[X] My Assistant

[dvandorn]

The Messenger posted an interesting comment over on the asteroid/comet board, in a discussion about how comets are turning out to have much less water in them than we once thought:

I think it is very legitimate, at this time, to rise the question: If we need an explanation for why the earth has oceans, the collision of objects like Saturns icy moons, rather than comets as we now know them, seems a little more likely.

Now then -- something with a mass like a large outer-planet moon hitting Earth most likely *did* happen -- it's called the Big Whack theory.

I have never seen any speculations on the *composition* of the impactor that created the Moon -- the only comments I have seen are those relating to oxygen isotope abundances which indicate that Moon rocks formed from roughly the same solar nebula composition as Earth did (which I've always thought could be in doubt, since the material that made up the Moon must have a pretty large admixture of material from the proto-Earth).

If the impactor (which had a mass roughly equivalent to Mars) was something like an oversized Ganymede, with a large percentage of ice around a chondritic rocky core, is it possible that *all* of the water ended up back on Earth, while the ring from which the Moon formed lost all of it?

I don't have the proper education or tools, myself, to model a Big Whack with an impactor composed of, say, 25% water ice by mass, to see where the water ends up and how it can be disassociated from the materials that made up the Moon. But it seems to me that the question of how water in the debris cloud could have been so thoroughly removed before the Moon formed must be answered, in any event, since there had to have been terrestrial water involved -- unless we postulate that neither proto-Earth nor impactor had any water. And *that* means we have to postulate no ice-body impacts onto Earth before the Big Whack and a whole bunch afterwards, to explain where the oceans came from.

Assuming, of course, that the oceans came from impacts of ice-body objects in the first place...

-the other Doug