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This one puzzles me:

"One of the leading hypotheses on Earth’s formation is that it was so hot when it formed 4.6 billion years ago that any original water content should have boiled off. "

So where would it actually have boil to? I thought gravity affected water also in gas state. Just like the atmosphere doesn't "boil off", why would the water?

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Actually, some fractions of the atmosphere do "boil off":

https://en.wikipedia.org/wiki/Atmospheric_escape

Essentially, because the distribution of thermal energy has a fairly long tail, it is possible for some lighter particles to reach escape velocity. The hotter the atmosphere, the more (and heavier) particles escape.

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