All planets in the solar system except Venus and Mercury have at least one moon orbiting them,with the two largest jovian (giant) planets,Jupiter and Saturn,each having over sixty.Most of these moons probably formed in the following way:As the jovian planets formed,their gravity drew surrounding gas toward them until each jovian planet was surrounded by a swirling disk of gas that spun in the same direction as the planet rotated.Within the disk,solid particles of ice and rock began to stick together,growing into larger and larger bodies called planetisimals.This process of gradual growth is called accretion.Moons that accreted from planetesimals within these disks ended up with nearly circular orbits going in the same direction as their planet’s rotation and lying close to their planet’s equatorial plane.
However,some moons in the solar system have less orderly orbits.These moons are probably planetesimals that originally orbited the Sun but were then captured into planetary orbit.It is not easy for a planet to capture a moon.An object cannot switch from an unbound orbit (for example,an asteroid moving quickly past Jupiter)to a bound orbit (for example,a moon orbiting Jupiter) unless it somehow loses orbital energy.For the jovian planets, captures probably occurred when passing planetesimals lost energy passing through the extended and relatively dense gas that surrounded these planets as they formed.The planetesimals would have been slowed by friction with the gas,just as artificial satellites are slowed by encounters with Earth’s atmosphere.If friction reduced a passing planetesimal’s orbital energy enough,it could have become an orbiting moon.Because of the random nature of the capture process,captured moons do not necessarily orbit around their planets in the same direction that their planet is rotating in.Most of the small moons of the jovian planets are a few kilometers across,supporting the idea that they were captured in this way.Mars may have similarly captured its two small moons, Phobos and Deimos,at a time when the planet had a much more extended atmosphere than it does today.
Capture processes cannot explain our own Moon,because it is much too large to have been captured by a small planet like Earth. We can also rule out the possibility that our Moon formed simultaneously with Earth,because if both had formed together, they would have accreted from planetesimals of the same type and should therefore have approximately the same composition and density.But this is not the case:The Moon’s density is considerably lower than Earth’s,indicating that it has a very different average composition.Today,the leading hypothesis suggests that it formed as the result of a giant impact between Earth and a huge planetesimal.According to models,a few planetesimals left over after planet formation may have been as large as Mars.If one of these Mars-size objects struck a young planet,the blow might have tilted the planet’s axis,changed the planet’s rotation rate,or completely shattered the planet.The giant impact hypothesis holds that a Mars-size object hit Earth at a speed and angle that blasted Earth’s outer layers into space.According to computer simulations, this material could have collected into orbit around our planet,and accretion of materials within this ring of debris could have formed the Moon.
Strong support for the giant impact hypothesis comes from two features of the Moon’s composition.First,the Moon’s overall composition is quite similar to that of Earth’s outer layers-just as we should expect if it were made from material blasted away from those layers.Second,the Moon has a much smaller proportion of easily vaporized ingredients (such as water)than Earth.This fact supports the hypothesis because the heat of the impact would have vaporized these ingredients.As gases,they would not have participated in the process of accretion that formed the Moon.Giant impacts may also explain other exceptions to the general trends. For example,Pluto’s moon Charon shows signs of having formed in a giant impact similar to the one thought to have formed our Moon, and Mercury’s surprisingly high density may be the result of a giant impact that blasted away its outer,lower-density layers.
1
All planets in the solar system except Venus and Mercury have at least one moon orbiting them,with the two largest jovian (giant) planets,Jupiter and Saturn,each having over sixty.Most of these moons probably formed in the following way:As the jovian planets formed,their gravity drew surrounding gas toward them until each jovian planet was surrounded by a swirling disk of gas that spun in the same direction as the planet rotated.Within the disk,solid particles of ice and rock began to stick together,growing into larger and larger bodies called planetisimals.This process of gradual growth is called accretion.Moons that accreted from planetesimals within these disks ended up with nearly circular orbits going in the same direction as their planet’s rotation and lying close to their planet’s equatorial plane.
According to paragraph 1,which of the following is NOT true of the formation of the jovian planets and their moons?
AAs the planets were forming,they pulled surrounding gas into a swirling disk around them.
BIce and rock particles accreted within the disk to form planetesimals.
CMoons formed from materials in orbit around planetesimals.
DThe planets’moons began orbiting in the same direction as their planets’ rotation.
2
However,some moons in the solar system have less orderly orbits.These moons are probably planetesimals that originally orbited the Sun but were then captured into planetary orbit.It is not easy for a planet to capture a moon.An object cannot switch from an unbound orbit (for example,an asteroid moving quickly past Jupiter)to a bound orbit (for example,a moon orbiting Jupiter) unless it somehow loses orbital energy.For the jovian planets, captures probably occurred when passing planetesimals lost energy passing through the extended and relatively dense gas that surrounded these planets as they formed.The planetesimals would have been slowed by friction with the gas,just as artificial satellites are slowed by encounters with Earth’s atmosphere.If friction reduced a passing planetesimal’s orbital energy enough,it could have become an orbiting moon.Because of the random nature of the capture process,captured moons do not necessarily orbit around their planets in the same direction that their planet is rotating in.Most of the small moons of the jovian planets are a few kilometers across,supporting the idea that they were captured in this way.Mars may have similarly captured its two small moons, Phobos and Deimos,at a time when the planet had a much more extended atmosphere than it does today.
According to paragraph 2,why is it not easy for a planet to capture a moon?
AMost planetesimals do not have enough energy to stay in an orderly orbit
BPlanetesimals do not necessarily orbit in the same direction as planets.
CMost planetesimals are prevented from reaching a planet’s orbit by friction with the gas surrounding the planet.
DSomething must slow the planetesimal down significantly in order for it to enter a bound orbit around the planet.
3
However,some moons in the solar system have less orderly orbits.These moons are probably planetesimals that originally orbited the Sun but were then captured into planetary orbit.It is not easy for a planet to capture a moon.An object cannot switch from an unbound orbit (for example,an asteroid moving quickly past Jupiter)to a bound orbit (for example,a moon orbiting Jupiter) unless it somehow loses orbital energy.For the jovian planets, captures probably occurred when passing planetesimals lost energy passing through the extended and relatively dense gas that surrounded these planets as they formed.The planetesimals would have been slowed by friction with the gas,just as artificial satellites are slowed by encounters with Earth’s atmosphere.If friction reduced a passing planetesimal’s orbital energy enough,it could have become an orbiting moon.Because of the random nature of the capture process,captured moons do not necessarily orbit around their planets in the same direction that their planet is rotating in.Most of the small moons of the jovian planets are a few kilometers across,supporting the idea that they were captured in this way.Mars may have similarly captured its two small moons, Phobos and Deimos,at a time when the planet had a much more extended atmosphere than it does today.
Paragraph 2 suggests that a planetesimal that is captured as a moon generally has which of the following characteristics?
ASubstantial density
BIncreasing orbital energy
CAn orbit opposite of the rotational direction of their planet
DA very small size in comparison to the capturing planet
4
Capture processes cannot explain our own Moon,because it is much too large to have been captured by a small planet like Earth. We can also rule out the possibility that our Moon formed simultaneously with Earth,because if both had formed together, they would have accreted from planetesimals of the same type and should therefore have approximately the same composition and density.But this is not the case:The Moon’s density is considerably lower than Earth’s,indicating that it has a very different average composition.Today,the leading hypothesis suggests that it formed as the result of a giant impact between Earth and a huge planetesimal.According to models,a few planetesimals left over after planet formation may have been as large as Mars.If one of these Mars-size objects struck a young planet,the blow might have tilted the planet’s axis,changed the planet’s rotation rate,or completely shattered the planet.The giant impact hypothesis holds that a Mars-size object hit Earth at a speed and angle that blasted Earth’s outer layers into space.According to computer simulations, this material could have collected into orbit around our planet,and accretion of materials within this ring of debris could have formed the Moon.
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.
AWe can also rule out the possibility that our Moon and Earth have approximately the same composition and density.
BOur Moon could not have formed together with Earth because the similar type of planetesimals would have resulted in both having the same composition and density.
CWe know that our Moon and Earth could have accreted from planetesimals of the same type at about the same time.
DBecause the Moon and Earth accreted from planetesimals of the same type,it is surprising that they do not share the same composition and density.
5
Capture processes cannot explain our own Moon,because it is much too large to have been captured by a small planet like Earth. We can also rule out the possibility that our Moon formed simultaneously with Earth,because if both had formed together, they would have accreted from planetesimals of the same type and should therefore have approximately the same composition and density.But this is not the case:The Moon’s density is considerably lower than Earth’s,indicating that it has a very different average composition.Today,the leading hypothesis suggests that it formed as the result of a giant impact between Earth and a huge planetesimal.According to models,a few planetesimals left over after planet formation may have been as large as Mars.If one of these Mars-size objects struck a young planet,the blow might have tilted the planet’s axis,changed the planet’s rotation rate,or completely shattered the planet.The giant impact hypothesis holds that a Mars-size object hit Earth at a speed and angle that blasted Earth’s outer layers into space.According to computer simulations, this material could have collected into orbit around our planet,and accretion of materials within this ring of debris could have formed the Moon.
The word“shattered”in the passage is closest in meaning to
Abroken apart
Bchanged
Cshifted
Dcombined with
6
Capture processes cannot explain our own Moon,because it is much too large to have been captured by a small planet like Earth. We can also rule out the possibility that our Moon formed simultaneously with Earth,because if both had formed together, they would have accreted from planetesimals of the same type and should therefore have approximately the same composition and density.But this is not the case:The Moon’s density is considerably lower than Earth’s,indicating that it has a very different average composition.Today,the leading hypothesis suggests that it formed as the result of a giant impact between Earth and a huge planetesimal.According to models,a few planetesimals left over after planet formation may have been as large as Mars.If one of these Mars-size objects struck a young planet,the blow might have tilted the planet’s axis,changed the planet’s rotation rate,or completely shattered the planet.The giant impact hypothesis holds that a Mars-size object hit Earth at a speed and angle that blasted Earth’s outer layers into space.According to computer simulations, this material could have collected into orbit around our planet,and accretion of materials within this ring of debris could have formed the Moon.
According to paragraph 3,what is the leading hypothesis for how our Moon was formed?
AIt was one of a few planetesimals left over after planet formation that was pulled into Earth’s orbit because its density was lower than Earth’s.
BIt was a very small planet that was knocked off its orbit by a giant impact.
CIt was originally in orbit around Mars,but when Mars was struck by a large planetesimal,it was thrown into Earth’s orbit.
DIt formed from the debris left over from an impact between Earth and a Mars-size planetesimal.
7
Strong support for the giant impact hypothesis comes from two features of the Moon’s composition.First,the Moon’s overall composition is quite similar to that of Earth’s outer layers-just as we should expect if it were made from material blasted away from those layers.Second,the Moon has a much smaller proportion of easily vaporized ingredients (such as water)than Earth.This fact supports the hypothesis because the heat of the impact would have vaporized these ingredients.As gases,they would not have participated in the process of accretion that formed the Moon.Giant impacts may also explain other exceptions to the general trends. For example,Pluto’s moon Charon shows signs of having formed in a giant impact similar to the one thought to have formed our Moon, and Mercury’s surprisingly high density may be the result of a giant impact that blasted away its outer,lower-density layers.
Why does the author provide the information that“the Moon has a much smaller proportion of easily vaporized ingredients (such as water)than Earth”?
ATo argue against the claim that the Moon’s overall composition is quite similar to that of Earth’s outer layers
BTo provide evidence that supports the giant impact hypothesis
CTo provide a second hypothesis for how the Moon was formed
DTo explain why gases would not have been involved in the process of accretion that formed the Moon
8
Strong support for the giant impact hypothesis comes from two features of the Moon’s composition.First,the Moon’s overall composition is quite similar to that of Earth’s outer layers-just as we should expect if it were made from material blasted away from those layers.Second,the Moon has a much smaller proportion of easily vaporized ingredients (such as water)than Earth.This fact supports the hypothesis because the heat of the impact would have vaporized these ingredients.As gases,they would not have participated in the process of accretion that formed the Moon.Giant impacts may also explain other exceptions to the general trends. For example,Pluto’s moon Charon shows signs of having formed in a giant impact similar to the one thought to have formed our Moon, and Mercury’s surprisingly high density may be the result of a giant impact that blasted away its outer,lower-density layers.
According to paragraph 4,an impact between Mercury and a giant planetesimal might be responsible for which of the following?
AMercury’s lack of water vapor
BMercury’s high density
CMercury’s lower-density outer layers
DMercury’s similarity to Pluto
9
Capture processes cannot explain our own Moon,because it is much too large to have been captured by a small planet like Earth. [■] We can also rule out the possibility that our Moon formed simultaneously with Earth,because if both had formed together, they would have accreted from planetesimals of the same type and should therefore have approximately the same composition and density. [■] But this is not the case:The Moon’s density is considerably lower than Earth’s,indicating that it has a very different average composition. [■] Today,the leading hypothesis suggests that it formed as the result of a giant impact between Earth and a huge planetesimal. [■] According to models,a few planetesimals left over after planet formation may have been as large as Mars.If one of these Mars-size objects struck a young planet,the blow might have tilted the planet’s axis,changed the planet’s rotation rate,or completely shattered the planet.The giant impact hypothesis holds that a Mars-size object hit Earth at a speed and angle that blasted Earth’s outer layers into space.According to computer simulations, this material could have collected into orbit around our planet,and accretion of materials within this ring of debris could have formed the Moon.
Look at the four squaresthat indicate where the following sentence could be added to the passage
So how did we get our Moon?
Where would the sentence best fit？Click on a square sentence to the passage.
10
Planets in the solar system have several different types of moons, which formed in different ways.
AMany moons in the solar system accreted from the disk of gas surrounding their planets,giving them circular orbits that follow in the direction of their planet’s rotation.
BDifferences in the direction of the orbits of planetary moons and their planets occur as the moons lose orbital energy after capture.
CThe impact of a giant planetesimal likely blasted Earth’s outer layers into space,where our Moon accreted out of this debris,and Pluto’s moon Charon may have formed in a similar way.
DSome of the solar system’s smaller moons,including those of Mars as well as those of the jovian planets,were probably captured as they passed through the thick atmospheres of the planets.
EHigh-density moons may have formed from the capture of planetesimals whose lower-density outer layers were removed by collisions within the dense gas disk surrounding the planet.
FIt is now thought that Earth’s moon and Mercury may both have formed as the result of a single giant impact between Earth and a huge planetesimal.
