TF阅读真题第933篇The Collision Model of the Moon’s Formation

 

 

The Collision Model of the Moon’s Formation

 

 

 

The collection and analysis of rocks brought back from the Moon during the Apollo spaceflight program in 1969 renewed interest in the process by which the Moon formed. One theory, the collision model, was that Earth, while it was still in the process of forming, was hit by a projectile that was Mars-sized (half the diameter of Earth). While some debris from this collision was ejected into space, other debris remained in Earth’s orbit, where it formed a thin ring of rocks. The Moon then formed from this ring by accretion, the processes of collision and sticking responsible for building most bodies in the solar system. Consistent with this model was the finding that lunar samples were depleted of elements that vaporize readily, such as zinc, as would have occurred as a result of a violent impact. Stuck in the gas phase, these elements would have been swept into space. Apollo’s samples were also exceedingly dry, water being among the lost substances.

Another remarkable property of lunar samples is that they are highly depleted in the siderophile, or iron-loving, elements that tend to concentrate in the metallic iron cores of planets. When planetary cores form by the molten iron sinking to the planet’s center, the siderophile elements (such as platinum, gold, and iridium) are incorporated into the falling iron and are highly depleted from the crustal and mantle materials left above (the mantle is the layer above the core, while the crust is the thin outermost layer). That the lunar rocks were depleted in siderophile elements was peculiar, however, because the Moon cannot have a substantial iron core. The mean density of the Moon is 3.4 times the density of water, very similar to that of rocks on the lunar surface and much lower than that of Earth which is 5.5 times the mean density of water. If it had a substantial core of dense metallic iron, the mean density of the Moon would be higher than is observed.

The collision model solves the siderophile mystery by suggesting that metallic cores had formed in both Earth and the impact projectile before the collision. In the collision, the iron cores of both bodies ended up in the center of Earth, and the debris ejected into orbit was mainly from the mantles of both bodies. The preceding burying of siderophiles in planetary cores explains why gold and platinum are so rare on the Moon and in the crustal rocks of Earth. The impact ejection of mantle materials from both the giant impactor and the target Earth is consistent with some of the remarkable similarities between the Moon’s trace elements (elements that exist in only low concentrations) and those of rocks from Earth’s mantle. It is also consistent with Earth and the Moon having identical ratios of the isotopes (variations of a single element) of the elements that are present on both of them.

A collisional origin is very appealing, but did it actually happen? For an impact to eject enough material to form the Moon, the colliding body has to be huge, a Mars-sized body. Theoretical modeling of planet formation by planetary scientist George Wetherill showed that a natural consequence of the accretion process is that several large bodies do form in each planet’s accretion zone (the area in which the planet-forming material accumulates). The growth process includes the impact of several bodies, each of which carries more than 10 percent of the mass of the final planet.

In the case of Earth, these big bodies were the size of Mars and larger. Their impact not only ejected material into space but also injected considerable amounts of heat into Earth’s mantle. This heat input and great violence led to the forging of Earth’s core during the accretion phase, before the planet was fully formed. Core formation requires high internal temperatures so that blobs of molten iron descend through the mantle to reach the core. Celestial objects that form through the accretion of small bodies form a core only after long-term buildup of radioactive heat from the decay of uranium, potassium, and thorium. In Earth’s case, the early heat from the accretion of large bodies led to core formation as accretion occurred. Both Earth and the large projectile that struck it had differentiated (separated into layers) and already had metal cores at the time of collision.

 

 

 

 

1

The collection and analysis of rocks brought back from the Moon during the Apollo spaceflight program in 1969 renewed interest in the process by which the Moon formed. One theory, the collision model, was that Earth, while it was still in the process of forming, was hit by a projectile that was Mars-sized (half the diameter of Earth). While some debris from this collision was ejected into space, other debris remained in Earth’s orbit, where it formed a thin ring of rocks. The Moon then formed from this ring by accretion, the processes of collision and sticking responsible for building most bodies in the solar system. Consistent with this model was the finding that lunar samples were depleted of elements that vaporize readily, such as zinc, as would have occurred as a result of a violent impact. Stuck in the gas phase, these elements would have been swept into space. Apollo’s samples were also exceedingly dry, water being among the lost substances.

According to the theory described in paragraph 1，which of the following occurred as a result of a Mars-sized projectile hitting Earth?

 

Factual Information Questions事实信息题

 

AEarth’s orbital direction was reversed.

BRocks began circling Earth.

CElements such as zinc accumulated on the Moon.

DEarth’s diameter was reduced by half.

2

Another remarkable property of lunar samples is that they are highly depleted in the siderophile, or iron-loving, elements that tend to concentrate in the metallic iron cores of planets. When planetary cores form by the molten iron sinking to the planet’s center, the siderophile elements (such as platinum, gold, and iridium) are incorporated into the falling iron and are highly depleted from the crustal and mantle materials left above (the mantle is the layer above the core, while the crust is the thin outermost layer). That the lunar rocks were depleted in siderophile elements was peculiar, however, because the Moon cannot have a substantial iron core. The mean density of the Moon is 3.4 times the density of water, very similar to that of rocks on the lunar surface and much lower than that of Earth which is 5.5 times the mean density of water. If it had a substantial core of dense metallic iron, the mean density of the Moon would be higher than is observed.

The word “peculiar”in the passage is closest in meaning to

 

Vocabulary Questions词汇题

 

Astrange

Brevealing

Cimpressive

Dimportant

3

Another remarkable property of lunar samples is that they are highly depleted in the siderophile, or iron-loving, elements that tend to concentrate in the metallic iron cores of planets. When planetary cores form by the molten iron sinking to the planet’s center, the siderophile elements (such as platinum, gold, and iridium) are incorporated into the falling iron and are highly depleted from the crustal and mantle materials left above (the mantle is the layer above the core, while the crust is the thin outermost layer). That the lunar rocks were depleted in siderophile elements was peculiar, however, because the Moon cannot have a substantial iron core. The mean density of the Moon is 3.4 times the density of water, very similar to that of rocks on the lunar surface and much lower than that of Earth which is 5.5 times the mean density of water. If it had a substantial core of dense metallic iron, the mean density of the Moon would be higher than is observed.

In paragraph 2， why does the author compare the mean density of the Moon with the mean density of Earth?

 

Factual Information Questions事实信息题

 

ATo provide evidence that siderophile elements are concentrated in planetary iron cores

BTo explain why molten iron was able to sink to Earth’s center

CTo explain why lunar rocks largely lack siderophile elements

DTo support the claim that the Moon lacks a sizable iron core

4

The collision model solves the siderophile mystery by suggesting that metallic cores had formed in both Earth and the impact projectile before the collision. In the collision, the iron cores of both bodies ended up in the center of Earth, and the debris ejected into orbit was mainly from the mantles of both bodies. The preceding burying of siderophiles in planetary cores explains why gold and platinum are so rare on the Moon and in the crustal rocks of Earth. The impact ejection of mantle materials from both the giant impactor and the target Earth is consistent with some of the remarkable similarities between the Moon’s trace elements (elements that exist in only low concentrations) and those of rocks from Earth’s mantle. It is also consistent with Earth and the Moon having identical ratios of the isotopes (variations of a single element) of the elements that are present on both of them.

The phrase”the siderophile mystery”in the passage refers to

 

Rhetorical Purpose Questions修辞目的题

 

Auncertainty about the exact location of Earth’s siderophiles before its collision with an impact projectile

Buncertainty about how Earth’s siderophiles were affected by Earth’s collision with an impact projectile

Cthe absence of siderophiles in lunar rocks even though the Moon lacks a substantial iron core

Dthe presence of siderophiles in both Earth’s iron core and the iron core of the impact projectile that struck it

5

The collision model solves the siderophile mystery by suggesting that metallic cores had formed in both Earth and the impact projectile before the collision. In the collision, the iron cores of both bodies ended up in the center of Earth, and the debris ejected into orbit was mainly from the mantles of both bodies. The preceding burying of siderophiles in planetary cores explains why gold and platinum are so rare on the Moon and in the crustal rocks of Earth. The impact ejection of mantle materials from both the giant impactor and the target Earth is consistent with some of the remarkable similarities between the Moon’s trace elements (elements that exist in only low concentrations) and those of rocks from Earth’s mantle. It is also consistent with Earth and the Moon having identical ratios of the isotopes (variations of a single element) of the elements that are present on both of them.

Paragraph 3 suggests all of the following are true of rocks from Earth’s upper layers and rocks from the Moon EXCEPT:

 

Inference Questions推理题

 

AThey contain very little gold or platinum.

BThey contain much the same trace elements.

CThey have similar ratios of the isotopes of elements that they have in common.

DThey contain most of the siderophiles from the planetary cores that were involved in a collision.

6

A collisional origin is very appealing, but did it actually happen? For an impact to eject enough material to form the Moon, the colliding body has to be huge, a Mars-sized body. Theoretical modeling of planet formation by planetary scientist George Wetherill showed that a natural consequence of the accretion process is that several large bodies do form in each planet’s accretion zone (the area in which the planet-forming material accumulates). The growth process includes the impact of several bodies, each of which carries more than 10 percent of the mass of the final planet.

The word”appealing”in the passage is closest in meaning to

 

Vocabulary Questions词汇题

 

Acomplicated

Bpowerful

Cattractive

Dlogical

7

A collisional origin is very appealing, but did it actually happen? For an impact to eject enough material to form the Moon, the colliding body has to be huge, a Mars-sized body. Theoretical modeling of planet formation by planetary scientist George Wetherill showed that a natural consequence of the accretion process is that several large bodies do form in each planet’s accretion zone (the area in which the planet-forming material accumulates). The growth process includes the impact of several bodies, each of which carries more than 10 percent of the mass of the final planet.

According to paragraph 4, George Wetherill’s studies of planetary formation led to conclusions about which of the following?

 

Factual Information Questions事实信息题

 

AThe size of the accretion zone needed to create a final planet

BThe amount of time that it took for large bodies to form in Earth’s accretion zone

CThe percentage of bodies in Earth’s accretion zone that became Mars-sized or larger

DThe relative mass of objects causing impacts on a planet during the accretion process

8

In the case of Earth, these big bodies were the size of Mars and larger. Their impact not only ejected material into space but also injected considerable amounts of heat into Earth’s mantle. This heat input and great violence led to the forging of Earth’s core during the accretion phase, before the planet was fully formed. Core formation requires high internal temperatures so that blobs of molten iron descend through the mantle to reach the core. Celestial objects that form through the accretion of small bodies form a core only after long-term buildup of radioactive heat from the decay of uranium, potassium, and thorium. In Earth’s case, the early heat from the accretion of large bodies led to core formation as accretion occurred. Both Earth and the large projectile that struck it had differentiated (separated into layers) and already had metal cores at the time of collision.

According to paragraph 5， which of the following occurred as a result of Earth’s collision with a Mars-sized body?

 

Factual Information Questions事实信息题

 

ASmall bodies within Earth’s accretion zone began colliding with each other.

BTemperatures within Earth’s mantle rose.

CRadioactive heat was released from Earth’s core.

DEarth’s accretion phase ended.

9

Another remarkable property of lunar samples is that they are highly depleted in the siderophile, or iron-loving, elements that tend to concentrate in the metallic iron cores of planets.  [■]  When planetary cores form by the molten iron sinking to the planet’s center, the siderophile elements (such as platinum, gold, and iridium) are incorporated into the falling iron and are highly depleted from the crustal and mantle materials left above (the mantle is the layer above the core, while the crust is the thin outermost layer).  [■]  That the lunar rocks were depleted in siderophile elements was peculiar, however, because the Moon cannot have a substantial iron core.  [■]  The mean density of the Moon is 3.4 times the density of water, very similar to that of rocks on the lunar surface and much lower than that of Earth which is 5.5 times the mean density of water.  [■]  If it had a substantial core of dense metallic iron, the mean density of the Moon would be higher than is observed.

Look at the four squares [■]  that indicate where the following sentence could be added to the passage

This quality can result from the process by which materials in a planet’s interior separate to form distinct parts.Insert Text Questions句子插入题

Where would the sentence best fit？Click on a square  sentence to the passage.

10

Lunar samples brought back by the Apollo spacecraft helped scientists understand how the Moon formed.

 

Prose Summary Questions概要小结题

 

Select 3 answers

AUnlike most bodies in the solar system, the Moon did not form through an accretion process, and lunar samples show that it is mostly made up from elements that vaporize readily.

BSimilarities between rocks from the Moon and rocks from Earth’s mantle support the idea that the Moon formed from debris ejected into space from the mantles of Earth and a colliding body.

CModeling indicates that bodies large enough to create a lot of debris when they collided were in Earth’s accretion zone, and their impact could have created enough heat to form a metal core.

DThe lack of siderophiles in the lunar samples is best explained by the theory that Earth and the large projectile that struck it had already formed iron cores by the time that they collided.

EEarth’s accretion process produced small bodies that collided with each other, producing radioactive materials that became Earth’s main source of heat.

FThe bodies in Earth’s accretion zone were the size of Mars rather than the 10 percent of the mass of the final planet predicted by George Wetherill, so a collisional origin probably did not actually happen.