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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. It 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 m 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 field, 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.
本文是托福tpo42阅读原文文本及翻译Part3：Callisto and Ganymede。
译文：木卫四和木卫三
从1996年到1999年，伽利略号宇宙飞船围绕木星飞行，提供了很多关于木星的卫星的信息。木卫四，木星的四个最大的卫星中最外层的一个，围绕木星转一圈需要17天，轨道半径200万公里。像地球的卫星月球一样，木卫四在同一周期公转和自转，所以它总是与木星保持固定的相对位置。它中午的表面温度大约只有零下140°C，所以表面常年都是冰。木卫四的直径为4820千米，几乎与水星(的直径)相同，但它的质量仅为水星的三分之一，这也意味着它的密度应该也只有水星的三分之一。这就告诉我们，木卫四内核的岩石金属材料比较少，内核的大部分应该是冰。
木卫四还没有完全分化，也就意味着还没分离成几层不同密度的材料。在几次近地的飞行中，通过伽利略号宇宙飞船的重力探测，天文学家可以知道木卫四缺乏一个高密度的内核。这使科学家们感到吃惊，因为他们认为所有大的冰卫星都会有分层现象。一个冰质天体要比一个岩石天体更容易分层，因为冰的融化温度很低。只要有一点热量，冰就开始软化，分化便开始了。这使得岩石和金属向中心下沉，融化的冰会浮到表面。然而木卫四似乎在分化过程完成之前就已经冻成固体了。
就像地球的卫星月球的高地一样，木卫四表面到处都是撞击坑。这些撞击坑的存在告诉我们，一个冰体可以形成并保留其表面的撞击坑。在考虑距离太阳如此之远的冰体时，不要根据我们在地球上所了解的温度更高的冰体来判断其行为是很重要的;在外太阳系的温度下，表面的冰几乎就像岩石一样坚硬，且性质也相似。木卫四上的冰不像地球上的冰川那样会变形或流动。作为太阳系一个行星大小的天体，木卫四是唯一一个无法依靠内部力量推动地质演变的星体。卫星一形成就是死的，它保持地质不变的时间超过了4亿年。
木卫三是木星的另一个卫星，也是太阳系中最大的卫星，其表面也是坑坑洼洼的，但不如木卫四那么多坑。木卫三有大约四分之一的表面似乎是很老了，布满了坑洞;其余的四分之三形成的时间更近一些，这一点我们可以从稀疏分布的撞击坑和相对较新的坑判断出来。像类地行星一样，木卫三已经分化了。通过测量它的引力场，我们可以知道下沉的岩石和金属形成了一个月球大小般的核，而冰形成地幔和地壳浮在其上面。此外，伽利略号宇宙飞船发现木卫三有一个磁场，意味着它的内部已经部分熔融了。木卫三并非一个死的星体，其内部的热能会持续地引发地质活动。它的表面大部分的年龄可能是五亿年左右。
较年轻的地形是地质和火山活动形成的。当地壳破裂时，一些地理特征便形成了，内部涌出的洪水淹没了大部分的地壳。巨大的山脉是由地壳挤压形成的，形成了长长的山脊和间隔一到两千米的平行山谷。在一些地方，更早的撞击坑被拉扯开来。甚至有迹象表明木卫三存在类似于板块构造的大型地壳运动。
为什么木卫三和木卫四有这么多不同?可能体积上和内部热量上的小差异导致了他们在进化中的差距。但更可能的原因是，木星的引力造成了木卫三持续的地质活动。木卫三离木星较近，这个巨行星的潮汐力偶尔会使得木卫三的内部温度升高，引发重大的地壳运动。
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