Habitability of Planets

Magnetic fields deflect (turn away) charged particles. Earth has a magnetic field generated by the movement of liquid in its core. One reason that Earth’s magnetic field is considered important for Earth’s habitability (ability to support life) is that it helps protect the planet from bombardment by cosmic rays. Cosmic rays are energetic charged particles that originate from space, either from the Sun or from farther away in the galaxy. Solar cosmic rays are relatively low-energy particles that are released in a stream of charged particles (the “solar wind”) from the upper atmosphere of the Sun. Galactic cosmic rays are high-energy particles and gamma rays that originate from outside the solar system. Prolonged exposure to either of these types of radiation can create health risks, including cancer-a serious consideration for astronauts out in space.

Although Earth’s magnetic field helps protect the planet from cosmic rays, that protection is only partial: the magnetic field barely deflects the most energetic cosmic rays that have energies of many gigaelectron volts. What then shields Earth’s surface from high-energy cosmic rays? The answer is the atmosphere itself. Most cosmic rays, including those falling at the poles, are absorbed by the atmosphere at altitudes of 80 km or more. The high-energy ones create cascades of secondary particles, and some of these do indeed make it down to the surface. By the time they get there, however, they have lost most of their force, and so they account for less than 10 percent of the radiation exposure that an average person receives during the course of a year.

Planetary magnetic fields have another effect that may actually be more important in keeping a planet habitable. Earth’s magnetic field prevents the solar wind from interacting directly with its atmosphere. (This statement is equivalent to saying that it protects Earth from solar cosmic rays.) Mars, which lacks an intrinsic magnetic field, does not have such protection, and consequently its atmosphere may have been sputtered away by the solar wind “Sputtering” is the term used to describe the process whereby collisions between solar wind particles and electrically charged atoms in a planet’s upper atmosphere can lead to loss of atmospheric gases. Based partly on this observation, scientists Peter Ward and Donald Brownlee suggest that planets that lack magnetic fields are unlikely to be habitable.

This argument, though, overlooks something obvious: Venus, which also has no intrinsic magnetic field. has a very dense atmosphere-100 times thicker than Earth’s. Furthermore. Venus is closer to the Sun than is either Earth or Mars, and hence it is exposed to a denser, more energetic solar wind. If solar wind sputtering is so effective, then why didn’t Venus lose its atmosphere? The answer probably has to do with planetary size. Venus is roughly the same mass as Earth, and so its upper atmosphere is less extended than that of Mars. That makes it harder for the solar wind to strip away atmospheric gases. Both Venus and Mars do have induced magnetic fields (fields generated in their upper atmospheres by their interaction with solar wind), and in Venus’s case this induced field is evidently sufficient to protect its atmosphere. Mars’s problem is the combination of its lack of an intrinsic magnetic field and its small size, which is clearly not conducive to planetary habitability.

This brings us to the issue of planetary size itself. Mars’s small mass (1/9 of Earth’s mass) also allowed the planet to cool more quickly. Without enough internal heat to sustain volcanism, it was unable to recycle carbonates (consisting of one carbon and three oxygen atoms) back into CO2 (carbon dioxide, a gas that traps heat at the planet’s surface). Mars appears to have already cooled off by about 3.8 billion years ago. No one knows exactly how big a planet needs to be to stay volcanically active for 4.5 billion years, as Earth has done, but one can guess that it needs to be 1/3 of Earth’s mass or more. A higher mass should also give a planet a better chance of sustaining a magnetic field because its core should take longer to solidify.

 

 

1

Magnetic fields deflect (turn away) charged particles. Earth has a magnetic field generated by the movement of liquid in its core. One reason that Earth’s magnetic field is considered important for Earth’s habitability (ability to support life) is that it helps protect the planet from bombardment by cosmic rays. Cosmic rays are energetic charged particles that originate from space, either from the Sun or from farther away in the galaxy. Solar cosmic rays are relatively low-energy particles that are released in a stream of charged particles (the “solar wind”) from the upper atmosphere of the Sun. Galactic cosmic rays are high-energy particles and gamma rays that originate from outside the solar system. Prolonged exposure to either of these types of radiation can create health risks, including cancer-a serious consideration for astronauts out in space.

According to paragraph 1, which of the following is true of galactic cosmic rays?

AThey originate closer to Earth than do any other types of cosmic rays.

BTheir particles are carried primarily by the solar wind.

CTheir particles are higher in energy than are those of solar cosmic rays

DThey cause less harm to humans than do other cosmic rays.

 

2

Magnetic fields deflect (turn away) charged particles. Earth has a magnetic field generated by the movement of liquid in its core. One reason that Earth’s magnetic field is considered important for Earth’s habitability (ability to support life) is that it helps protect the planet from bombardment by cosmic rays. Cosmic rays are energetic charged particles that originate from space, either from the Sun or from farther away in the galaxy. Solar cosmic rays are relatively low-energy particles that are released in a stream of charged particles (the “solar wind”) from the upper atmosphere of the Sun. Galactic cosmic rays are high-energy particles and gamma rays that originate from outside the solar system. Prolonged exposure to either of these types of radiation can create health risks, including cancer-a serious consideration for astronauts out in space.

Which of the following questions about Earth’s magnetic field is answered in paragraph 1?

AWhat types of cosmic rays other than solar and galactic cosmic rays are deflected by Earth’s magnetic field?

BWhat produces Earth’s magnetic field?

CWhat parts of Earth’s surface tend to be least protected by Earth’s magnetic field?

DWhat types of health risks does Earth’s magnetic field pose to astronauts in space?

 

3

Although Earth’s magnetic field helps protect the planet from cosmic rays, that protection is only partial: the magnetic field barely deflects the most energetic cosmic rays that have energies of many gigaelectron volts. What then shields Earth’s surface from high-energy cosmic rays? The answer is the atmosphere itself. Most cosmic rays, including those falling at the poles, are absorbed by the atmosphere at altitudes of 80 km or more. The high-energy ones create cascades of secondary particles, and some of these do indeed make it down to the surface. By the time they get there, however, they have lost most of their force, and so they account for less than 10 percent of the radiation exposure that an average person receives during the course of a year.

According to paragraph 2, all of the following are true about the particles of high-energy cosmic rays EXCEPT:

AThey lose most of their energy by the time they reach Earth’s surface.

BThey account for a relatively small percentage of individuals’ annual radiation exposure.

CThey do not reach low-altitude areas of Earth’s surface

DThey break up into secondary particles when they reach the atmosphere.

 

4

Planetary magnetic fields have another effect that may actually be more important in keeping a planet habitable. Earth’s magnetic field prevents the solar wind from interacting directly with its atmosphere. (This statement is equivalent to saying that it protects Earth from solar cosmic rays.) Mars, which lacks an intrinsic magnetic field, does not have such protection, and consequently its atmosphere may have been sputtered away by the solar wind “Sputtering” is the term used to describe the process whereby collisions between solar wind particles and electrically charged atoms in a planet’s upper atmosphere can lead to loss of atmospheric gases. Based partly on this observation, scientists Peter Ward and Donald Brownlee suggest that planets that lack magnetic fields are unlikely to be habitable.

According to paragraph 3, which of the following factors may have contributed to Mars’s loss of atmospheric gases?

AMars’s atmosphere was particularly rich in electrically charged atoms that interact with solar wind particles.

BMars had a weak magnetic field that was destroyed by the solar wind.

CMars’s upper atmosphere was not extensive enough to prevent solar wind particles from colliding with atmospheric charged atoms.

DMars did not have a magnetic field that could protect its atmosphere from sputtering.

 

5

This argument, though, overlooks something obvious: Venus, which also has no intrinsic magnetic field. has a very dense atmosphere-100 times thicker than Earth’s. Furthermore. Venus is closer to the Sun than is either Earth or Mars, and hence it is exposed to a denser, more energetic solar wind. If solar wind sputtering is so effective, then why didn’t Venus lose its atmosphere? The answer probably has to do with planetary size. Venus is roughly the same mass as Earth, and so its upper atmosphere is less extended than that of Mars. That makes it harder for the solar wind to strip away atmospheric gases. Both Venus and Mars do have induced magnetic fields (fields generated in their upper atmospheres by their interaction with solar wind), and in Venus’s case this induced field is evidently sufficient to protect its atmosphere. Mars’s problem is the combination of its lack of an intrinsic magnetic field and its small size, which is clearly not conducive to planetary habitability.

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

Amisses

Breveals

Csuggests

Dcontradicts

 

6

This argument, though, overlooks something obvious: Venus, which also has no intrinsic magnetic field. has a very dense atmosphere-100 times thicker than Earth’s. Furthermore. Venus is closer to the Sun than is either Earth or Mars, and hence it is exposed to a denser, more energetic solar wind. If solar wind sputtering is so effective, then why didn’t Venus lose its atmosphere? The answer probably has to do with planetary size. Venus is roughly the same mass as Earth, and so its upper atmosphere is less extended than that of Mars. That makes it harder for the solar wind to strip away atmospheric gases. Both Venus and Mars do have induced magnetic fields (fields generated in their upper atmospheres by their interaction with solar wind), and in Venus’s case this induced field is evidently sufficient to protect its atmosphere. Mars’s problem is the combination of its lack of an intrinsic magnetic field and its small size, which is clearly not conducive to planetary habitability.

Why does the author provide the information that “Venus, which also has no intrinsic magnetic field. has a very dense atmosphere-100 times thicker than Earth’s”?

ATo provide evidence that challenges the argument made by Peter Ward and Donald Brownlee

BTo show that some planets with atmospheres are not habitable

CTo support the claim that planets lacking intrinsic magnetic fields are unlikely to be habitable

DTo illustrate an effect of Venus being closer to the Sun than is Earth or Mars

 

7

This argument, though, overlooks something obvious: Venus, which also has no intrinsic magnetic field. has a very dense atmosphere-100 times thicker than Earth’s. Furthermore. Venus is closer to the Sun than is either Earth or Mars, and hence it is exposed to a denser, more energetic solar wind. If solar wind sputtering is so effective, then why didn’t Venus lose its atmosphere? The answer probably has to do with planetary size. Venus is roughly the same mass as Earth, and so its upper atmosphere is less extended than that of Mars. That makes it harder for the solar wind to strip away atmospheric gases. Both Venus and Mars do have induced magnetic fields (fields generated in their upper atmospheres by their interaction with solar wind), and in Venus’s case this induced field is evidently sufficient to protect its atmosphere. Mars’s problem is the combination of its lack of an intrinsic magnetic field and its small size, which is clearly not conducive to planetary habitability.

What can be inferred from paragraph 4 about the reason why Mars lost atmospheric gases while Venus maintained a dense atmosphere?

AMars has a weaker intrinsic magnetic field than does Venus.

BMars is significantly smaller than Venus.

CMars does not have an induced magnetic field.

DMars is exposed to denser solar wind than is Venus.

 

8

This brings us to the issue of planetary size itself. Mars’s small mass (1/9 of Earth’s mass) also allowed the planet to cool more quickly. Without enough internal heat to sustain volcanism, it was unable to recycle carbonates (consisting of one carbon and three oxygen atoms) back into CO2 (carbon dioxide, a gas that traps heat at the planet’s surface). Mars appears to have already cooled off by about 3.8 billion years ago. No one knows exactly how big a planet needs to be to stay volcanically active for 4.5 billion years, as Earth has done, but one can guess that it needs to be 1/3 of Earth’s mass or more. A higher mass should also give a planet a better chance of sustaining a magnetic field because its core should take longer to solidify.

According to paragraph 5, all of the following are associated with increased planetary mass EXCEPT

Alower levels of CO2 in the planet’s atmosphere

Bthe ability to maintain a liquid core for a longer time

Cthe ability to remain volcanically active for a longer time

Dan increased likelihood of keeping a magnetic field

 

9

Look at the four squaresthat indicate where the following sentence could be added to the passage

Such high-energy rays can pass right through.

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

10

A number of factors affect the ability of a planet to support life

AAlthough solar cosmic rays are more likely to reach Earth, galactic cosmic rays are much more harmful to life on Earth and expose people to large amounts of radiation.

BA planet’s magnetic field can help preserve the planet’s atmosphere by preventing the solar wind from stripping away atmospheric gases

CPlanets that are small in size are less likely to support life, because they cool off quickly and are particularly vulnerable to sputtering.

DEarth’s magnetic field and atmosphere make the planet habitable by shielding it from most cosmic rays

EWard and Brownlee argue that frequent collisions between solar wind particles and charged atoms in the atmosphere can weaken a planet’s magnetic field over time, making the planet less habitable.

FVenus has remained volcanically active despite its lack of a magnetic field, which helps to explain why Venus has kept its atmosphere.

 

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