自然杂志文章：土星光环形成的秘密【英文】

Recipe for making Saturn’s rings

Simulations show that the still-mysterious origin of Saturn’s vast, icy rings could be explained by the ‘peeling’ by Saturn’s tides of the icy mantle of a large satellite migrating towards the planet. See Letter p.943

Since Christiaan Huygens realized in 1655 that the planet Saturn is encircled by a ring, this “jewel of the Solar System” has defied researchers’ best efforts to explain its origin. Saturn’s rings are made of centimetreto metre-sized boulders of almost pure water ice[sup]1[/sup] — a unique characteristic among Solar System bodies (comets and planetary satellites contain about 50% silicates and metals). The total mass of Saturn’s rings is thought to be equivalent to that of a satellite about 500 kilometres across [sup]2,3[/sup]. But how and when the rings formed, and why they are so clean of silicates, is not understood. Several mechanisms for their formation have been proposed [sup]4–6[/sup], but none has provided a convincing explanation for their observed peculiarities. On page 943 of this issue, Canup[sup]7[/sup] offers an attractive solution to the problem that answers several questions at once*.

Saturn’s rings are in a location that is dominated by tides. Generally, boulders in circular orbits, such as those making up the rings of Saturn, merge and grow to form larger bodies because of their own gravity — this process is thought to be the way in which planets and asteroids form around the Sun and satellites form around giant planets. But because Saturn’s rings are so close to the planet (below the planet’s Roche limit, which is 2.5 times the planetary radius), the tidal forces that the planet exerts on them prevent the boulders from accreting material. Just as the Moon’s tides stretch the oceans on Earth, Saturn’s tides stretch any boulder aggregates in the rings, and separate their constituents. Similarly, if a pre-existing icy body were to be placed within Saturn’s Roche limit, it would be destroyed by the planet’s tides.

These considerations suggest a simple recipe for making Saturn’s rings: take a large body and put it into a close orbit around the planet; tides should then destroy it, and the resulting fragments should form the rings. But what sort of body? A large, differentiated satellite — with a core composed of silicates and iron, and a lighter mantle made of water ice — would be a good choice. The rings could then be formed simply by ‘peeling off ’ the satellite’s icy mantle from the core using the planet’s tides as a knife [sup]4–6[/sup]. Although appealing, however, this recipe has never been investigated owing to computational limitations.

Enter Canup[sup]7[/sup], who describes the details of a numerical and analytical model for tidal splitting of a differentiated satellite around Saturn. She demonstrates that the planet’s tidal forces can indeed be strong enough to take water ice away from the satellite, but not to tear its dense silicate core. But how can the differentiated satellite be brought so close to the planet, within the Roche limit? When could this have occurred? And what mechanism can get rid of the silicate core that has not been broken by the tides? Canup solves these problems using a single phenomenon: planetary migration.

The planets of our Solar System formed 4.5 billion years ago in a disk of gas and dust that surrounded the young Sun for a few million years. At the same time, the satellites of the giant planets formed in gaseous and dusty disks around their hosts. But when a body orbits inside a gaseous disk, its orbit shrinks and the body spirals gradually towards the planet as a result of the gravitational interaction of the body with the surrounding gas. In the circum-Saturn disk, satellites grew and fell into the planet in this way, until the last generation formed and escaped inward migration because the disk had vanished[sup]8[/sup]. Canup suggests that the last migrating differentiated satellite, about the size of Titan (Saturn’s largest moon), had its icy mantle pulled to pieces by Saturn’s tides as it crossed the Roche limit. Beyond this limit, the satellite’s silicate core carried on migrating inwards and eventually disappeared into Saturn, leaving behind the ice boulders that make up the rings (Fig. 1). This process would have produced icy rings about 1,000 times more massive than the rings are today.

This elegant model[sup]7[/sup] could provide the missing links between a suite of observational and theoretical results that have changed our understanding of Saturn’s rings. Such massive rings would be less sensitive to the darkening effect of meteoroid bombardment [sup]3,9[/sup], thus explaining their brightness today. In addition, because of their mass they should spread more rapidly than the present ones, leading, over the age of the Solar System, to lighter rings like those seen today [sup]7,10[/sup]. During this spreading, the material expanding beyond the Roche limit may have given birth to satellites; such a mechanism has recently been proposed for the formation of Saturn’s small moons[sup]11[/sup]. Indeed, observations made with the Cassini spacecraft have shown that accretion processes are still active at the outer edge of Saturn’s rings [sup]12,13[/sup] and on the satellites Pan and Atlas[sup]14[/sup]. Canup suggests that the inner satellites of Saturn, up to and including Tethys, could also have formed by accretion of spreading ring material beyond the Roche limit.

Canup’s model[sup]7[/sup] offers, for the first time, a convincing starting point for a consistent theory of the origin of Saturn’s rings and satellites. It shows that the rings and satellites are intimately linked, and that Saturn’s system, despite being made of ice, is not frozen but is constantly evolving. The origin of the rings and satellites must be understood in the wider framework of models of planet formation, and this work is one step in that direction. One may question whether the specific conditions required for such rings to form have also been met around other Solar System planets and exoplanets. The details and the consequences of the formation of the satellites at the outer edge of the rings, and their outward migration to their present positions, are still to be explored. Establishing these details could change our understanding of Saturn’s satellites, and more generally of giant planets and their environments. ■  


Figure 1 | Canup’s model for the formation of Saturn’s icy rings. a, A differentiated satellite in the
gaseous circumplanetary disk around Saturn migrates towards the planet. When the satellite crosses the
planet’s Roche limit, its icy mantle starts to be pulled into pieces by the planet’s tidal forces. b, The silicate
core carries on migrating towards Saturn and eventually falls into it, leaving behind the ice boulders that
give birth to the rings and icy satellites of the planet.

————————————————————————————————————————
*This article and the paper under discussion7 were published online on 12 December 2010.

1. nicholson, P. D. et al. Icarus 193, 182–212(2008).
2. Esposito, L. W., O’callaghan, M. & West R. a. Icarus 56, 439–452 (1983).
3. Robbins, s. J., stewart, G. R., Lewis, M. c., colwell, J. E. & sremčević, M. Icarus 206, 431–445 (2010).
4. harris, a. in Planetary Rings (eds Greenberg, R. & Brahic, a.) 641–659 (univ. arizona Press, 1984).
5. Dones, L. Icarus 92, 194–203 (1991).
6. charnoz, s., Morbidelli, a., Dones, L. & salmon, J. Icarus 199, 413–428 (2009).
7. canup, R. M. Nature 468, 943–946 (2010).
8. canup, R. M. & Ward, W. R. Astron. J. 124, 3404–3423 (2002).
9. cuzzi, J. n. & Estrada, P. R. Icarus 132, 1–35 (1998).

http://www.nature.com/nature/journal/v468/n7326/full/nature09738.html

感兴趣的可以翻译此文，悬赏300MFB！（版主除外）

我抛砖引玉，先翻译个标题：《土星光环的制作方法》。
拿1点牧夫币走人～

回复 3# bearcat

小心我把这1分给扣回去

回复 4# positron



（非表情贴，只是提前把被扣的一分赚回来～）

土星环在一个地区充斥着潮汐。一般来说,巨石圆形的轨道,如在土星环、合并和生长形成更大的组织,因为他们的自己的引力, 这个过程被认为是行星的方法及小行星形成周围太阳和卫星形成周围巨行星。但由于土星环是如此接近地球(下这个行星的罗氏极限,这是2.5倍的行星范围), 潮汐力,地球在一条直线上从大石块,防范accreting材料。就像月亮的潮汐弹性海洋在地球上,土星的潮汐弹性任何巨石在星环聚集,分裂他们的选民。同样,如果一个已经存在的结冰的身体被放置在土星罗氏限制,它会被这个行星的潮汐。

这些考虑建议一个简单的配方使土星环:找一个大身体,然后放进密切绕地球,潮将摧毁它,产生的碎片应该形成了奥运五环标志。但是何种的身体吗?一个大的分化卫星——有一个核心由硅酸盐和铁,更轻的地幔的水冰)将是一个不错的选择。戒指可以然后形成的简单的剥卫星的冰雪覆盖从核心使用这个行星的潮汐作为刀4 - 6。虽然吸引人的,然而,这配方从未被调查由于计算的局限性。

进入,她说Canup7的细节分析模型与数值潮汐的分裂分化的卫星绕土星。她表明这个行星的潮汐力的确可以足够强大的力量可以将冰水离开这颗卫星,但不要撕裂以致密堆积硅酸盐核心。但是怎样才能的差异性卫星是带来如此接近地球在罗氏限制吗?当这可能发生的?和什么机构能够克服硅酸盐中心,至今未被打破受到潮汐的吗?解决这些问题Canup使用单个的现象:行星的迁移。

我们太阳系的行星形成了45亿年前在磁盘的气体和尘埃围在年轻的太阳,而太阳原几百万年了。与此同时,卫星形成了以气态巨行星盘和灰尘在他们的主人。但当身体内轨道,其运动轨迹气体盘萎缩,身体逐渐朝着地球螺旋由于重力的相互作用的身体与周围的气体。在circum-Saturn磁盘,卫星长大了、落在行星这样,直到最后一代形成和逃脱了内在的迁移因为磁盘有vanished8。Canup表明,最后迁移分化卫星泰坦大小的土星最大卫星),可能是它的结冰的地幔撕成碎片了土星的潮汐跨入这种罗氏限制。超过这一极限,卫星的硅酸盐核心进行迁移的脏腑与最终消亡了到土星,剩下的是一些冰组成环巨石发生(图1)。这个过程会产生结冰的戒指约1000倍还要大安环子的今天。

这个漂亮model7可以提供失踪的之间联系的一套观测和理论结果已经改变,土星的理解。这些大规模的戒指会较不敏感的影响这才是重点叙述流星轰炸三个、九个,从而解释它们的亮度今天。此外,由于它们的质量要比当前传播的更快,领先、在太阳系的年龄,就像那些更轻松的环今天看到的7,10。在这传播、材料扩张超越这种罗氏限额就生了卫星,这样的机制提出了最近的形成moons11土星的小。事实上,观察和卡西尼号飞船拍摄表明吸积过程仍然是活跃的土星环外围、12、13 Atlas14油底壳和卫星。Canup表明,内在的土星卫星,直到包括特提斯域油气形成的,可能会传播累积的戒指资料超出了罗氏限制。

Canup 的model7第一次提供一个令人信服的起点理论一致的起源的土星环和卫星。结果表明,戒指和卫星间具有紧密的关联,土星的系统,尽管是冰做的,而不是冻结,但在不断变化。戒指和起源的人造卫星必须明白的,在更广泛的框架模型,而且这项工作行星只是第一步,在这个方向努力。你可以具体情况是否需要这样的戒指也会形成其他太阳系的行星和系外行星。的详细信息,结果形成的外缘的卫星的环内,他们外在的迁移到现在的位置,仍然可以进行了探讨。建立这些细节可能会改变我们的理解的土星卫星,而更通常的巨型行星及其环境。

回复 6# agitoyxn


    请勿机器翻译！再次发机器翻译将扣分！

等。。。。。。。。。。。。。

机器翻译真是乱七八糟

这么有趣的文章怎么没人翻译? 我来翻译两段,剩下的大家补充.

题目就用熊猫的翻译了
《土星光环的制作方法》

Simulations show that the still-mysterious origin of Saturn’s vast, icy rings could be explained by the ‘peeling’ by Saturn’s tides of the icy mantle of a large satellite migrating towards the planet. See Letter p.943

计算模拟表明土星的让人琢磨不透的巨大的冰质光环的起源可能是因为土星对一个逐渐靠近的大卫星的冰质幔层的潮汐剥离作用.

Since Christiaan Huygens realized in 1655 that the planet Saturn is encircled by a ring, this “jewel of the Solar System” has defied researchers’ best efforts to explain its origin. Saturn’s rings are made of centimetreto metre-sized boulders of almost pure water ice1 — a unique characteristic among Solar System bodies (comets and planetary satellites contain about 50% silicates and metals). The total mass of Saturn’s rings is thought to be equivalent to that of a satellite about 500 kilometres across 2,3. But how and when the rings formed, and why they are so clean of silicates, is not understood. Several mechanisms for their formation have been proposed 4–6, but none has provided a convincing explanation for their observed peculiarities. On page 943 of this issue, Canup7 offers an attractive solution to the problem that answers several questions at once*.

自从惠更斯1655年发现土星被环围绕后, 这个"太阳系的璀灿明珠"就激起了无数研究者投入他们的才智去探求它的起源. 土星光环是由厘米到米级的几乎是纯(水)冰的漂砾组成. 这个特征在太阳系里是独一无二的(彗星,行星的卫星都含有约50%的硅化物和金属). 土星环系的总质量大概等于一个直径500公里的卫星的质量.  但这些环是什么时候如何形成的, 为什么硅化物的含量如此少, 这些问题目前还没有答案. 过去提出过几种机制来解释它们的形成,但没有一个能对光环的独特性质给出令人信服的解释. 在这期的943页, Canup对该问题提出了一个很有吸引力的答案, 能一下子解释(上面提到的)很多困惑.

不是300MFB么 怎么才6啊！

嗯……我回头试试看能不能翻出来

一个午休的时间，翻译的不怎么样，但大意应该是没错的。
Recipe for making Saturn’s rings
Simulations show that the still-mysterious origin of Saturn’s vast, icy rings could be explained by the ‘peeling’ by Saturn’s tides of the icy mantle of a large satellite migrating towards the planet. See Letter p.943
模拟的结果表明，土星潮汐力剥离巨型卫星表层冰面并引向主星的理论是可以解释土星巨大的冰质光环的神秘来源。
Since Christiaan Huygens realized in 1655 that the planet Saturn is encircled by a ring, this “jewel of the Solar System” has defied researchers’ best efforts to explain its origin. Saturn’s rings are made of centimetreto metre-sized boulders of almost pure water ice— a unique characteristic among Solar System bodies (comets and planetary satellites contain about 50% silicates and metals). The total mass of Saturn’s rings is thought to be equivalent to that of a satellite about 500 kilometres across. But how and when the rings formed, and why they are so clean of silicates, is not understood. Several mechanisms for their formation have been proposed, but none has provided a convincing explanation for their observed peculiarities. On page 943 of this issue, Canup offers an attractive solution to the problem that answers several questions at once*.
自1655年Christiaan Huygens确认土星周边环绕着一个光环后，这个“太阳系的宝石”就吸引了众多研究者致力于光环起源的研究。土星光环是由尺度在厘米至米的几乎是纯水的冰质漂砾构成。而这是太阳系众多天体中独一无二的（彗星和行星卫星包含50%左右的硅酸盐和金属）。光环的总质量一般认为与500km尺度卫星相当。但光环是何时，如何产生的，为何不含硅酸盐，我们不得而知。目前有多种机理被提出，但无一能有力的解释这些。在这期的943页, Canup对该问题提出了一个很有吸引力的答案, 能一下子解释(上面提到的)很多困惑。
Saturn’s rings are in a location that is dominated by tides. Generally, boulders in circular orbits, such as those making up the rings of Saturn, merge and grow to form larger bodies because of their own gravity — this process is thought to be the way in which planets and asteroids form around the Sun and satellites form around giant planets. But because Saturn’s rings are so close to the planet (below the planet’s Roche limit, which is 2.5 times the planetary radius), the tidal forces that the planet exerts on them prevent the boulders from accreting material. Just as the Moon’s tides stretch the oceans on Earth, Saturn’s tides stretch any boulder aggregates in the rings, and separate their constituents. Similarly, if a pre-existing icy body were to be placed within Saturn’s Roche limit, it would be destroyed by the planet’s tides.
土星光环处在潮汐力控制区。通常，圆形轨道上的漂砾，比如构成光环的这些，会因引力逐渐合并生长。这样的过程也是太阳所属行星和巨行星所属卫星的形成过程。但土星光环太靠近主星（低于洛希极限，也即2.5倍行星半径），潮汐力将阻止漂砾的聚合。就像月球的潮汐力拉扯海洋一样，土星的潮汐力会拉伸并搅碎环中任何已聚集的漂砾。同样的，如果在土星洛希极限内存在一个冰质天体，它必然的会被潮汐力摧毁。
These considerations suggest a simple recipe for making Saturn’s rings: take a large body and put it into a close orbit around the planet; tides should then destroy it, and the resulting fragments should form the rings. But what sort of body? A large, differentiated satellite — with a core composed of silicates and iron, and a lighter mantle made of water ice — would be a good choice. The rings could then be formed simply by ‘peeling off ’ the satellite’s icy mantle from the core using the planet’s tides as a knife 4–6. Although appealing, however, this recipe has never been investigated owing to computational limitations.
这些考虑暗示了一个简单的土星光环制作方法：俘获一个大的天体并将其送入近土轨道；潮汐力将其摧毁，碎片随即就会形成光环。那么可能的天体有哪些？拥有硅酸盐金属核心与冰质幔层的大量已分化小行星是最佳选择。那么，光环可以通过像刀一样的潮汐力将小行星的冰质幔层剥离破碎形成。虽然这样的解释很吸引人，但因计算能力的缺乏，它还未被验证。
Enter Canup, who describes the details of a numerical and analytical model for tidal splitting of a differentiated satellite around Saturn. She demonstrates that the planet’s tidal forces can indeed be strong enough to take water ice away from the satellite, but not to tear its dense silicate core. But how can the differentiated satellite be brought so close to the planet, within the Roche limit? When could this have occurred? And what mechanism can get rid of the silicate core that has not been broken by the tides? Canup solves these problems using a single phenomenon: planetary migration.
Enter Canup，在数值与解析模型中详细的描述了潮汐力快速变化对环土星卫星的作用。她证明了土星的潮汐力强度能够满足将冰质幔层剥离而不伤害到硅酸盐核心的条件。但这些已分化的卫星是怎么进入土星洛希极限内的？何时会发生？硅酸盐核心又是在何种机理作用下不被潮汐力摧毁？Canup只用了一个简单现象来解释，那就是行星运动。
The planets of our Solar System formed 4.5 billion years ago in a disk of gas and dust that surrounded the young Sun for a few million years. At the same time, the satellites of the giant planets formed in gaseous and dusty disks around their hosts. But when a body orbits inside a gaseous disk, its orbit shrinks and the body spirals gradually towards the planet as a result of the gravitational interaction of the body with the surrounding gas. In the circum-Saturn disk, satellites grew and fell into the planet in this way, until the last generation formed and escaped inward migration because the disk had vanished8. Canup suggests that the last migrating differentiated satellite, about the size of Titan (Saturn’s largest moon), had its icy mantle pulled to pieces by Saturn’s tides as it crossed the Roche limit. Beyond this limit, the satellite’s silicate core carried on migrating inwards and eventually disappeared into Saturn, leaving behind the ice boulders that make up the rings. This process would have produced icy rings about 1,000 times more massive than the rings are today.
太阳系的行星形成于45亿年前环绕太阳的星云盘中，并围绕太阳运转的数百万年。与此同时，巨行星的卫星也逐渐的在主星周边的气态尘埃云中形成。当一个天体的轨道处在尘埃云内时，由于与周边物质的引力作用，它的轨道将逐渐收缩，以螺旋渐进的方式接近主星。在环绕土星的尘埃圆盘中，卫星以同样的方式生长，然后坠入主星。该过程直到尘埃圆盘消失殆尽，最后一代卫星形成并摆脱这样的命运。Canup认为最后分化出来的尺度与泰坦相当的卫星在掠过洛希极限时被潮汐力剥离了冰质幔层，形成大量的碎片从而构成了光环。而硅酸盐核心在越过极限后，继续漂移直至坠落消失在土星上。这一过程将形成一个1000倍于现在的光环。
This elegant model could provide the missing links between a suite of observational and theoretical results that have changed our understanding of Saturn’s rings. Such massive rings would be less sensitive to the darkening effect of meteoroid bombardment，thus explaining their brightness today. In addition, because of their mass they should spread more rapidly than the present ones, leading, over the age of the Solar System, to lighter rings like those seen today. During this spreading, the material expanding beyond the Roche limit may have given birth to satellites; such a mechanism has recently been proposed for the formation of Saturn’s small moons. Indeed, observations made with the Cassini spacecraft have shown that accretion processes are still active at the outer edge of Saturn’s rings and on the satellites Pan and Atlas. Canup suggests that the inner satellites of Saturn, up to and including Tethys, could also have formed by accretion of spreading ring material beyond the Roche limit.
这一优秀的模型密切了观测与理论之间的联系，改变了我们对土星光环的认识。如此厚重的光环降低了流星撞击引起的暗化效应，因此也可以用来解释目前光环亮度的问题。另外，也因质量巨大，光环伸展的要比现在快的多，N多年后光环会扩展的比现在还薄。在扩展的过程中，越过洛希极限的物质可能会再形成新的卫星；这种机理最近被认为是小卫星的形成原因。确实，卡西尼探测器的观测表明在土星光环外沿生长过程仍作用在土卫Pan和Atlas上。Canup认为内侧的土星卫星，包括Tethys在内，也是由极限之外的环内物质在增长过程中形成的。
Canup’s model offers, for the first time, a convincing starting point for a consistent theory of the origin of Saturn’s rings and satellites. It shows that the rings and satellites are intimately linked, and that Saturn’s system, despite being made of ice, is not frozen but is constantly evolving. The origin of the rings and satellites must be understood in the wider framework of models of planet formation, and this work is one step in that direction. One may question whether the specific conditions required for such rings to form have also been met around other Solar System planets and exoplanets. The details and the consequences of the formation of the satellites at the outer edge of the rings, and their outward migration to their present positions, are still to be explored. Establishing these details could change our understanding of Saturn’s satellites, and more generally of giant planets and their environments.
Canup的模型提供了一个新的统一土星光环与卫星形成的令人信服的起点。它表明了光环与卫星是密切关联。而且，土星卫星系尽管是由冰质物质构成，但并非一成不变的。光环与卫星的起源模型必须在一个更广泛的范围被考虑并认知，而本文的该方向的第一步。一个可能的问题是：这样的光环一旦必要条件满足是否会在其它太阳系或外星系天体上形成。光环外沿卫星的形成以及时如何运行至现在位置的问题依然在研究。确立这些内容将有助于我们更加了解土星的卫星以及更多巨行星与它们的环境。

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