【RC提问】有一个关键的句子读不懂-OG13第四篇（OG18第七篇））

LcR-

Io and Europa, the inner two of Jupiter's four largest moons, are about the size of Earth`s moon and are composed mostly or entirely of rock and metal. Ganymede and Callisto are larger and roughly half ice. Thus, these four moons are somewhat analogous to the planets of the solar system, in which the rock- and metal-rich inner planets are distinct from the much larger gas- and ice-rich outer planets. Jupiter's moons are, however, more "systematic" :many of their properties vary continuously with distance from Jupiter. For example, lo is ice-free, Europa has a surface shell of ice, and while Ganymede and Callisto are both ice-rich, outermost Callisto has more.

This compositional gradient has geological parallels. lo is extremely geologically active, Europa seems to be active on a more modest scale, and Ganymede has undergone bouts of activity in its geological past. Only Callisto reveals no geological activity. In similar fashion, Callisto's surface is very heavily cratered from the impact of comets and asteroids; Ganymede, like Earth`s moon, is heavily cratered in parts; Europa is very lightly cratered; and no craters have been detected on lo, even though Jupiter's gravity attracts comets and asteroids passing near it, substantially increasing the bombardment rate of the inner moons compared to that of the outer ones. But because of lo's high degree of geological activity, its surface undergoes more-or-less continuous volcanic resurfacing.

请教一下各位！这篇文章题目都会做，就是第二段主题句This compositional gradient has geological parallels.不是很懂。如果大家没时间看文章我大概说一下内容，第一段描述了4个卫星并且提出它与太阳系有相似性，而且它们比太阳系更加“systematic”，然后举了一些systematic的例子，这些例子体现了它们的compositional gradient。
OG说第二段的内容是further证明systematic的，我的问题是：第二段的主题句里面的parallel是取什么意思？是平行还是相似处？从这句话怎么看出第二段是要继续讲systematic的？

谢谢！

梦季

看一下！               

LcR-

practidealist 发表于 2018-5-19 21:09
个人愚见，这不是一个主题句，而是承接句，compositional gradient 指的是上文说的那一段， geological说的 ...

谢谢！！理解多了！

practidealist

个人愚见，这不是一个主题句，而是承接句，compositional gradient 指的是上文说的那一段， geological说的是下文的地理活动，理解起来就是：地理活动和成分组成一样也成梯度分布。然后下文就开始介绍地里活动的梯度现象。希望有帮助