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 是土星已知衛(wèi)星中距土星第十二近的一顆：
公轉(zhuǎn)軌道：距土星377,400 千米
衛(wèi)星直徑：1120 千米
質(zhì)量：1.05e21 千克
在希臘神話中，Dione與宙斯（木星）生下了阿佛洛狄特（金星）。（阿佛洛狄特即維納斯是克洛諾斯把他的父親烏拉諾斯的肢體投入大海中時(shí)從泡沫中誕生出來(lái)的。摘自《羅馬神話故事》，有出入。－－譯注）
它是在1684年由卡西尼發(fā)現(xiàn)的。
Dione是土星衛(wèi)星中密度者(另外，土衛(wèi)六由于地心引力對(duì)衛(wèi)星的壓縮，使它的密度不斷增大)。它主要由冰水混合物組成，但可能由有待考慮的更質(zhì)密的硅酸鹽石組成。
盡管它有一點(diǎn)兒小，但Dione還是很類似于土衛(wèi)五。它們都有相近的物質(zhì)組成、反照率和多種地形。它們自轉(zhuǎn)同步，但朝自轉(zhuǎn)方向的半球卻不相同。
在背朝自轉(zhuǎn)方向的半球（逆半球）上，有一個(gè)在暗背景下的由亮條紋組成的網(wǎng)狀物和一些明顯的隕石坑，這些條紋覆蓋在隕石坑上，表明它們比較新。
朝自轉(zhuǎn)方向的半球（順半球）上有著很深的隕石坑并且有同樣的光亮反射。像木衛(wèi)四一樣，這些隕石坑缺少在月球和水星上可看到的高度起伏的表面。
以下是一些解釋：Dione形成后不久是十分活躍的，一些變化過(guò)程（冰火山現(xiàn)象）使Dione的大部分換了新顏，順半球保留了條紋的地形，也可能在整個(gè)地表都有。不久之后，當(dāng)內(nèi)部活動(dòng)和和地表變化結(jié)束后，發(fā)生了一系列很不強(qiáng)烈的撞擊。留下的隕石坑小到在旅行者上的照片都看不到，這主要集中于順半球，并除去了條紋地形，然而卻完整地保留在了逆半球。
Helene（土衛(wèi)十二）的軌道是在Dione的拉格朗日前點(diǎn)上。
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 是土星已知衛(wèi)星中距土星第十二近的一顆：
公轉(zhuǎn)軌道：距土星377,400 千米
衛(wèi)星直徑：1120 千米
質(zhì)量：1.05e21 千克
在希臘神話中，Dione與宙斯（木星）生下了阿佛洛狄特（金星）。（阿佛洛狄特即維納斯是克洛諾斯把他的父親烏拉諾斯的肢體投入大海中時(shí)從泡沫中誕生出來(lái)的。摘自《羅馬神話故事》，有出入。－－譯注）
它是在1684年由卡西尼發(fā)現(xiàn)的。
Dione是土星衛(wèi)星中密度者(另外，土衛(wèi)六由于地心引力對(duì)衛(wèi)星的壓縮，使它的密度不斷增大)。它主要由冰水混合物組成，但可能由有待考慮的更質(zhì)密的硅酸鹽石組成。
盡管它有一點(diǎn)兒小，但Dione還是很類似于土衛(wèi)五。它們都有相近的物質(zhì)組成、反照率和多種地形。它們自轉(zhuǎn)同步，但朝自轉(zhuǎn)方向的半球卻不相同。
在背朝自轉(zhuǎn)方向的半球（逆半球）上，有一個(gè)在暗背景下的由亮條紋組成的網(wǎng)狀物和一些明顯的隕石坑，這些條紋覆蓋在隕石坑上，表明它們比較新。
朝自轉(zhuǎn)方向的半球（順半球）上有著很深的隕石坑并且有同樣的光亮反射。像木衛(wèi)四一樣，這些隕石坑缺少在月球和水星上可看到的高度起伏的表面。
以下是一些解釋：Dione形成后不久是十分活躍的，一些變化過(guò)程（冰火山現(xiàn)象）使Dione的大部分換了新顏，順半球保留了條紋的地形，也可能在整個(gè)地表都有。不久之后，當(dāng)內(nèi)部活動(dòng)和和地表變化結(jié)束后，發(fā)生了一系列很不強(qiáng)烈的撞擊。留下的隕石坑小到在旅行者上的照片都看不到，這主要集中于順半球，并除去了條紋地形，然而卻完整地保留在了逆半球。
Helene（土衛(wèi)十二）的軌道是在Dione的拉格朗日前點(diǎn)上。