The Origin of Earth’s Atmosphere
Paragraph 1：In order to understand the origin of Earth’s atmosphere, we must go back to the earliest days of the solar system, before the planets themselves were formed from a disk of rocky material spinning around the young Sun. This material gradually coalesced into lumps called planetesimals as gravity and chance smashed smaller pieces together, a chaotic and violent process that became more so as planetesimals grew in size and gravitational pull. Within each orbit, collisions between planetesimals generated immense heat and energy. How violent these processes were is suggested by the odd tilt and spin of many of the planets, which indicate that each of the planets was, like a billiard ball, struck at some stage by another large body of some kind. Visual evidence of these processes can be seen by looking at the Moon. Because the Moon has no atmosphere, its surface is not subject to erosion, so it retains the marks of its early history. Its face is deeply scarred by millions of meteoric impacts, as you can see on a clear night with a pair of binoculars. The early Earth did not have much of an atmosphere. Before it grew to full size, its gravitational pull was insufficient to prevent gases from drifting off into space, while the solar wind (the great stream of atomic particles emitted from the Sun) had already driven away much of the gaseous material from the inner orbits of the solar system. So we must imagine the early Earth as a mixture of rocky materials, metals, and trapped gases, subject to constant bombardment by smaller planetesimals and without much of an atmosphere.
1. The word chaotic in the passage is closest in meaning to
O rapid
O disorganized
O intense
O long-lasting
2. All of the following are true of the planetesimals mentioned in paragraph 1 EXCEPT:
O They were formed of rocky material spinning around the early Sun.
O They collided violently with each other.
O They gradually grew in size.
O They lost their atmospheres as they were hit by larger bodies.
3. The author discusses the Moon in paragraph 1 in order to
O help explain why Earth had fewer meteoric impacts than other planets in the solar system
O show why it is difficult to understand how the first planetary atmospheres developed
O help explain the processes that took place in the formation of large planetary bodies in the solar system
O illustrate why the Moon’s spin and tilt are unique among other planetary bodies in the solar system
Paragraph 2：As it began to reach full size, Earth heated up, partly because of collisions with other planetesimals and partly because of increasing internal pressures as it grew in size. In addition, the early Earth contained abundant radioactive materials, also a source of heat. As Earth heated up, its interior melted. Within the molten interior, under the influence of gravity, different elements were sorted out by density. By about 40 million years after the formation of the solar system, most of the heavier metallic elements in the early Earth, such as iron and nickel, had sunk through the hot sludge to the center, giving Earth a core dominated by iron. This metallic core gives Earth its characteristic magnetic field, which has played an extremely important role in the history of our planet.
4. Paragraph 2 answers which of the following questions about early Earth?
O What caused materials on Earth to become radioactive?
O What percentage of Earth’s core was nickel?
O What internal pressures caused Earth to heat up as it grew in size?
O What caused Earth’s magnetic field?
5. According to paragraph 2, Earth’s core is mostly iron because, compared to most other elements on early Earth, iron
O was denser
O melted more easily
O was more radioactive
O was more plentiful
Paragraph 3：As heavy materials headed for the center of Earth, lighter silicates (such as the mineral quartz) drifted upward. The denser silicates formed Earth’s mantle, a region almost 3,000 kilometers thick between the core and the crust. With the help of bombardment by comets, whose many impacts scarred and heated Earth’s surface, the lightest silicates rose to Earth’s surface, where they cooled more rapidly than the better-insulated materials in Earth’s interior.
Paragraph 4：These lighter materials, such as the rocks we call granites, formed a layer of continental crust about 35 kilometers thick. Relative to Earth as a whole, this is as thin as an eggshell. Seafloor crust is even thinner, at about 7 kilometers; thus, even continental crust reaches only about 1/200th of the way to Earth’s core. Much of the early continental crust has remained on Earth’s surface to the present day.
6. Select the TWO answer choices that, according to paragraph 3 and 4, indicate true statements about Earth’s formation.
To obtain credit, you must select TWO answer choices.
O Comets hitting Earth helped the lightest silicates to reach Earth’s surface.
O Silicates such as mineral quartz drifted downward and mixed with denser materials as they reached Earth’s core.
O When Earth’s mantle became approximately 3,000 kilometers thick, the heaviest materials in it began to cool.
O Lighter materials reaching Earth’s surface formed Earth’s continental crust.
7. According to paragraph 4, Earth’s continental crust
O has changed significantly in composition over time
O was as thick as Earth’s mantle in its early stages
O is very thin relative to Earth’s size
O caused the temperatures of Earth’s early core and mantle to gradually increase
Paragraph 5：The lightest materials of all, including gases such as hydrogen and helium, bubbled through Earth’s interior to the surface. So we can imagine the surface of the early Earth as a massive volcanic field. And we can judge pretty well what gases bubbled up to that surface by analyzing the mixture of gases emitted by volcanoes. These include hydrogen, helium, methane, water vapor, nitrogen, ammonia, and hydrogen sulfide. Other materials, including large amounts of water vapor, were brought in by cometary bombardments. Much of the hydrogen and helium escaped; but once Earth was fully formed, it was large enough for its gravitational field to hold most of the remaining gases, and these formed Earth’s first stable atmosphere.
8. What can be inferred from paragraph 5 about Earth’s first stable atmosphere?
O It existed before Earth was yet fully formed.
O It contained very little hydrogen and helium.
O It contained only materials that had bubbled up through Earth’s surface.
O It lacked water vapor.
Paragraph 4：These lighter materials, such as the rocks we call granites, formed a layer of continental crust about 35 kilometers thick. ■Relative to Earth as a whole, this is as thin as an eggshell. ■Seafloor crust is even thinner, at about 7 kilometers; thus, even continental crust reaches only about 1/200th of the way to Earth’s core. ■Much of the early continental crust has remained on Earth’s surface to the present day.■
9. Look at the four squares that indicate where the following sentence could be added to the passage.
Even some of its oldest portions as old as 3.8 billion years can still be found in parts of Canada, Australia, South Africa, and Greenland.
Where would the sentence best fit Click on a square to add the sentence to the passage.
10. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text.
Answer Choices
O Early Earth’s lack of an atmosphere explains why it was bombarded with much more frequency and violence than other planetesimals.
O Continued bombardments and internal pressures made the growing Earth hotter, causing its interior to melt and the heavier elements to sink and form Earth’s core.
O Lighter elements from Earth’s interior rose and formed the mantle, a denser layer of silicates around the core, and the crust, a thinner layer of silicates at Earth’s surface.
O The formation of Earth’s crust protected the inner layers of Earth from the high-energy particles in space, reducing the temperatures of the mantle and the core.
O Once Earth had gone through the final stages of its formation, gases bubbled to the surface and were held by Earth’s gravitational field to form the atmosphere.
O Volcanoes today are the result of gases that were trapped in Earth’s interior during the planet’s early stages of formation.
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