Expert: Courtney Seligman - 1/26/2011

Question
Courtney,
I very much appreciated a previous answer from you.
My question today is related to water on earth in its early stages.
My understanding of current theory is that the early earth formed where it was too hot for liquid water, therefore any existing water would have been in the form of vapor and/or been blown into space by accretion events; and that most of the earth's water came from the asteroid belt.
My question is:  Why didn't whatever water molecules which were in the solar neighborhood of earth at the time re-accrete and continue to do so until it was eventually all back on the planet?  And if this did happen, would that have been a significant portion of the earth's current water?
Thanks for your time,
Paul V

Answer
If there were water molecules in space, they would be blown away from our orbit by the solar wind, so they wouldn't have a chance to re-accrete. So the water we have has to have been somehow retained and not lost in the first place, or acquired from elsewhere later on.

As you have probably learned, there are a number of theories about how this happened, and there are serious objections to every single one of them. So we don't really know why the Earth has so much water.

My personal opinion, and therefore one that should only be viewed as an opinion and not as a solid scientific theory, is that during the accretion of the Earth small amounts of hydrogen were trapped on the surfaces of grains that stuck together to build up the Earth; and that despite theories to the contrary, the amount of hydrogen trapped in this way was far more than adequate to explain the current hydrosphere.

The reason I believe this is that a lot of the hydrogen trapped inside the Earth during its formation should have made its way to the surface (primarily as steam) when the Earth melted and differentiated; yet even today, oceanic volcanic vents release enormous quantities of water vapor whose source lies within the Earth's mantle. If there is still such a large amount of water inside the Earth, the original amount must have been much larger; and although how it could have got there in the first place might be open to question, I have no doubt that the water now present on the Earth was already inside it at the time that it formed.