土星的外貌与构成(第1页)
土星的外貌与构成
土星的视直径过小,且它的亮度不及那些更靠近太阳的行星的亮度,这两点使得土星观测活动非常棘手:必须要有足够好的可见度以及光学放大条件才能清晰辨别土星所呈现的细节。土星上有一些平行于赤道分布的带纹,类似木星的带纹;或因为土星总体就很微弱的亮度,或因为这些带纹本身就较浅,总之土星带纹看起来没有那么明显,但我们同样能模糊辨认出这些带纹区域上的各种亮斑或暗斑。
卡西尼于1676年首次发现了其中一条带纹。
同木星一样,土星上的可见细节也在不断地变化,这意味着两颗行星在本质上明显类似,且它们在斑纹方面的相似度更甚。通过观察斑纹,我们得以确定土星不同区域的不同转速——转速从赤道向两极递减。赤道的自转周期为10小时12分钟53秒,介于纬度17度与37度之间的区域的自转周期减慢到10小时14分钟45秒,位于纬度36度的一处亮斑被测到的自转周期为10小时38分钟,所以我们不得不面对土星极不稳定的可视面积,这也意味着我们对土星各种现象的细致研究还有待继续推进。
此处就不再赘述前一章对木星提出的所有推论,但我们应该也能对土星做出如下假设:土星上找不到任何具备坚固表面的星球会呈现出的特征,土星多少是由液态或胶态物质构成的团状物,在它上面遇不到任何固体成分。
所有这些由天文望远镜观测而来的推论又得到了光谱分析或物理学各类研究的佐证。相较于木星,土星的温度更低,在可供光谱分析的土星大气层中,我们发现了氨水、甲烷以及一些尚未被得知的气体。
同木星一样,我们依旧无法构想出土星上的环境。任何可被人类目前的认知所证实的数据,都不足以使人们尝试过去的一贯做法——以确定的方式再现土星世界,谈论它的地貌或居民。
卡西尼于1676年绘制的土星图。图显示了土星球体表面的一圈带纹以及土星环之间的缝隙。
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