海王星的卫星家族

发布时间：2015-07-10 00:00:00 点击：601

[导读] 海王星，冰冷的气态巨行星，按照同太阳的平均距离由近及远排列，是环绕太阳运行的第八颗行星，有14颗已知的天然卫星，让我们跟随本文一起了解海王星的卫星们。

中国科技网11月13日报道（张微 编译）海王星，冰冷的气态巨行星，按照同太阳的平均距离由近及远排列，是环绕太阳运行的第八颗行星，在1846年被两位天文学家奥本·勒维耶和约翰 加勒发现。根据行星命名的惯例，海王星以罗马海洋之神(相当于希腊神话中的海神)的名字命名。在它被发现的17天后，天文学家注意到它也有一个卫星系统。

最初，只有海王星最大的卫星海卫一可以被观测到。但在20世纪中叶以后，随着地面望远镜的改进和太空探测器的发展，更多的卫星被发现。海王星现在有14颗已知的天然卫星，由于海王星得名于罗马神话的海神，它的卫星都以低等的海神命名。

发现和命名：

海卫一，最大的、是唯一拥有足够质量成为球体的，也是第一个被发现的海王星的卫星。在海王星被发现的17天以后，也就是1846年10月10日，就被威廉·拉塞尔发现了。而其它卫星被发现就是一个世纪以后了。

首先是海卫二，它是1949年5月1日热拉尔·柯伊伯利用位于德克萨斯，戴维斯堡的麦克唐纳天文台拍摄的照片(柯伊伯带就是以他的名字命名的)发现的。第三个卫星，后来被命名为海卫七(拉里萨)，是在1981年5月24日被Harold J. Reitsema，威廉 哈伯德，拉里A.列波夫斯基和戴维J.托伦发现的。

这颗卫星的发现纯属偶然，它的发现的过程是持续搜索光环的结果，与四年前发现的围绕着天王星的光环类似。如果光环确实存在，那么当行星靠近的时候，恒星的亮度就稍微下降。而在观测恒星接近海王星的时候，恒星的亮度减弱，虽然只有几秒钟。这就表示存在一个卫星而不是光环。

直到1989年旅行者2号飞越海王星才有新的卫星被发现。在飞越卫星系统的过程中，太空探测器有一次观测到了海卫七，而且发现了五颗内卫星：海卫三，海卫四，海卫五，海卫六和海卫八。

2001年，使用大型地面望远镜——智利托洛洛山美洲际天文台和加拿大-法国-夏威夷望远镜——进行的两项调查，发现另外五颗卫星，使得卫星总数达到了十三颗。研究团队在2002和2003年的接下来的调查中，分别重新观测到这五颗卫星，分别是海卫九，海卫十一，海卫十，海卫十二和海卫十三。

然后，在2013年7月15日，地外文明搜索研究所(SETI)的马克 R肖沃尔特领导的天文学家研究团队宣布，他们从哈勃望远镜2004-2009年拍摄的图片中发现了之前未知的第十四颗卫星。尚未命名的第十四颗卫星，目前确定为S/2004 N 1，直径不超过16-20公里。

由于天文学的惯例，海王星卫星的名字都是取自希腊和罗马神话中。在这个情况中，所有卫星的名字都是以海洋之神或海神孩子(包括海卫一，海卫八，海卫五，海卫四)，低等希腊海神(海卫三和海卫二)，或涅瑞伊得斯，希腊神话中的水仙女(海卫九，海卫六，海卫十三，海卫十一，海卫十二，海卫十)名字命名的。

许多卫星直到20世纪才得到命名。海卫一(特里同)的名字，最初由卡米伊·弗拉马利翁在他1880年的书《Astronomie Populaire》中提出建议，但不是一个常用的称呼，直到20世纪30年代这个状况才得到改变。

内圈(规则)卫星：

海王星的规则卫星靠近行星，而且位于行星赤道平面做圆形顺行轨道运转。它们距离海王星的距离依次为：海卫三(48,227公里)，海卫四(50,074公里)，海卫五(52,526公里)，海卫六(61,953公里)，海卫七(73,548公里)，S/2004 N 1 (105,300 ± 50 km)和海卫八(117,646公里)。除了外圈的两颗，其余的规则卫星都位于海王星同步轨道内，因此存在潮汐减速现象。

内圈卫星与海王星窄环系统密切相关。最里面的两颗卫星，海卫三和海卫四，在加勒和勒威耶环之间运转，而海卫五在勒威耶环内运转。下一个卫星，海卫六，运转在最著名的亚当斯环内，而且它通过重力作用包含颗粒，维持光环的存在。

根据观测数据和假设的密度，海卫三的测量数据是96 × 60 × 52公里，重量大约是.9 x 1017 kg公斤。海卫四的数据是 108 x 100 × 52 公里，重量是 3.5 x 1017 公斤，海卫五的数据是180 x 148 x 128 而且重量是21 x 1017公斤，海卫六的数据是 204 x 184 x 144 而且重量是37.5 x 1017 公斤， 海卫七的数据216 x 204 x 168 而且重量是 49.5 x 1017公斤，编号S/2004 N1 的数据是直径16-20公里，重量是0.5 ± 0.4 x 1017 公斤，海卫八是 436 x 416 x 402，重量是50.35 x 1017公斤。

只有两个最大的规则卫星照片的分辨率足以辨别它们的形状和表面特征。然而，除了海卫七和海卫八之外，所有海王星的内圈卫星都被认为是细长形状的。此外，所有内圈卫星都是暗物体，几何反照率从7%-10%不等。

它们的光谱也表明，它们是由被一些暗色物质(也可能是有机化合物)污染的水冰构成的。在这方面，海王星内圈卫星与天王星内圈卫星相似。

外圈(不规则)卫星：

海王星的不规则卫星由该行星的其余卫星(包括海卫一)组成。它们大都是倾斜偏心的，而且经常远离海王星逆行运转，唯一例外的是海卫一，虽然也是逆行和偏心的，但是它的运转是靠近海王星的圆形轨道。

按照它们距离行星的距离顺序，这些不规则行星分别是海卫一，海卫二，海卫九，海卫十一，海卫十二，海卫十三和海卫十，这组卫星包括顺行和逆行的天体。除了例外的海卫一和海卫二，海王星的不规则卫星与其他巨行星类似，被认为是由海王星的引力捕获的。

就规模和质量来说，不规则卫星大体相当，从直径大约40公里，质量4 x 1016公斤(海卫十)到直径62公里，质量16 x 1016公斤的海卫九。

海卫一和海卫二：

海卫一和海卫二是两个异常的不规则卫星，因此与其他五个不规则的海王星卫星区别对待。在这两个卫星和其他不规则卫星之间，有四个重要的差异。

首先，他们是太阳系中已知最大的两个不规则卫星。海卫一比其他已知的不规则卫星大一个数量级，质量是海王星已知轨道卫星的99.5%(包括海王星光环和13颗其他已知卫星)。

其次，它们都有非典型的小半长轴，海卫一的半长轴比所有已知不规则卫星要小一个数量级。再次，它们都有不寻常的轨道偏心率：海卫二的轨道偏心率是所有不规则卫星中最高的，海卫一的轨道接近完美的圆形。最后，海卫二在所有已知不规则卫星中有着最低的倾角。

平均直径约2700公里，质量为214080±520 x 1017公斤，海卫一是海王星的最大卫星，也是唯一大到可以静力平衡的星球(例如它的形状是球形)。与海王星的距离是354,759公里，海卫一也是位于行星内圈和外圈之间的卫星。

海卫一是一颗逆行而且有着标准的圆形轨道的卫星，主要由氮气，甲烷，二氧化碳和水冰组成。几何反照率超过70%以上，球面反照率高达90%，它也是太阳系中最明亮的天体。由于紫外线辐射和甲烷相互作用，形成的索林(发现于星际空间的粘性固体物质)，使得海卫一的表面呈现出红色。

海卫一也是太阳系中最冷的卫星之一，表面温度38K。而且，由于卫星的地质活动活跃(导致了冰火山化)，表面温度变动引发了升华，海卫一是太阳系中唯一的两颗拥有大气层的卫星。与它的表面类似，海卫一的大气层主要由氮气和少量甲烷以及一氧化碳组成，估计压力是14?巴。

海卫一密度较高，大约2克/立方厘米，这表明海卫一质量的三分之二是由岩石构成的，冰(主要是水冰)占了剩余的三分之一。海卫一的地表下可能有一层液态水，形成了一个地下海洋。表面特征包括大型南极冰盖，由地堑和峭壁交叉横切的坑洞平面，以及由于其内生重修作用造成其表面非常年轻化的特征。

由于它的逆行轨道而且与海王星的距离很近(比月亮和地球的距离还要近)，海卫一被分到了行星的不规则卫星群。此外，它被认为是一个颗被捕获的天体，也可能曾经是柯伊伯带的矮行星，同时，这些轨道特征正是海卫一经历潮汐减速的原因。这颗卫星最终将盘旋向内靠拢，在36亿年后与海王星相撞。

海卫二是海王星的第三大卫星。它有着顺行但是偏心率非常高的轨道，被认为一个规则卫星通过与海卫一的引力相互作用被捕捉到当前轨道。光谱分析探测到其表面含有水冰。海卫二是一个变化多端的卫星，这可能是由于其强迫进动或无序旋转与细长形状以及表面或明或暗的点结合所导致的。

形成：

鉴于其卫星的质量分布不均，人们普遍认为，海卫一是海王星的原始卫星系统形成之后被捕获的，在这个过程中海卫一已经被破坏。多年来，科学们提出许多理论来分析其捕获的机制。

关于它的身世，最被广泛接受的说法是，之前有一个柯伊伯带的双星系统与海王星近距离相遇时，海王星的引力便破坏了双星体系，其中的一个幸存的星体被海王星俘获，就是现在的海卫一。海卫一的捕获是三个天体碰撞的结果，由于双星系统的残余影响和海王星的引力共同作用，海卫一的轨道旋转方向就变成和海王星自转方向相反，其他天体在这个过程中，天体或者被毁灭或者变成喷射物。

刚被捕获的时候，海卫一就是非常奇特的高度偏心轨道，而且在原始的海王星内圈卫星轨道上引起了混沌扰动，使得这些卫星相互碰撞，体积减小成为盘状碎石结构星体。只有海卫一轨道再一次成为圆形以后，有些碎石结构才能重新结合形成目前的规则卫星。这意味着，海王星目前的内圈卫星并不是与海王星一起形成的原始天体。

数值模拟研究表明，过去的某个时间，海卫九与海卫二有0.41的碰撞概率。虽然现在还不知道是否发生过碰撞，但是两颗卫星都呈现出类似(灰色)的颜色，这表明海卫九可能是海卫二的一个碎片。

由于距离太阳非常遥远，唯一的一次曾研究海王星及其卫星的探测任务是旅行者2号。虽然目前没有探测海王星的研究计划，但是科学家们已经提出了几个建议，在21世纪20年代晚期或30年代早期的某个时候，将会有机器人探测器被派遣到海王星的卫星系统上执行研究任务。

英文原文：

The moons of Neptune

Neptune, that icy gas giant that is the eight planet from our Sun, was discovered in 1846 by two astronomers – Urbain Le Verrier and Johann Galle. In keeping with the convention of planetary nomenclature, Neptune was named after the Roman god of the sea (the equivalent to the Greek Poseidon). And just seventeen days after it was discovered, astronomers began to notice that it too had a system of moons.

Initially, only Triton – Neptune's largest moon – could be observed. But by the mid-20th century and after, thanks to improvements in ground-based telescopes and the development of robotic space probes, many more moons would be discovered. Neptune now has 14 recognized satellites, and in honor of of their parent planet, all are named for minor water deities in Greek mythology.

Discovery and Naming:

Triton, being the largest and most massive of Neptune's moons, was the first to be discovered. It was observed by William Lassell on October 10th, 1846, just seventeen days after Neptune was discovered. It would be almost a century before any other moons would be discovered.

The first was Nereid, which was discovered on May 1st, 1949, by Gerard P. Kuiper (for whom the Kuiper Belt is named) using photographic plates from the McDonald Observatory in Fort Davis, Texas. The third moon, later named Larissa, was first observed by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Tholen on May 24th, 1981.

The discovery of this moon was purely fortuitous, and occurred as a result of the ongoing search for rings similar to those discovered around Uranus four years earlier. If rings were in fact present, the star's luminosity would decrease slightly just before the planet's closest approach. While observing a star's close approach to Neptune, the star's luminosity dipped, but only for several seconds. This indicated the presence of a moon rather than a ring.

No further moons were found until Voyager 2 flew by Neptune in 1989. In the course of passing through the system, the space probe rediscovered Larissa and discovered five additional inner moons: Naiad, Thalassa, Despina, Galatea and Proteus.

In 2001, two surveys using large ground-based telescopes – the Cerro Tololo Inter-American Observatory and the Canada-France-Hawaii telescopes – found five additional outer moons bringing the total to thirteen. Follow-up surveys by two teams in 2002 and 2003 respectively re-observed all five of these moons – which were Halimede, Sao, Psamathe, Laomedeia, and Neso.

And then on July 15th, 2013, a team of astronomers led by Mark R. Showalter of the SETI Institute revealed that they had discovered a previously unknown fourteenth moon in images taken by the Hubble Space Telescope from 2004–2009. The as yet unnamed fourteenth moon, currently identified as S/2004 N 1, is thought to measure no more than 16–20 km in diameter.

In keeping with astronomical convention, Neptune's moons are all taken from Greek and Roman mythology. In this case, all are named for gods of the sea, or for the children of Poseidon (which include Triton, Proteus, Depsina and Thalassa), minor Greek water dieties (Naiad and Nereid) or Nereids , the water nymphs in Greek mythology (Halimede, Galatea, Neso, Sao, Laomedeia and Psamathe).

However, many of the moons were not officially named until the 20th century. The name Triton, which was originally suggested by Camille Flammarion in his 1880 book Astronomie Populaire, but not into common usage until at least the 1930s.

Inner (Regular) Moons:

Neptune's Regular Moons are those located closest to the planet and which follow circular prograde orbits that lie in the planet's equatorial plane. They are, in order of distance from Neptune: Naiad (48,227 km), Thalassa (50,074 km), Despina (52,526 km), Galatea (61,953 km), Larissa (73,548 km), S/2004 N 1 (105,300 ± 50 km), and Proteus (117,646 km). All but the outer two are within Neptune-synchronous orbit and thus are being tidally decelerated.

The inner moons are closely associated with Neptune's narrow ring system. The two innermost satellites, Naiad and Thalassa, orbit between the Galle and LeVerrier rings, whereas Despina orbits just inside the LeVerrier ring. The next moon, Galatea, orbits just inside the most prominent Adams ring and its gravity helps maintaining the ring by containing its particles.

Based on observational data and assumed densities, Naiad measures 96 × 60 × 52 km and weighs approximately 1.9 x 1017 kg. Meanwhile, Thalassa measures 108 x 100 × 52 km and weighs 3.5 x 1017 kg; Despina measures 180 x 148 x 128 and weighs 21 x 1017 kg; Galatea measures 204 x 184 x 144 and weighs 37.5 x 1017 kg; Larissa measures 216 x 204 x 168 and weighs 49.5 x 1017 kg; S/2004 N1 measures 16-20 km in diameter and weighs 0.5 ± 0.4 x 1017 kg; and Proteus measures 436 x 416 x 402 and weighs 50.35 x 1017 kg.

Only the two largest regular moons have been imaged with a resolution sufficient to discern their shapes and surface features. Nevertheless, with the exception of Larissa and Proteus (which are largely rounded) all of Neptune's inner moons are believed to be elongated in shape. In addition, all the inner moons dark objects, with geometric albedo ranging from 7 to 10%.

Their spectra also indicated that they are made from water ice contaminated by some very dark material, probably organic compounds. In this respect, the inner Neptunian moons are similar to the inner moons of Uranus.

Outer (Irregular) Moons:

Neptune's irregular moons consist of the planet's remaining satellites (including Triton). They generally follow inclined eccentric and often retrograde orbits far from Neptune; the only exception is Triton, which orbits close to the planet following a circular orbit, though retrograde and inclined.

In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Neso and Psamathe, a group that includes both prograde and retrograde objects. With the exception of Triton and Nereid, Neptune's irregular moons are similar to those of other giant planets and are believed to have been gravitationally captured by Neptune.

In terms of size and mass, the irregular moons are relatively consistent, ranging from approximately 40 km in diameter and 4 x 1016 kg in mass (Psamathe) to 62 km and 16 x 1016 kg for Halimede.

Triton and Nereid:

Triton and Nereid are unusual irregular satellites and are thus treated separately from the other five irregular Neptunian moons. Between these two and the other irregular moons, four major differences have been noted.

First of all, they are the largest two known irregular moons in the Solar System. Triton itself is almost an order of magnitude larger than all other known irregular moons and comprises more than 99.5% of all the mass known to orbit Neptune (including the planet's rings and thirteen other known moons).

Secondly, they both have atypically small semi-major axes, with Triton's being over an order of magnitude smaller than those of all other known irregular moons. Thirdly, they both have unusual orbital eccentricities: Nereid has one of the most eccentric orbits of any known irregular satellite, and Triton's orbit is a nearly perfect circle. Finally, Nereid also has the lowest inclination of any known irregular satellite

With a mean diameter of around 2700 km and a mass of 214080 ± 520 x 1017 kg, Triton is the largest of Neptune's moons, and the only one large enough to achieve hydrostatic equilibrium (i.e. is spherical in shape). At a distance of 354,759 km from Neptune, it also sits between the planet's inner and outer moons.

Triton follows a retrograde and quasi-circular orbit, and is composed largely of nitrogen, methane, carbon dioxide and water ices. With a geometric albedo of more than 70% and a Bond albedo as high as 90%, it is also one of the brightest objects in the Solar System. The surface has a reddish tint, owning to the interaction of ultraviolet radiation and methane, causing tholins.

Triton is also one of the coldest moons in the Solar System, with surface temperature of about 38 K (235.2 °C). However, owing to the moon being geologically active (which results in cryovolcanism) and surface temperature variations that cause sublimation, Triton is one of only two moons in the Solar System that has a substantial atmosphere. Much like it's surface, this atmosphere is composed primarily of nitrogen with small amounts of methane and carbon monoxide, and with an estimated pressure of about 14 ?bar.

Triton has a relatively high density of about 2 g/cm3 indicating that rocks constitute about two thirds of its mass, and ices (mainly water ice) the remaining one third. There also may be a layer of liquid water deep inside Triton, forming a subterranean ocean. Surface features include the large southern polar cap, older cratered planes cross-cut by graben and scarps, as well as youthful features caused by endogenic resurfacing.

Because of its retrograde orbit and relative proximity to Neptune (closer than the Moon is to Earth), Triton is grouped with the planet's irregular moons (see below). In addition, it is believed to be a captured object, possibly a dwarf planet that was once part of the Kuiper Belt. At the same time, these orbital characteristics are the reason why Triton experiences tidal deceleration. and will eventually spiral inward and collide with the planet in about 3.6 billion years.

Nereid is the third-largest moon of Neptune. It has a prograde but very eccentric orbit and is believed to be a former regular satellite that was scattered to its current orbit through gravitational interactions during Triton's capture. Water ice has been spectroscopically detected on its surface. Nereid shows large, irregular variations in its visible magnitude, which are probably caused by forced precession or chaotic rotation combined with an elongated shape and bright or dark spots on the surface.

Formation:

Given the lopsided distribution of mass in its moons, it is widely believed that Triton was captured after the formation of Neptune's original satellite system – much of which would have been destroyed in the process of capture. Many theories have been offered regarding the mechanisms of its capture over the years.

The most widely-accepted is that Triton is a surviving member of a binary Kuiper Belt Object that was disrupted with an encounter with Neptune. In this scenario, Triton's captured was the result of a three-body encounter, where it fell into a retrograde orbit while the other object was either destroyed or ejected in the process.

Triton's orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble. Only after Triton's orbit became circular again could some of the rubble re-accrete into the present-day regular moons. This means it is likely that Neptune's present inner satellites are not the original bodies that formed with Neptune.

Numerical simulations show that there is a 0.41 probability that the moon Halimede collided with Nereid at some time in the past. Although it is not known whether any collision has taken place, both moons appear to have similar ("grey") colors, implying that Halimede could be a fragment of Nereid.

Given its distance from the Sun, the only mission to ever study Neptune and its moons up close was the Voyager 2 mission. And though no missions are currently being planned, several proposals have been made that would see a robotic probe dispatched to the system sometime in the late 2020s or early 2030s.

来源：中国科技网 2015年11月13日　13:37

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