[II2II]

Multiple impacts is the standard hypothesis for a source of Earth's water. If I recall correctly, outgassing from volcanos is another source.

Keep in mind, the solar system formed from a relatively homogenous nebula. It was the formation of the sun that forced lighter elements to migrate outwards, and that only happens if the lighter elements aren't already part of a larger object. There isn't much of a difference between a 10 km chunk of ice and a 10 km chunk of iron gravitationally speaking. Bouancy doesn't play a role here, so density doesn't matter. Outgassing does matter, but that is a slow process for large object, like the Earth, or for smaller objects on Earth crossing orbits that don't get too close to the sun.

It's also worth considering that each planet's situation is unique. There is much more water ice on the moons of the outer solar system because there was more water at the time of formation and the lower temperatures mean the water that was there stayed there. As for Mars, even though it is colder than the Earth, it is much less massive. As such, its atmosphere bleeds away lighter molecules (never mind lighter elements).

[andrewflnr]

> Multiple impacts is the standard hypothesis for a source of Earth's water.

Right, which is why it's baffling to me that everyone in this thread seems to be losing their mind over this result, thinking it affects the Drake equation and rewrites solar system dynamics. The multiple impacts thing might not have actually happened to earth, but there's still no reason to believe it wouldn't work.