为什么大多数天体都是球形的

【为什么大多数天体都是球体?】

一般只有达到某个直径上限的天体才具有球体状态,这个值取决于天体本身的物质属性。土卫一就是一个很好的例子:土卫一为太阳系岩石球体中体积最小的。简单的说,这和引力势能的差值(质量)以及流体静力平衡相关,本文将为您简单介绍这个原理。

首先来了解下我们熟悉而陌生的太阳,太阳不仅仅是球体,而且非常接近于圆球,估计扁率大概只有900万分之一,这意味着其极直径和赤道直径的差别不到10千米,这和太阳约140万千米的直径比起来,都不算什么了。我们再来看看其他天体,月亮的扁率约为0.0012,地球约为0.003

我们在前言所提到的土卫一,它是目前太阳系最小的岩石球体,可以根据自身的引力以及自转形成球体。土卫一的直径大约为400千米,自转周期大约为22小时。但是比土卫一更小的天体,却不是球体的了。比如土卫七,形状诡异,其直径约为270千米,自转周期为混沌状态。又如木卫五,直径约为200千米,其形状也是不规则的。如果比土卫一直径大的话,那么天体就成球体了。如土卫二,直径约为500千米,形状为球体。

卡西尼号拍摄的土卫一的高分辨率照片,照片中显示了深达6公里的撞击坑以及右下方的一个深达1公里的峡谷。图片:NASA/JPL/Space Science Institute

如果太阳系天体的质量大于10的21次方千克,那么已知或预期天体就会近似球体。当天体质量的引力足以克服其自身材料的结构强度时(钢性力),天体就会不受约束的形成球形(椭球体),从而达到流体静力学平衡。由冰制成的物体比由岩石制成的物体更容易变为球体,而且许多冰冷的物体在更小的直径下也会是球状的。其大小边界在半径100公里到200公里之间,但大多数还是不规则的。

质量范围在10的18次方千克到10的21次方千克之间的较大物体,如土卫三,谷神星和土卫一,由于它们的引力强大到足以形成流体静力平衡状态,所以可以形成球体(椭球体),而较小质量的碎石堆(例如土卫十和木卫五)可以形成类似的状态,但形状不是球形的,因此称这类天体形状为“不规则形状”。

由于自身旋转产生了离心力,所以球体通常具有一些极性扁率,有时甚至可能具有很大差异的赤道直径(斜率椭球体,如妊神星)。与妊神星等天体不同,不规则形状天体与椭圆体的形状明显不同。

这是曙光号在2015年5月拍摄,以原色呈现的谷神星。图片:NASA-JPL-California Institute of Technology

流体静力平衡简介

在流体力学中,当流体静止时,或者当每个点的流速随时间恒定时,流体被认为处于流体静力平衡或静水压平衡。当重力等外力通过压力梯度力平衡时,会发生这种情况。例如,压力梯度力可防止重力使地球大气层坍塌成薄而致密的外壳体,而重力则可以阻止压力梯度力将大气扩散到太空中。

流体静力学平衡是矮行星与太阳系小天体的现行区分标准,在天体物理学和行星地质学中还有其他作用。这种限定通常意味着物体被对称地圆化成球体或椭球体,其中任何不规则的表面特征是由于相对薄的固体地壳造成的。在太阳系中,目前具备该条件的天体有32颗,并已经得到观测证实(除了太阳外),其中有7颗天体几乎是可以确定的,但是还有100颗或更多的候选者。

在恒星的任何给定层中,从下方(核心)向外的热压(辐射)和上面物质向内挤压的引力之间存在一个流体静力学平衡。各向同性引力场将恒星压缩成最紧凑的形状。当达到一定的(临界)角速度时,在静力平衡下旋转的恒星会形成一个扁圆球体。这种现象的一个极端例子便是恒星织女一,它的旋转周期为12.5小时。因此,织女一在赤道处的直径比两极处的要大20%左右。如果角速度高于临界角速度,那么这颗恒星将会变成雅可比(斜率)椭球体,在旋转速度更快时,它将不再是椭球体,而是梨形或卵形体,还可能变成其他诡异形状,如甜甜圈形状,但是超过雅可比(斜率)椭球体的形状是不稳定的。

如果恒星附近有一个巨大的伴星时,那么潮汐力会发挥作用,当仅靠旋转就使恒星变成一个球体时,就会把恒星扭曲成不等边的形状(具有很大斜率的椭球体),恒星渐台二就是一个例子。

流体静力学平衡对于星系团内的介质也很重要,在那里它限制了存在于星系团核心中的流体数量。

此外,有足够的质量,能以自身的重力克服刚体力,以呈现流体静力平衡的形状也是行星或矮行星的定义要素之一。根据国际天文联会(IAU)的行星定义,行星有足够的质量能以自身的重力达到流体静力平衡的状态,这意味着它们的形状要成为球体,或类球体。

人们一直认为,直径大于约400公里的冰天体通常处于流体静力平衡状态,而那些小于此直径的冰天体则没有。冰冻天体比岩石天体需要更小的静水平衡质量。已知具有平衡形状的最小天体是直径约397公里的冰卫星土卫一,而已知具有明显非平衡形状的最大天体是直径约为532公里(582×556×500±18公里)的岩石小行星智神星。然而,土卫一实际并不处于流体静力平衡状态。已知能够确认处于静水平衡的最小天体是直径约945公里的矮行星谷神星,而已知未处于静水平衡的最大天体是直径约1470公里的冰卫星土卫八。

要注意谷神星虽然是球体,但是古柏带的妊神星比谷神星重数倍,却是太阳系内已知最大的非球体天体,因为妊神星被快速的自转拉成了椭球体。木星和土星也是因为快速的自转成为扁圆形,土卫一、土卫二、和天卫五是因为潮汐力拉长变成类球体。

美国阿波罗17号宇航员在前往月球途中拍摄的“蓝色弹珠”地球照片(摄于1972年12月7日,非原始照片),图:NASA/Apollo 17 crew; taken by either Harrison Schmitt or Ron Evans

参考资料

1.WJ百科-英文版(List of Solar System objects by size/Definition of planet

2.WJ百科-英文版(Hydrostatic equilibrium/Jacobi ellipsoid

 

文章作者:零度星系(天文在线)

审核人员:北极光

最终审核:零度星系

编辑用时:2018年09月26日-2018年09月27日

审核用时:四十分钟

最后更新2018年12月4日星期二

注意:所有信息数据庞大,难免出现错误,还请指出错误所在好加以改之。

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