The Origin of Mars’s Moons
Mars has two tiny moons, Phobos and Deimos. The orbits of both these satellites are nearly circular and close to the planet: Phobos orbits at a distance of about 9,378 kilometers (5,627 miles) from the center of Mars, and Deimos at about 23,459 kilometers (14,075 miles). Their orbital periods are short: 7 hours, 37 minutes for Phobos, and 30 hours, 18 minutes for Deimos. Both satellites orbit in the same direction as the planet rotates, and they have low- inclination orbits (meaning their orbits are tilted at a small angle from Mars’s equator). The origin of Phobos and Deimos is unknown, but there are two currently popular theories. One theory holds that these bodies are captured asteroids (rocky objects smaller than planets, that orbit the Sun). The other theory suggests that they are asteroid-sized pieces of debris left over from the initial formation of Mars from the cloud of rock and dust from which all the planets in the solar system formed
Astronomers generally agree that small bodies (asteroids, comets, or other debris)that wander the solar system, if captured by a planet, would typically be trapped in a highly elliptical (oval- shaped), inclined orbit. Consequently, when astronomers see a small body that has a circular and low-inclination orbit around a planet, they generally regard this as evidence that the small satellite was not a captured wanderer but a piece of debris left over from the formation of the planet.
Some scientists argue that wandering solar-system bodies can be captured in an initially highly elliptical orbit and then later have their orbits circularized. This requires the delicate balance of many conditions to work, making many scientists uneasy about its validity. The circularization of an initially elliptical orbit can be done through a mechanism akin to aerobraking, used by modern-day space-flight controllers to slowly circularize highly elliptical orbits of some spacecraft. With this mechanism, under the right conditions if an asteroid was initially captured into just the right highly elliptical orbit around Mars, the lowest part of its path could have carried it repeatedly through the upper part of the massive early Martian atmosphere, slowing it by atmospheric drag and slowly circularizing the orbit.
Circularization of an orbit can also be done through a mechanism involving tidal forces (gravitational forces whose strength periodically rises and falls)tugging on a satellite Depending on the orbital period and spin period of Mars at the time of their capture, it has been calculated that tidal forces could act on these satellites in such a way that would eventually drag them from their initial highly elliptical orbits into circular ones.
Though these mechanisms require special conditions to work. some astronomers suggest that there is evidence that these moons are asteroids captured when they wandered from the outer solar system: their density and composition. The densities of Phobos and Deimos are surprisingly low, too low for them to be made of the materials that formed Mars. However, the density of some carbonaceous chondrite meteorites-meteorites rich in carbon and formed with particles present in the early solar system-is a much closer match, and with the addition of water ice, would be identical. Spectral measurements (taken from properties of reflected light) show that these moons are composed of nearly black rock, similar to carbonaceous chondrites and asteroids found in the outer part of the main asteroid belt, though very different from asteroids found closer to the Sun or from rocks on Mars
The counter-argument to this holds that if Phobos and Deimos formed in place around Mars, then they should be composed of the same high-density material as the planet. To account for their low density, some scientists have speculated either that these moons are composed of high-density Mars rock and low-density ice trapped in their interiors, or that they contain enough pore spaces (air-filled sacs) to bring their density down to the observed values In addition, to account for the spectral(reflected) properties of the bodies, these scientists argue that Phobos and Deimos are covered by a veneer (thin surface layer) of higher-density carbonaceous chondritic material delivered from the outer solar system, evidenced by the numerous impact craters seen on these moons.
1
Mars has two tiny moons, Phobos and Deimos. The orbits of both these satellites are nearly circular and close to the planet: Phobos orbits at a distance of about 9,378 kilometers (5,627 miles) from the center of Mars, and Deimos at about 23,459 kilometers (14,075 miles). Their orbital periods are short: 7 hours, 37 minutes for Phobos, and 30 hours, 18 minutes for Deimos. Both satellites orbit in the same direction as the planet rotates, and they have low- inclination orbits (meaning their orbits are tilted at a small angle from Mars’s equator). The origin of Phobos and Deimos is unknown, but there are two currently popular theories. One theory holds that these bodies are captured asteroids (rocky objects smaller than planets, that orbit the Sun). The other theory suggests that they are asteroid-sized pieces of debris left over from the initial formation of Mars from the cloud of rock and dust from which all the planets in the solar system formed
Which of the following statements about the moons of Mars is supported by paragraph 1?
Factual Information Questions事实信息题
ADeimos and Phobos move in opposite directions around Mars.
BBoth moons originally were asteroids captured by Mars
CThe orbital periods of both moons are surprisingly long.
DDeimos is farther from Mars than Phobos is
2
Some scientists argue that wandering solar-system bodies can be captured in an initially highly elliptical orbit and then later have their orbits circularized. This requires the delicate balance of many conditions to work, making many scientists uneasy about its validity. The circularization of an initially elliptical orbit can be done through a mechanism akin to aerobraking, used by modern-day space-flight controllers to slowly circularize highly elliptical orbits of some spacecraft. With this mechanism, under the right conditions if an asteroid was initially captured into just the right highly elliptical orbit around Mars, the lowest part of its path could have carried it repeatedly through the upper part of the massive early Martian atmosphere, slowing it by atmospheric drag and slowly circularizing the orbit.
The phrase “akin to” in the passage is closest in meaning to
Vocabulary Questions词汇题
Asimilar to
Bknown as
Crequired for
Dbased on
3
Some scientists argue that wandering solar-system bodies can be captured in an initially highly elliptical orbit and then later have their orbits circularized. This requires the delicate balance of many conditions to work, making many scientists uneasy about its validity. The circularization of an initially elliptical orbit can be done through a mechanism akin to aerobraking, used by modern-day space-flight controllers to slowly circularize highly elliptical orbits of some spacecraft. With this mechanism, under the right conditions if an asteroid was initially captured into just the right highly elliptical orbit around Mars, the lowest part of its path could have carried it repeatedly through the upper part of the massive early Martian atmosphere, slowing it by atmospheric drag and slowly circularizing the orbit.
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.
Sentence Simplification Questions句子简化题
AThe massive early Martian atmosphere could have slowed the speed of captured asteroids. causing their orbits to become highly elliptical.
BAn asteroid would not necessarily enter the dense Martian atmosphere, unless it was captured into just the right highly elliptical orbit.
CWith this mechanism, the original circular orbits of Mars’s moons are maintained by the dragging effect of Mars’s massive atmosphere
DThe initial elliptical orbit of a captured asteroid could have been circularized by drag if the asteroid repeatedly passed through the thick early Martian atmosphere.
4
Some scientists argue that wandering solar-system bodies can be captured in an initially highly elliptical orbit and then later have their orbits circularized. This requires the delicate balance of many conditions to work, making many scientists uneasy about its validity. The circularization of an initially elliptical orbit can be done through a mechanism akin to aerobraking, used by modern-day space-flight controllers to slowly circularize highly elliptical orbits of some spacecraft. With this mechanism, under the right conditions if an asteroid was initially captured into just the right highly elliptical orbit around Mars, the lowest part of its path could have carried it repeatedly through the upper part of the massive early Martian atmosphere, slowing it by atmospheric drag and slowly circularizing the orbit.
Which of the following can be inferred from paragraph 3 about the circularization of elliptical orbits?
Inference Questions推理题
AIt is more likely to occur with large satellites than with small ones
BIf it does occur, it would be a rare event.
CScientists have been unable to demonstrate the process with modern-day spacecraft.
DConditions are more favorable for this process now than they were early in the history of the solar system.
5
Circularization of an orbit can also be done through a mechanism involving tidal forces (gravitational forces whose strength periodically rises and falls)tugging on a satellite Depending on the orbital period and spin period of Mars at the time of their capture, it has been calculated that tidal forces could act on these satellites in such a way that would eventually drag them from their initial highly elliptical orbits into circular ones.
According to paragraph 4, which of the following describes a way in which a satellite’s elliptical orbit could be circularized after capture by Mars?
Factual Information Questions事实信息题
AThe spinning motion of a satellite could accelerate during capture, pushing its orbit into a circular path.
BTidal forces from one satellite might have pulled the other into a more circular orbit
CGravitational forces from Mars could pull the satellite into a circular orbit.
DIf the strength of tidal forces decreases, the satellite would no longer be pulled into an elliptical orbit, and would be circular.
6
Though these mechanisms require special conditions to work. some astronomers suggest that there is evidence that these moons are asteroids captured when they wandered from the outer solar system: their density and composition. The densities of Phobos and Deimos are surprisingly low, too low for them to be made of the materials that formed Mars. However, the density of some carbonaceous chondrite meteorites-meteorites rich in carbon and formed with particles present in the early solar system-is a much closer match, and with the addition of water ice, would be identical. Spectral measurements (taken from properties of reflected light) show that these moons are composed of nearly black rock, similar to carbonaceous chondrites and asteroids found in the outer part of the main asteroid belt, though very different from asteroids found closer to the Sun or from rocks on Mars
In paragraph 5, why does the author provide the information that the density of Phobos and Deimos is almost the same as that of some carbonaceous chondrites?
Rhetorical Purpose Questions修辞目的题
ATo support the idea that Phobos and Deimos are captured asteroids
BTo transition to a discussion of carbonaceous chondrites and other types of rock
CTo emphasize that the density of rock changes when water ice is added
DTo argue that Phobos and Deimos are, in fact, nearly as dense as Mars
7
The counter-argument to this holds that if Phobos and Deimos formed in place around Mars, then they should be composed of the same high-density material as the planet. To account for their low density, some scientists have speculated either that these moons are composed of high-density Mars rock and low-density ice trapped in their interiors, or that they contain enough pore spaces (air-filled sacs) to bring their density down to the observed values In addition, to account for the spectral(reflected) properties of the bodies, these scientists argue that Phobos and Deimos are covered by a veneer (thin surface layer) of higher-density carbonaceous chondritic material delivered from the outer solar system, evidenced by the numerous impact craters seen on these moons.
According to paragraph 6, which of the following is a possible explanation for the observed low density of Phobos and Deimos?
Factual Information Questions事实信息题
AThe moons may have a lot of air or ice pockets in their interiors.
BThe presence of carbonaceous chondrites may interfere with measurements of the moons’ density.
CThe moons may be composed of the same material as Mars.
DMeteorites may have deposited low-density material on the moons’ surfaces
8
The counter-argument to this holds that if Phobos and Deimos formed in place around Mars, then they should be composed of the same high-density material as the planet. To account for their low density, some scientists have speculated either that these moons are composed of high-density Mars rock and low-density ice trapped in their interiors, or that they contain enough pore spaces (air-filled sacs) to bring their density down to the observed values In addition, to account for the spectral(reflected) properties of the bodies, these scientists argue that Phobos and Deimos are covered by a veneer (thin surface layer) of higher-density carbonaceous chondritic material delivered from the outer solar system, evidenced by the numerous impact craters seen on these moons.
According to paragraph 6, some scientists believe that the surface layer materials of Phobos and Deimos
Factual Information Questions事实信息题
Arose to the surface from deep within their interiors
Bcame from the outer solar system
Cwere largely lost after numerous impacts
Dare of lower density than Mars
9
Astronomers generally agree that small bodies (asteroids, comets, or other debris)that wander the solar system, if captured by a planet, would typically be trapped in a highly elliptical (oval- shaped), inclined orbit. ⬛Consequently, when astronomers see a small body that has a circular and low-inclination orbit around a planet, they generally regard this as evidence that the small satellite was not a captured wanderer but a piece of debris left over from the formation of the planet.
⬛Some scientists argue that wandering solar-system bodies can be captured in an initially highly elliptical orbit and then later have their orbits circularized.⬛ This requires the delicate balance of many conditions to work, making many scientists uneasy about its validity. ⬛The circularization of an initially elliptical orbit can be done through a mechanism akin to aerobraking, used by modern-day space-flight controllers to slowly circularize highly elliptical orbits of some spacecraft. With this mechanism, under the right conditions if an asteroid was initially captured into just the right highly elliptical orbit around Mars, the lowest part of its path could have carried it repeatedly through the upper part of the massive early Martian atmosphere, slowing it by atmospheric drag and slowly circularizing the orbit.
Look at the four squaresthat indicate where the following sentence could be added to the passage
However, this type of evidence is not necessarily conclusive.Insert Text Questions句子插入题
Where would the sentence best fit？Click on a square sentence to the passage.
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Two theories have been proposed to explain the origin of Mars’s moons, Phobos and Deimos.
Prose Summary Questions概要小结题
Select 3 answers
AIn terms of their size, orbital direction, and period, Mars’s moons are more like asteroids or comets than like the moons of other planets i our solar system
BStudies of the light reflected from Phobos and Deimos show that they cannot be composed of a carbonaceous chondrite material because they contain too much water ice.
CMars’s moons are made of a nearly black rock that is much less dense than the rock on Mars, suggesting that they may be captured asteroids.
DMars’s moons may have been made from the cloud and dust remaining after Mars was formed, as suggested by their orbits circular shape and low angle of inclination.
EBecause Mars has particularly strong tidal forces and a dense atmosphere, any object caught in its orbit tends to develop a circular, low-inclination orbit.
FNone of the evidence is decisive because the moons’ orbits may have changed shape over time, or the material on their surfaces may differ from that of their interiors.
