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Message: <blockquote data-quote="Umbran" data-source="post: 6254153" data-attributes="member: 177">Well, the entire planet is imported from space, now isn't it?  We're only quibbling over what part of space the water came from, and when.The thing is, water isn't just water.  Hydrogen and oxygen have isotopes.  You can look at the water on Earth, and measure the ratio of isotopes in our water, and compare it to water seen in asteroids, and that seen in comets.  If the isotope ratios match one or the other, one is apt to think there's a bit of a link.It was only in the 1980s that we started getting an idea of the isotope ratios in the water of comets.  And the results seemed to indicate that comets had notably different ratios than seen in our oceans, while water in asteroids looked a lot more like our water.  But more recent measurements (of comet Hartley 2, for example) have shown comets with rather Earthlike water.The basic reason folks sometimes wonder if our water came from asteroids or comets is simple:  heat.  The early Earth was molten - what water we had on the surface would be forced to vapor - hot vapor.  And hot vapor has a tendency to escape into space.  So, many figure that any water we had would have boiled off, and would have needed to be replaced.  As far as I know, there's no consensus, though.  Some think our water came from comets and/or asteroids, others think it was outgassed or created (by burning hydrogen) on Earth after it had cooled enough to hold onto liquid water.  Others think there's a mix of sources.Water is pretty common stuff - there's lots of it out in the Universe that we can see.  Life is less obvious...There's a basic tenet that runs through all of science - one place in the universe is pretty much the same as any other. We are not the center of the universe, the galaxy, or the solar system.  We aren't the center of *anything*.  Earth is not really special.  So, if you think that the origin of life is an unlikely event, you have to ask if or why it happened here, specifically.</blockquote>

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