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Water on earth has been theorized to have come through comets trapped inside crystals. But why wouldn't that water evaporate on impact, and wouldn't the atmosphere at that time allow the vapours to escape Earth?

Also, what is the current scientific opinion of the validity of this theory ?

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    $\begingroup$ Why would the rate of water loss in the Hadean era be much greater than the rate of water loss today? There would be some increase because the temperature was higher, but I can't see why it would have been dramatically higher. $\endgroup$ – John Rennie May 23 '14 at 6:58
  • $\begingroup$ Of course it would evaporate, but then later the vast majority of it would condense again and fall as rain. As far as I'm aware it's not completely accepted that comets brought a substantial proportion of Earth's water, but it's a pretty mainstream theory. $\endgroup$ – Nathaniel May 23 '14 at 7:38
  • $\begingroup$ -1, please mention the relevant difference in atmospheric composition. $\endgroup$ – Blackbody Blacklight May 23 '14 at 8:06
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There's vapour in our atmosphere all the time but it doesn't escape, at least not quickly enough to be of any concern. With the exceptions of hydrogen and helium, which really are escaping (not heavy enough), our atmosphere is stable in the medium-long term. And that includes water (you may recall that water vapour eventually comes back down as rain).

A worse problem is the "splash" in the atmosphere, created by the impact, but that's also a small efect, and also strips away more "native" atmosphere than the newly acquired water.

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