While Theia may explain the origin of the moon, where did all the water come from? If we assume that the pre-impact Earth had similar amounts of water (percentage-wise) as Venus & Mars, must we assume that the percentage of water on the smaller body was the same or greater than Earth’s? If greater, then it is most likely the smaller body was not from our system. If the percentage is similar, that does not explain why Earth has the high percentage of water that we do which is far more than twice the percentage of our planetary neighbors.
Interesting, is it not?
Check your premise!
To begin with, the Earth contains only a paltry 1.4 million cubic km of H2O.
Venus and Mars likely once contained comparable amounts of water. (And Mars still contains an estimated 5 million cubic km of surface and subsurface ice.)
However, Venus and Mars lack a significant magnetosphere to protect their atmospheres from depletion by the Solar Wind.
Secondly, Venus is a lot hotter - so the H2O (volatile) was essentially "baked off."
The gravity field of Mars is weaker than Earth's further contributing blah-blah-blah...
The water probably came from cometary impacts.
Regards,
Sorry for the misplaced decimal point!
Regards,