From 1996 to 1999, the Galileospacecraftpassedthroughthe Jovian system, providing much information aboutJupiter's satellites. Callisto, theoutermostofJupiter's four largest satellites, orbits theplanetin seventeen days at a distance fromJupiterof two million kilometers. Like our own Moon, Callisto rotates in the same period as it revolves, so it always keeps the same face towardJupiter. Its noontime surfacetemperatureis only about -140°C, so water ice isstableon its surface year-round. Callisto has adiameterof 4.820 kilometers, almost the same as that ofMercury. Itsmassis only one-third as great, which means itsdensitymust be only one-third as great as well. This tells us that Callisto has far less of therockymetallicmaterials found in theinnerplanets and mustinsteadbe an icy bodythroughmuch of itsinterior.

Callisto has not fully differentiated, meaning separated into layers of differentdensitymaterials. Astronomers can tell that it lacks adensecorefrom the details of itsgravitationalpull on the Galileospacecraftduring several very close flybys. This fact surprised scientists, who expected that all the big icy moons would be differentiated. It is much easier for an icy body todifferentiatethan for arockyone, since themeltingtemperatureof ice is so low. Only a little heating will soften the ice and get theprocessstarted, allowing the rock and metal to sink to the center and the slushy ice tofloatto the surface. Yet Callisto seems to have frozensolidbefore theprocessofdifferentiationwas complete.

Like our Moon's highlands, the surface of Callisto is covered withimpactcraters. Thesurvivalof these craters tells us that an icy object can form andretainimpactcraters in its surface. In thinking of ice so far from the Sun, it is important not to judge itsbehaviorfrom that of the much warmer ice we know on Earth; at the temperatures of the outersolarsystem, ice on the surface is nearly as hard as rock, and behaves similarly. Ice on Callisto does notdeformor flow like ice in glaciers on Earth. Callisto isuniqueamong the planet-sized objects of thesolarsystem in itsabsenceofinteriorforces todrivegeologicalevolution. Thesatellitewas born dead and has remained geologically dead for more than four billion years.

Ganymede, another ofJupiter's satellites and the largest in oursolarsystem, is also cratered, but less so than Callisto. About one-quarter of its surface seems to be as old and heavily cratered; the rest formed more recently, as we can tell by thesparsecovering ofimpactcraters as well as therelativefreshness of the craters. Ganymede is a differentiated world, like theterrestrialplanets. Measurements of itsgravityfieldtell us that the rock and metal sank to form acoreabout the size of our Moon, with amantleandcrustof ice floating above it. Inaddition, the Galileospacecraftdiscovered that Ganymede has amagneticfield, the signature of apartiallymolteninterior. Ganymede is not a dead world, but rather a place of continuing geologicalactivitypowered by aninternalheatsource. Much of its surface may be as young as half a billion years.

The youngerterrainis the result oftectonicandvolcanicforces. Some features formed when thecrustcracked, flooding many of the craters with water from theinterior. Extensive mountain ranges were formed from compression of thecrust, forming long ridges withparallelvalleys spaced one to two kilometers apart. In some places olderimpactcraters weresplitand pulled apart. There are even indications of large-scalecrustalmovements that aresimilarto theplatetectonicsof Earth.

Why is Ganymede different from Callisto? Possibly the smalldifferencein size andinternalheating between the two led to this divergence in theirevolution. But more likely thegravityofJupiteris to blame for Ganymede's continuing geologicalactivity. Ganymede is close enough toJupiterthattidalforces from thegiantplanetmay have episodically heated itsinteriorand triggered major convulsions on itscrust.

Questions:
question_stem: According to paragraph 1, which of the following statements about Callisto is true?
choices: tag: A
text: It is the satellite closest to Jupiter's surface

tag: B
text: Its surface temperature is constant at all times of the day.

tag: C
text: It has the same mass and diameter as the planet Mercury

tag: D
text: It completes one rotation every seventeen days.
answer_key: D
explanation: 根据原文 Callisto, the outermost of Jupiter's four largest satellites, orbits the planet in seventeen days at a distance from Jupiter of two million kilometers.可知，orbits the planet in seventeen days at a distance from Jupiter 对应 D 选项中的 one rotation every seventeen days. 所以 D 选项正确。

question_stem: According to paragraph 1, how do scientists know that Callisto is made up largely of ice?
choices: tag: A
text: A sample of its interior was taken by the Galileo spacecraft in the late 1990s

tag: B
text: It has too low a density to contain much rocky metallic material

tag: C
text: With a noontime surface temperature of only about .140° C. the ice on it never melts

tag: D
text: All of the bodies in the Jovian system are icy, because they are so far from the Sun.
answer_key: B
explanation: 根据 ice 定位至原文 This tells us that Callisto has far less of the rocky metallic materials found in the inner planets and must instead be an icy body through much of its interior. 其中 far less of the rocky metallic materials 对应 B 选项中的 too low a density to contain much rocky metallic material. 所以 B 选项正确。

question_stem: Why does the author provide the information that “It is much easier for an icy body to differentiate than for a rocky one”?
choices: tag: A
text: To support the claim that all of the big icy moons are differentiated

tag: B
text: To suggest that Callisto may be a rocky body rather than an icy one

tag: C
text: To explain why scientists expected Callisto to be differentiated

tag: D
text: To refute the claim that Callisto could not differentiate because it was frozen solid
answer_key: C
explanation: 根据原文 This fact surprised scientists, who expected that all the big icy moons would be differentiated.可知 scientists expected 对应 C 选项中的 This fact surprised scientists. 所以 C 选项正确。

question_stem: All of the following questions are answered in paragraph 2 EXCEPT
choices: tag: A
text: Why was Callisto frozen solid before differentiation was complete?

tag: B
text: What allows the process of differentiation to get started?

tag: C
text: Why is it easier for an icy body to differentiate than a rocky one?

tag: D
text: How do astronomers know that Callisto lacks a dense core?
answer_key: A
explanation: 原文 Only a little heating will soften the ice and get the process started, allowing the rock and metal to sink to the center and the slushy ice to float to the surface.对应 B 选项。It is much easier for an icy body to differentiate than for a rocky one, since the melting temperature of ice is so low.对应 C 选项。Astronomers can tell that it lacks a dense core from the details of its gravitational pull on the Galileo spacecraft during several very close flybys. 对应 D 选项。所以选择 A 选项。

question_stem: 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.
choices: tag: A
text: It should not be assumed that surface ice has the same characteristics wherever it is found

tag: B
text: Surface ice and rock are much more similar in temperature in the outer solar system than they are on Earth.

tag: C
text: The further away surface ice is from the Sun, the more its temperature differs from that of the warmer ice on Earth

tag: D
text: In the cold of the outer solar system, surface ice is so hard it behaves more like rock than like the warmer ice on Earth
answer_key: D
explanation: highlighted 句子主干在后半部分，at the temperatures of the outer solar system, ice on the surface is nearly as hard as rock, and behaves similarly.注意之间的比较关系，参考前半句话，much warmer ice we know on Earth. 所以只有 D 选项符合句意。

question_stem: According to paragraph 3, how is Callisto different from all other planet-sized objects in the solar system?
choices: tag: A
text: It can form and retain impact craters on its icy surface

tag: B
text: It has ice glaciers that do not flow or deform.

tag: C
text: It has never had the interior forces required for geological evolution.

tag: D
text: It is more than four billion years old.
answer_key: C
explanation: 根据 planet-sized objects 定位至原文 its absence of interior forces to drive geological evolution 可知，木卫星缺少内部动力来引发地质演化，其中 its absence 对应 C 选项的 It has never had, 所以 C 选项正确。

question_stem: The word “signature” in the passage is closest in meaning to
choices: tag: A
text: primary cause

tag: B
text: end result

tag: C
text: identifying mark

tag: D
text: by-product
answer_key: C
explanation: signature, 签名，信号，标志。
Primary cause, 主因。End result, 最终结果。Identifying mark, 识别标记。By-product, 副产品。所以 C 选项正确。

question_stem: According to paragraph 4, each of the following provides evidence about Ganymede's interior EXCEPT
choices: tag: A
text: Ganymede's large size

tag: B
text: measurements of Ganymede's gravity field

tag: C
text: the fact that Ganymede has a magnetic field

tag: D
text: the fact that Ganymede continues to be geologically active
answer_key: A
explanation: 原文 Measurements of its gravity field tell us that the rock and metal sank to form a core about the size of our Moon,对应 B 选项。the Galileo spacecraft discovered that Ganymede has a magnetic field, 对应 C 选项。but rather a place of continuing geological activity powered by an internal heat source, 对应 D 选项。所以选择 A 选项。

question_stem: The word “Extensive” in the passage is closet in meaning to
choices: tag: A
text: unusual

tag: B
text: large

tag: C
text: new

tag: D
text: steep
answer_key: B
explanation: extensive, 大量的，广阔的
Unusual, 与众不同的。Large, 大的。New, 新的。Steep, 陡峭的。所以 B 选项正确。

question_stem: Paragraph 5 supports each of the following statements about Ganymede EXCEPT
choices: tag: A
text: All water on it has always been frozen solid.

tag: B
text: There is evidence that part of its crust once broke open

tag: C
text: Its crust has been subject to forces that have created mountains and valleys.

tag: D
text: Some of its older craters have been split apart by more recent geological activity
answer_key: A
explanation: 原文 Some features formed when the crust cracked 对应 B 选项，其中 crust cracked 对应 B 选项中的 crust once broke open.原文 Extensive mountain ranges were formed from compression of the crust, forming long ridges with parallel valleys spaced one to two kilometers apart.对应 C 选项。原文 In some places older impact craters were split and pulled apart.对应 D 选项。所以选择 A 选项。

question_stem: According to paragraph 6, the differences in how Callisto and Ganymede evoked are most probably due to differences in their
choices: tag: A
text: size and internal heating

tag: B
text: distance from Jupiter

tag: C
text: chemical and physical composition

tag: D
text: age
answer_key: B
explanation: 根据原文 Ganymede is close enough to Jupiter that tidal forces from the giant planet may have episodically heated its interior and triggered major convulsions on its crust.可知，Ganymede 距离木星更近，其中 Ganymede is close enough to Jupiter 对应 B 选项的 distance from Jupiter, 所以选项 B 正确。