【词汇】托福同义词串串烧(111)- Callisto and Ganymede

TPO-42-3 Callisto and Ganymede

From 1996 to 1999, the Galileo spacecraft passed through the Jovian system, providing much information about Jupiter’s satellites. Callisto, the outermost of Jupiter’s four largest satellites, orbits the planet in seventeen days at a distance from Jupiter of two million kilometers. Like our own Moon, Callisto rotates in the same period as it revolves, so it always keeps the same face toward Jupiter. Its noontime surface temperature is only about -140°C, so water ice is stable on its surface year-round. Callisto has a diameter of 4,820 kilometers, almost the same as that of Mercury. Its mass is only one-third as great, which means its density must be only one third as great as well. 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.

Callisto has not fully differentiated, meaning separated into layers of different density materials. 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. This fact surprised scientists, who expected that all the big icy moons would be differentiated. lt is much easier for an icy body to differentiate than for a rocky one, since the melting temperature of ice is so low. 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. Yet Callisto seems to have frozen solid before the process of differentiation was complete.

Like our Moon’s highlands, the surface of Callisto is covered with impact craters. The survival of these craters tells us that an icy object can form and retain impact craters in its surface. In thinking of ice so far from the Sun, it is important not to judge its behavior from that of the much warmer ice we know on Earth; at the temperatures of the outer solar system, ice on the surface is nearly as hard as rock, and behaves similarly. Ice on Callisto does not deform or flow like ice in glaciers on Earth. Callisto is unique among the planet-sized objects of the solar system in its absence of interior forces to drive geological evolution. The satellite was born dead and has remained geologically dead for more than four billion years.

Ganymede, another of Jupiter’s satellites and the largest in our solar system, 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 the sparse covering of impact craters as well as the relative freshness of the craters. Ganymede is a differentiated world, like the terrestrial planets. Measurements of its gravity field tell us that the rock and metal sank to form a core about the size of our Moon, with a mantle and crust of ice floating above it. In addition, the Galileo spacecraft discovered that Ganymede has a magnetic held, the signature of a partially molten interior. Ganymede is not a dead world, but rather a place of continuing geological activity powered by an internal heat source. Much of its surface may be as young as half a billion years.

The younger terrain is the result of tectonic and volcanic forces. Some features formed when the crust cracked, flooding many of the craters with water from the interior. Extensive mountain ranges were formed from compression of the crust, forming long ridges with parallel valleys spaced one to two kilometers apart. In some places older impact craters were split and pulled apart. There are even indications of large-scale crustal movements that are similar to the plate tectonics of Earth.

Why is Ganymede different from Callisto? Possibly the small difference in size and internal heating between the two led to this divergence in their evolution. But more likely the gravity of Jupiter is to blame for Ganymede’s continuing geological activity. 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.


题目

【题目】1. The word “signature” in the passage is closest in meaning to

○primary cause

○end result

○identifying mark

○by-product

【题目】2. The word “Extensive” in the passage is closest in meaning to

○Unusual

○Large

○New

○Steep


单词解释和答案

1、signature

[sig·na·ture || ‘sɪgnɪtʃə(r)]

n.  签名, 签字, 签署

【同义词】:  key signature / signature tune / theme song / touch

【原句】: In addition, the Galileo spacecraft discovered that Ganymede has a magnetic held, the signature of a partially molten interior.

【题目】1. The word “signature” in the passage is closest in meaning to

○primary cause

○end result

○identifying mark★

○by-product


2、Extensive

[ex·ten·sive || ɪk’stensɪv]

adj.  广的, 多方面的, 广泛的

【反义词】:  intensive

【原句】: Extensive mountain ranges were formed from compression of the crust, forming long ridges with parallel valleys spaced one to two kilometers apart.

【题目】2. The word “Extensive” in the passage is closest in meaning to

○Unusual

○Large★

○New

○Steep


参考译文

木卫四和木卫三

从1996到1999,伽利略号飞船通过木星系统,提供很多关于木星的卫星信息。木卫四,木星的四大卫星之一,它轨道在最外层,在距离木星二百万公里处,以17天的周期,环绕着行星运行。像我们自己的月球,木卫四在绕行的同时自身也在旋转所以它总是用同一面朝向木星。它午时的表面温度大约只有零下140°C,所以水冰稳定常年稳定存在于它的表面。木卫四直径有4,820公里,和水星的直径差不多。它的质量仅为三分之一,这意味着它的密度必须只有三分之一。这告诉我们,在内行星中,木卫四岩石的金属的物质的含量远远小于在内行星上发现的,他必定有着冰态主体,覆盖大部分的内体。

木卫四尚未完全分化,意味着它被分成不同密度的材料层。从伽利略号太空船几次非常近距离飞越中获得的引力的细节,天文学家可以推断出它缺乏一个致密核心。这一事实使科学家们感到惊讶,他们认为所有的冰体大卫星都会有区别。冰状的(行星)要比岩石状的更容易分化,因为冰的融化温度是如此之低。只有一个小小的加热就会使冰软化,并开始分化的过程一使岩石和金属下沉到中心,泛起泥泞的冰浮到表层。然而,木卫四似乎冻结在分化过程完成前就已经冻结实了。

就像我们的月球高地,木卫四的表面布满了陨石坑。这些陨石坑的存在告诉我们,一个冰冷的物体可以形成并保留其表面的撞击坑。在考虑到远离太阳的冰时,重要的并不是通过我们熟知的地球上的暖冰作来判断它的行为;在外部太阳系的温度下,表面上的冰几乎像岩石一样坚硬,并且表现相似。木卫四的冰会不变形,也不会像地球上的冰川冰那样流动。在太阳系卫星大小的物体中,木卫四是独一无二的,因为其缺失力推动地质演化的内部力量。卫星在出生时就死了,并且保持地质学死亡超过四十亿年。

木卫三是木星的另一个卫星,且他在太阳系中是最大的,它也是坑坑洼洼的,但比木卫四要少一些。他表面约1/4的面积有着大量的老坑洞;其余的是在最近形成的,我们可以通过撞击坑的稀疏覆盖以及相对新的陨石坑来判断。木卫三是一个差异化的世界,像类地行星。对它重力场的测量告诉我们,岩石和金属下沉形成了一个像我们月球一样大的核心,地幔和地売的冰漂浮在上面。此外,伽利略号飞船发现木卫三有磁场,是内部部分熔化的鲜明特征。它不是一个死的世界,而是一个由由内部热源驱动,有着持续地质活动的地方。其表面的大部分可能只有五亿年的历史。

年轻的地形是构造和火山力量的产物。一些特征是在地売破裂时形成的,用内部的水将许多陨石坑淹没。广泛的山脉是由地売挤压形成的,形成了长长的山脊,有平行的山谷,间隔1到2千米的距离。在一些地方,旧的撞击坑被分开,并被分开。甚至有迹象表明,有类似于地球板块构造的大型地売运动发生。

为什么木卫四和木卫三不同呢?可能是大小上的小差异以及他们内热上的差别是他们在进化上产生差异。但这个更有可能是木星引力使得木卫三有着持续的地质活动。木卫三足够接近木星,木星的潮汐力可能会间歇得加热木卫三的内部,并可能在其地壳上触犯振动。

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