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Dione 土卫四(中英)

2006-02-21 00:00

  Dione is the twelfth of Saturn's known satellites:

  orbit: 377,400 km from Saturn

  diameter: 1120 km

  mass: 1.05e21 kg

  In Greek mythology Dione was the mother of Aphrodite (Venus) by Zeus (Jupiter).

  Discovered by Cassini in 1684.

  Dione is the densest of Saturn's moons (aside from Titan, whose density is increased by gravitational compression). It is composed primarily of water ice but must have a considerable fraction of denser material like silicate rock.

  Though somewhat smaller, Dione is otherwise very similar to Rhea. They both have similar compositions, albedo features and varied terrain. Both rotate synchronously and have dissimilar leading and trailing hemispheres.

  On the trailing hemisphere there is a network of bright streaks on a dark background and few visible craters. The streaks overlay the craters, indicating that they are newer.

  The leading hemisphere is heavily cratered and uniformly bright. Like Callisto, the craters lack the high relief features seen on the Moon and Mercury.

  This was interpreted as follows: shortly after its formation Dione was active. Some processes (ice volcanism?) resurfaced much of Dione leaving the pattern of streaks, probably on the whole surface. Later, after the internal activity and resurfacing ceased, a much less intense series of impacts (which left craters too small to be seen in Voyager's images) occurred. This was concentrated on the leading hemisphere and wiped out the streak patterns but left them intact on the trailing hemisphere.

  And as is often the case in science and to the delight of all involved, newer data shows that the previous hypothesis was wrong. Higher resolution images from Cassini clearly show that the streaks are not ice flows but rather a complex network tectonic fractures that are younger than most of the craters.

  Helene orbits in Dione's leading Lagrange point. The tiny moon Polydeuces (S/2004 S5), discovered by Cassini in 2004, occupies the trailing Lagrange point.

  Dione 是土星已知卫星中距土星第十二近的一颗:

  公转轨道:距土星377,400 千米

  卫星直径:1120 千米

  质量:1.05e21 千克

  在希腊神话中,Dione与宙斯(木星)生下了阿佛洛狄特(金星)。(阿佛洛狄特即维纳斯是克洛诺斯把他的父亲乌拉诺斯的肢体投入大海中时从泡沫中诞生出来的。摘自《罗马神话故事》,有出入。--译注)

  它是在1684年由卡西尼发现的。

  Dione是土星卫星中密度最大者(另外,土卫六由于地心引力对卫星的压缩,使它的密度不断增大)。它主要由冰水混合物组成,但可能由有待考虑的更质密的硅酸盐石组成。

  尽管它有一点儿小,但Dione还是很类似于土卫五。它们都有相近的物质组成、反照率和多种地形。它们自转同步,但朝自转方向的半球却不相同。

  在背朝自转方向的半球(逆半球)上,有一个在暗背景下的由亮条纹组成的网状物和一些明显的陨石坑,这些条纹覆盖在陨石坑上,表明它们比较新。

  朝自转方向的半球(顺半球)上有着很深的陨石坑并且有同样的光亮反射。像木卫四一样,这些陨石坑缺少在月球和水星上可看到的高度起伏的表面。

  以下是一些解释:Dione形成后不久是十分活跃的,一些变化过程(冰火山现象)使Dione的大部分换了新颜,顺半球保留了条纹的地形,也可能在整个地表都有。不久之后,当内部活动和和地表变化结束后,发生了一系列很不强烈的撞击。留下的陨石坑小到在旅行者上的照片都看不到,这主要集中于顺半球,并除去了条纹地形,然而却完整地保留在了逆半球。

  Helene(土卫十二)的轨道是在Dione的拉格朗日前点上。

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