MadSci Network: Earth Sciences |
Good question! The answer is -- by rain from the Earth's atmosphere.
This brings up the next question -- how did the atmosphere get here?
We have good reason to believe that the Earth's original atmosphere
(left over from the Earth's formation process) was about a thousand
times sparser than it is today. So our current atmosphere must have
come from volcanic gases, originating in the Earth's interior.
Measurements of the gas coming from volcanoes today (on Hawaii, for
example) have found that it is mostly composed of water vapor and
carbon dioxide. Voila, the oceans! (with vast amounts of carbon
dioxide locked up in them)
A recent twist on this scenario is a very controversial theory that
the Earth is being bombarded continually be millions of mini-comets
(house-sized) per year. As they are composed mostly of water-ice, these
comets would deposit a large amount of water on the Earth over long
periods of time. For more info, see
this page.
-Aaron
Try the links in the MadSci Library for more information on Earth Sciences.