The result, published in Nature, hints at the idea that much of the Earth's water could have initially came from cometary impacts.I haven't pulled out my slide rule, but I have a *problem* with that theory.
To wit: If you calculated, or estimated, the volume of all the water in the earth's oceans (not even all the lakes and rivers), then factored in the size of the 'average' comet head containing ice you would need a 'googol' of Comets hitting the earth during its 'formative' years - after it cooled but had no water.
Then after 'a while' (scientific term) some of that water would have formed a young atmosphere creating friction for the latter comets with water ice hitting the earth, which would evaporate the water into a gas. And that friction factor would then increase exponentially ('e') as our atmosphere increased. And keep in mind the comet(s) couldn't be too large or the debris field expelled 'up' would create an added friction factor for the other comets to pass through.
Granted, the young solar system was a violent place, just look at the moon. But comets being the source of all our water? I can't imagine the number ('n') of comets involved which would result in getting a '+' = 'water'.
Think I'll fire off an email to Stephan Hawking today for his take on this. 