Dark Terrain on Saturn's Iapetus(土卫八上的黑暗地带)

夜空

[center]Astronomy Picture of the Day
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2006 January 3


http://antwrp.gsfc.nasa.gov/apod ... us3_cassini_big.jpg

Dark Terrain on Saturn's Iapetus
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA
Explanation: Why are vast sections of Iapetus as dark as coal? No one knows for sure. Iapetus, the third largest moon of Saturn, was inspected again as the Saturn-orbiting robot Cassini spacecraft swooped past the enigmatic world again late last year. The dark material covers most of the surface visible in the above image, while the small portion near the top that appears almost white is of a color and reflectance more typical of Saturn's other moons. The unknown material covers about half of the 1,500 kilometer wide moon. The material is so dark that it reflects less than five percent of incident sunlight, yet overlays craters indicating that it was spread after the craters were formed. Iapetus has other unexplained features. The bright part of Iapetus is covered with unexplained long thin streaks. The orbit of Iapetus is also unusual, being tilted to the plane of Saturn's orbit by an unusually high fifteen degrees. A strange ridge about 13 kilometers high crosses much of Iapetus near the equator and is visible near the bottom. Oddly, this ridge is almost exactly parallel with Iapetus' equator. The exact shape of Iapetus remains undetermined, but images indicate that it is quite strange -- something like a walnut. Research into the formation and history of mysterious Iapetus is active and ongoing.

土卫八上的黑暗地带

版权：Cassini Imaging Team, SSI, JPL, ESA, NASA





说明：为什么土卫八上的广阔区域像煤球一样黑呢？没人知道确切答案。去年底，绕着土星运行的卡西尼号飞船再次经过这个谜一般的世界，又一次检查了土卫八Iapetus---土星的第三大卫星。从上图中可以看到，黑色的物质覆盖了土卫八表面的大部分，而顶部附近有一小块几乎白色的部分，其颜色和反射率则是土星其他卫星中比较典型的。不明物质大约覆盖了这个直径为1500千米卫星的整个半球，这种物质非常暗，只能反射不到5%的入射阳光，而遍布其上的陨石坑则表明这种物质在陨石坑形成以后蔓延开来。土卫八还有一些别的现在无法解释的特征。其表面明亮的部分覆盖着长长的窄条纹。土卫八的轨道也不寻常，与土星的轨道面倾角高达15度。一道约13千米高的奇异的山脊大概沿着赤道扫过土卫八表面的大部分，更奇怪的是，这道山脊几乎与土卫八的赤道平行，可以在图的底部看到。土卫八的确切形状还不清楚，不过图像显示它的形状十分奇怪——有点像一个胡桃。关于神秘的土卫八的结构和形成史的研究正在如火如荼的进行。（翻译：刘媛媛）

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[ Last edited by 夜空 on 2006-1-4 at 03:33 PM ]

夜空

迷一样的卫星，明暗非常分明，给人感觉好像只有半边

跨越地平线

这是一颗充满疑问的卫星，等卡西尼号绘制完土卫八的地图的时候，我们就可以进一步了解它的阴阳脸了

b777200

阴阳脸的星球！

夜猫子

土卫八  （Iapetus） 是土星已知卫星中距土星第十七近、第三大的一颗：

公转轨道: 距土星 3,561,300 千米
卫星直径: 1460 千米
质量: 1.88e21 千克

    它由卡西尼于1671年发现。

    密度仅为1.1，土卫八的大部分肯定是由冰水组成的。

    正对公转和反对公转的半球完全不同。正转半球的反照率在0.03到0.05之间，与煤烟一样暗，反面为1.5，几乎同木卫四一样亮。这些不同点太明显，使得卡西尼注意到了，说出了“我只能见到土卫八的一面，另一面则不能。”的话。