第1个回答 2009-11-15
也在散,比如氢气和氦气。地球引力还不能把它们给彻底束缚住。但是这个过程很缓慢。因为地球不仅有全球性的磁场保护,而且地球引力也还可以。
而火星就悲剧了,本来火星上可能甚至有过海洋,但是都因为引力太小,而且火星没有全球范围的磁场保护,所以水汽很多都逸散到太空里面了(有的是分解为氢气和氧气后,氢气跑了),于是成了现在的不毛之地。
地球大气层当然也跟着自转,因为地球引力的吸引。
可以参考一篇文章:
Mars’ volatile and climate history
Bruce M. Jakosky*† & Roger J. Phillips*‡
Synthesis of the observations and interpretations
It is clear that there are a number of strong constraints on the history of water on Mars. They tell us that water was present at the surface early in the planet’s history, and within the crust throughout time.
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2. A secondary atmosphere was created by water and CO2 released to the atmosphere as a direct result of Tharsis volcanism, and this may have had a strong influence on climate. It is likely that volatiles were also released by non-Tharsis Noachian volcanism presumed to have been responsible for forming the ancient highland crust.
3. Water and CO2 were lost from the surface and atmosphere system to space, to the polar caps and to carbonate deposits within the crust. There is compelling evidence for the existence of each of these sinks, as described above, although it is not possible at this time to determine uniquely the relative or absolute importance of each.
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第2个回答 2009-11-16
大气不是绝对的不向宇宙扩散,我们知道大气在越高的地方越稀薄,它和外太空没有明确的界限,在最靠近外太空的地方,地球的引力要小一些,气体分子运动的速度有快有慢,在这些离外太空很近的地方,某些速度特别快而且方向正对着外太空的空气分子,就会脱离地球的引力飞离地球.
事实上如果地球新产生的空气分子要是小于飞离地球的气体,迟早有一天,地球的大气会发生变化.(我们知道我们周围的空气是有速度不同空气分子组成,某些空气分子的速度是很高的,由于常压下,空气分子很多,所以每个分子平均速度虽然很高,但由于互相碰撞走的距离很短.在高空空气非常稀薄,所以空气分子走的距离比地球表面长很多,更容易脱离地球引力飞走)