The oldest Earth rocks known to date, found in Quebec, Canada in 2008， were determined to be about 4.3 billion years old. However, the oldest known materials of the solar system-Moon rocks and meteorites-are around 4.55 billion years old. Why the difference? Earth’s surface is constantly being recycled and remodeled due to the motion of tectonic plates. The Moon, by contrast, has a dead surface with no such movement. Meteorites that formed with the early solar system, like carbonaceous chondrites, have been unaltered since they cooled, so they give the oldest dates of all.
These are the dates for the oldest rocks on Earth, but they are not the oldest Earth materials known. That distinction goes to a handful of zircon sand grains from a much younger sandstone found in the Jack Hills of Western Australia. Each individual grain can be dated by uranium-lead methods, so they give a range of ages. But the oldest grains of all give an age of 4.404 billion years, at least 100 million years older than the 4.3-billion-year-old materials from Quebec. Thus, the current record holder for the oldest material from Earth (that is, not a Moon rock or meteorite) is 4.4 billion years. These sand grain dates put us closer and closer to the age of Moon rocks and meteorites, but we still have a gap of about 200 million years. This is about the same time span as the beginning of the Age of Dinosaurs (Late Triassic) until today, so it is not a trivial amount of time.
But those same tiny zircon sand grains held even more surprises. Not only did they give the oldest known dates, but when scientists analyzed the tiny bubbles of gases trapped inside them, they found evidence of the early atmosphere from over 4 billion years ago. These bubbles had oxygen isotopes (variants) in them that suggested Earth had liquid water on its surface as early as 4.4 billion years ago! This is surprising because when Earth formed 4.55 billion years ago, it was in a molten state (liquefied by heat). Prior to this discovery geologists had always assumed that Earth took a long time to cool from its molten state. Most thought that Earth took about 700 million years to cool down below the boiling point of water (100C), because that was the age of the oldest sedimentary rocks that would have been produced by running water (the Isua Supracrustals from Greenland, which are 3.8 billion years old). But the Jack Hills zircons turn that assumption inside out. If the zircons truly indicate the presence of liquid water on Earth 4.4 billion years ago, then it took only 200 million years for Earth to cool from its molten state to a condition that was below the boiling point of water. This also suggests that there were not as many meteorite impacts during this time interval, or the oceans would have been vaporized over and over again. Taken together, these data suggest what is now called the “cool early Earth hypothesis.”
So where did this early Earth water come from? Traditionally, geologists thought that it was water trapped inside Earth’s mantle (middle layer)when it cooled, gradually escaping through volcanoes in a process called degassing. But lately, chemical analyses of extraterrestrial objects match the chemistry of Earth’s oceans(especially carbonaceous chondrite meteorites). This suggests that there was a lot of water trapped in the debris of the early solar system (of which the chondrites are remnants), which represent the material from which Earth formed. The same is true of Moon rocks, which do not have much water in them today, but apparently they were pretty wet when the solar system formed. If this is so, then Earth was born with its water already present as it cooled and condensed. It only required its surface temperature to drop below 100C for that water to form the first oceans. One thing we can rule out is comets. Although comets are often called “dirty snowballs”because they are made mostly of dust and water ice, chemical analyses of four comets now show that their geochemistry is very different from Earth’s water. Thus, the popular idea that comets impacted early Earth and melted to form its oceans can be dismissed.
1
The oldest Earth rocks known to date, found in Quebec, Canada in 2008， were determined to be about 4.3 billion years old. However, the oldest known materials of the solar system-Moon rocks and meteorites-are around 4.55 billion years old. Why the difference? Earth’s surface is constantly being recycled and remodeled due to the motion of tectonic plates. The Moon, by contrast, has a dead surface with no such movement. Meteorites that formed with the early solar system, like carbonaceous chondrites, have been unaltered since they cooled, so they give the oldest dates of all.
Paragraph 1 supports which of the following statements about carbonaceous chondrites?
AThey contain some of the oldest materials in the solar system.
BThey formed on the Moon’s surface before it cooled.
CThey were discovered in Quebec, Canada in 2008.
DThey are materials brought up to the surface by the activity of tectonic plates.
2
These are the dates for the oldest rocks on Earth, but they are not the oldest Earth materials known. That distinction goes to a handful of zircon sand grains from a much younger sandstone found in the Jack Hills of Western Australia. Each individual grain can be dated by uranium-lead methods, so they give a range of ages. But the oldest grains of all give an age of 4.404 billion years, at least 100 million years older than the 4.3-billion-year-old materials from Quebec. Thus, the current record holder for the oldest material from Earth (that is, not a Moon rock or meteorite) is 4.4 billion years. These sand grain dates put us closer and closer to the age of Moon rocks and meteorites, but we still have a gap of about 200 million years. This is about the same time span as the beginning of the Age of Dinosaurs (Late Triassic) until today, so it is not a trivial amount of time.
According to paragraph 2， which of the following is NOT true of the zircon sand grains found in the Jack Hills of Western Australia?
ASome of them are the oldest known materials from Earth.
BThe oldest of them are 4.404 billion years old.
CThe oldest of them are 200 million years younger than the oldest Moon rocks and meteorites.
DSome of them formed during the Late Triassic.
3
But those same tiny zircon sand grains held even more surprises. Not only did they give the oldest known dates, but when scientists analyzed the tiny bubbles of gases trapped inside them, they found evidence of the early atmosphere from over 4 billion years ago. These bubbles had oxygen isotopes (variants) in them that suggested Earth had liquid water on its surface as early as 4.4 billion years ago! This is surprising because when Earth formed 4.55 billion years ago, it was in a molten state (liquefied by heat). Prior to this discovery geologists had always assumed that Earth took a long time to cool from its molten state. Most thought that Earth took about 700 million years to cool down below the boiling point of water (100C), because that was the age of the oldest sedimentary rocks that would have been produced by running water (the Isua Supracrustals from Greenland, which are 3.8 billion years old). But the Jack Hills zircons turn that assumption inside out. If the zircons truly indicate the presence of liquid water on Earth 4.4 billion years ago, then it took only 200 million years for Earth to cool from its molten state to a condition that was below the boiling point of water. This also suggests that there were not as many meteorite impacts during this time interval, or the oceans would have been vaporized over and over again. Taken together, these data suggest what is now called the “cool early Earth hypothesis.”
The phrase “this discovery”in the passage refers to the discovery that
Azircon sand grains contain tiny bubbles of trapped gases
Bliquid water existed on Earth’s surface 4.4 billion years ago
CEarth formed 4.55 billion years ago
DEarth used to be in a molten state
4
But those same tiny zircon sand grains held even more surprises. Not only did they give the oldest known dates, but when scientists analyzed the tiny bubbles of gases trapped inside them, they found evidence of the early atmosphere from over 4 billion years ago. These bubbles had oxygen isotopes (variants) in them that suggested Earth had liquid water on its surface as early as 4.4 billion years ago! This is surprising because when Earth formed 4.55 billion years ago, it was in a molten state (liquefied by heat). Prior to this discovery geologists had always assumed that Earth took a long time to cool from its molten state. Most thought that Earth took about 700 million years to cool down below the boiling point of water (100C), because that was the age of the oldest sedimentary rocks that would have been produced by running water (the Isua Supracrustals from Greenland, which are 3.8 billion years old). But the Jack Hills zircons turn that assumption inside out. If the zircons truly indicate the presence of liquid water on Earth 4.4 billion years ago, then it took only 200 million years for Earth to cool from its molten state to a condition that was below the boiling point of water. This also suggests that there were not as many meteorite impacts during this time interval, or the oceans would have been vaporized over and over again. Taken together, these data suggest what is now called the “cool early Earth hypothesis.”
Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
AMost thought that Earth had taken about 700 million years to produce the 3.8-billion-year-old sedimentary Isua Supracrustals from Greenland.
BThe oldest sedimentary rocks produced by running water are 3.8 billion years old, much older than the 700 million years most scientists thought.
CBased on the age of the oldest rocks formed by running water, most scientists thought Earth took about 700 million years to cool below water’s boiling point.
DOnce Earth cooled down below the boiling point of water, it started to produce sedimentary rocks like the Isua Supracrustals from Greenland.
5
But those same tiny zircon sand grains held even more surprises. Not only did they give the oldest known dates, but when scientists analyzed the tiny bubbles of gases trapped inside them, they found evidence of the early atmosphere from over 4 billion years ago. These bubbles had oxygen isotopes (variants) in them that suggested Earth had liquid water on its surface as early as 4.4 billion years ago! This is surprising because when Earth formed 4.55 billion years ago, it was in a molten state (liquefied by heat). Prior to this discovery geologists had always assumed that Earth took a long time to cool from its molten state. Most thought that Earth took about 700 million years to cool down below the boiling point of water (100C), because that was the age of the oldest sedimentary rocks that would have been produced by running water (the Isua Supracrustals from Greenland, which are 3.8 billion years old). But the Jack Hills zircons turn that assumption inside out. If the zircons truly indicate the presence of liquid water on Earth 4.4 billion years ago, then it took only 200 million years for Earth to cool from its molten state to a condition that was below the boiling point of water. This also suggests that there were not as many meteorite impacts during this time interval, or the oceans would have been vaporized over and over again. Taken together, these data suggest what is now called the “cool early Earth hypothesis.”
According to paragraph 3,what does the presence of liquid water on Earth’s surface 4.4 billion years ago indicate about meteorite impacts on Earth at the time?
AThey likely brought water to early Earth.
BThey likely caused the oceans to vaporize over and over again.
CThey likely contributed to Earth’s rapid cooling.
DThey were likely comparatively infrequent.
6
So where did this early Earth water come from? Traditionally, geologists thought that it was water trapped inside Earth’s mantle (middle layer)when it cooled, gradually escaping through volcanoes in a process called degassing. But lately, chemical analyses of extraterrestrial objects match the chemistry of Earth’s oceans(especially carbonaceous chondrite meteorites). This suggests that there was a lot of water trapped in the debris of the early solar system (of which the chondrites are remnants), which represent the material from which Earth formed. The same is true of Moon rocks, which do not have much water in them today, but apparently they were pretty wet when the solar system formed. If this is so, then Earth was born with its water already present as it cooled and condensed. It only required its surface temperature to drop below 100C for that water to form the first oceans. One thing we can rule out is comets. Although comets are often called “dirty snowballs”because they are made mostly of dust and water ice, chemical analyses of four comets now show that their geochemistry is very different from Earth’s water. Thus, the popular idea that comets impacted early Earth and melted to form its oceans can be dismissed.
The phrase “rule out”in the passage is closest in meaning to
Aeliminate
Bdebate
Cconsider
Dexplain
7
So where did this early Earth water come from? Traditionally, geologists thought that it was water trapped inside Earth’s mantle (middle layer)when it cooled, gradually escaping through volcanoes in a process called degassing. But lately, chemical analyses of extraterrestrial objects match the chemistry of Earth’s oceans(especially carbonaceous chondrite meteorites). This suggests that there was a lot of water trapped in the debris of the early solar system (of which the chondrites are remnants), which represent the material from which Earth formed. The same is true of Moon rocks, which do not have much water in them today, but apparently they were pretty wet when the solar system formed. If this is so, then Earth was born with its water already present as it cooled and condensed. It only required its surface temperature to drop below 100C for that water to form the first oceans. One thing we can rule out is comets. Although comets are often called “dirty snowballs”because they are made mostly of dust and water ice, chemical analyses of four comets now show that their geochemistry is very different from Earth’s water. Thus, the popular idea that comets impacted early Earth and melted to form its oceans can be dismissed.
In paragraph 4， why does the author discuss chemical analyses of four comets?
ATo support the hypothesis that Moon rocks once contained water introduced by comets
BTo provide evidence that the chemistry of Earth’s oceans matches that of extraterrestrial objects
CTo argue that the Moon and carbonaceous chondrite meteorites formed in a similar manner
DTo help support the claim that Earth formed with its water already present
8
So where did this early Earth water come from? Traditionally, geologists thought that it was water trapped inside Earth’s mantle (middle layer)when it cooled, gradually escaping through volcanoes in a process called degassing. But lately, chemical analyses of extraterrestrial objects match the chemistry of Earth’s oceans(especially carbonaceous chondrite meteorites). This suggests that there was a lot of water trapped in the debris of the early solar system (of which the chondrites are remnants), which represent the material from which Earth formed. The same is true of Moon rocks, which do not have much water in them today, but apparently they were pretty wet when the solar system formed. If this is so, then Earth was born with its water already present as it cooled and condensed. It only required its surface temperature to drop below 100C for that water to form the first oceans. One thing we can rule out is comets. Although comets are often called “dirty snowballs”because they are made mostly of dust and water ice, chemical analyses of four comets now show that their geochemistry is very different from Earth’s water. Thus, the popular idea that comets impacted early Earth and melted to form its oceans can be dismissed.
Paragraph 4 identifies which TWO of the following as possible sources of early Earth’s water? To receive credit, you must select TWO answer choices.
AWater released from Earth’s mantle through volcanic eruptions
BChemical reactions inside Earth’s mantle
CMoon rocks impacting Earth’s surface
DWater released in trapped debris of extraterrestrial objects
9
But those same tiny zircon sand grains held even more surprises. Not only did they give the oldest known dates, but when scientists analyzed the tiny bubbles of gases trapped inside them, they found evidence of the early atmosphere from over 4 billion years ago. [■] These bubbles had oxygen isotopes (variants) in them that suggested Earth had liquid water on its surface as early as 4.4 billion years ago! [■] This is surprising because when Earth formed 4.55 billion years ago, it was in a molten state (liquefied by heat).[■] Prior to this discovery geologists had always assumed that Earth took a long time to cool from its molten state. [■] Most thought that Earth took about 700 million years to cool down below the boiling point of water (100C), because that was the age of the oldest sedimentary rocks that would have been produced by running water (the Isua Supracrustals from Greenland, which are 3.8 billion years old). But the Jack Hills zircons turn that assumption inside out. If the zircons truly indicate the presence of liquid water on Earth 4.4 billion years ago, then it took only 200 million years for Earth to cool from its molten state to a condition that was below the boiling point of water. This also suggests that there were not as many meteorite impacts during this time interval, or the oceans would have been vaporized over and over again. Taken together, these data suggest what is now called the “cool early Earth hypothesis.”
Look at the four squaresthat indicate where the following sentence could be added to the passage
It turns out its composition at the time was not what geologists had expected.
Where would the sentence best fit？Click on a square sentence to the passage.
10
Scientists have made a number of important discoveries about early Earth.
AThe oldest rocks on Earth are not as old as the oldest Earth materials known to date, and both are hundreds of millions of years younger than the oldest materials in the solar system.
BAir bubbles trapped in 4.4 billion-year-old zircon sand grains revealed the presence of liquid water on Earth’s surface and suggested that Earth cooled much more quickly than earlier thought.
CEarth may have formed from materials containing water, but Earth’s surface temperature had to drop below 100C for the water to form its first oceans.
DThe difference in age between the oldest Earth rocks found in Quebec, Canada and the oldest Earth materials found in Australia is the same length of time as the Age of Dinosaurs.
EEarth experienced several meteorite impacts for millions of years after its formation, and this caused it to cool faster from its molten state to below the boiling point of water.
FImpact from comets made mostly of dust and ice water on early Earth contributed to changing the chemical composition of Earth’s first oceans.
答案：
