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It is known that the D/H-ratio of Earth's water inventory is very similar to that of asteroids. This is usually interpreted in a way that Earth's water must have come from similar, asteroid-like protoplanetary bodies.

However I find it hard to conceive Earth as a initially dry rock, being bombarded with a bunch of Ceres-like objects and then all the water just sticks.

It seems more plausible to me that asteroids would build the young Earth via classical collisional cascades, retain some water all the while, and degas this water later into the young atmosphere to form Oceans.

This scenario only remains plausible if the degassing process (I'm thinking of volcanism, but maybe there are more?) leaves the D/H ratio as it was in the original asteroids.

So my question: Is that so? Does degassing from a source material usually leave the D/H ratio untouched?

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