B.) has a slow rotation rate. (C.) closely orbits its star. (D.) has a large size for a terrestrial planet.

(D.) has a large size for a terrestrial planet. Correct. Large size means more internal heat. This internal heat drives active tectonics and volcanism, which is the source of outgassing.

The average temperature over the past 1000 years has been about 15∘C. From the graphs, you can conclude that Earth's average temperature during the past 800,000 years has __________. (A.) never been as high as it is today. (B.) stayed remarkable steady, never varying by more than about 2∘C. (C.) varied between about −10∘C and +4∘C. (D.) varied between about 7∘C and 19∘C.

(D.) varied between about 7∘C and 19∘C. Correct. The zero level on the graph represents the 15∘C average temperature over the past millennium, so the peaks near +4 on the graph represent a temperature of about 15∘C+4∘C=19∘C and the troughs near −8 represent 15∘C−8∘C=7∘C.

The following images show four types (wavelengths) of light. Rank these from left to right based on the amount of each that is emitted (as thermal radiation) by Earth's surface, from greatest to least. If you think that two (or more) types should be ranked as equal, drag one on top of the other(s) to show this equality.

Greatest amount = Infrared. Least amount = ultraviolet (UV), X-ray, visible. Correct. Earth emits thermal radiation characteristic of its surface temperature, which means it is almost entirely infrared (extending, in principle, down into the radio). For Earth, the surface temperature is too low to emit any visible, ultraviolet, or X-ray light, so those are all ranked equally. (Note: Technically, thermal emission extends over all wavelengths, so even at low temperatures there might be an occasional photon of visible or higher-energy radiation. However, this emission is negligible for Earth, which is why we rank them all equal to zero.)

The following images show the four terrestrial planets in our solar system. Rank these planets from left to right based on the total amount of gas in their atmospheres, from most to least. (Not to scale.)

Venus, Earth, Mars, Mercury. Correct. Note that this ranking is the same as the pressure ranking from Part A. This should not be surprising, because more atmospheric gas generally means more pressure (though the strength of gravity at a planet's surface also plays a role in determining the pressure).

The following images show the four terrestrial planets in our solar system. Rank the planets from left to right based on the strength of the greenhouse effect occurring at their surfaces, from strongest to weakest.

Venus, Earth, Mars, Mercury. Correct. The greenhouse effect is caused by greenhouse gases in the atmosphere, so more greenhouse gas means a stronger greenhouse effect. That is why the rankings here are the same as the rankings for Part B.

In Part A, you found that Earth emits only infrared light. This infrared light can be absorbed by greenhouse gases, such as carbon dioxide and water vapor, in the atmosphere. In fact, all the terrestrial planets emit infrared light from their surfaces. The following images show the four terrestrial planets in our solar system. Rank these planets from left to right based on the total amount of infrared-absorbing greenhouse gases in their atmospheres, from greatest to least.

Venus, Earth, Mars, Mercury. Correct. Venus has a thick atmosphere of carbon dioxide. Earth has greenhouse gases primarily in the form of water vapor, carbon dioxide, and methane. Mars has an atmosphere made mostly of carbon dioxide, but its atmosphere is so thin that it contains less total greenhouse gas than Earth's atmosphere. Mercury has essentially no atmosphere at all.