Dating of meteorites indicates that the Solar System is approximately 4.6 billion years old, and Earth appears to be approximately the same age. Yet the oldest sedimentary rocks -- those that formed by processes requiring liquid water -- are only approximately 3.9 billion years old. This observation suggests that at least some water was present on the surface of the Earth by that time, but the early conditions remain unclear.
If the aggregating planetesimals that formed the Earth resembled the most abundant type of meteorites, the ordinary chondrites, they contained approximately 0.1 percent water by weight. An early Earth consisting of 0.1 percent water would contain 4 times the amount of water now held in the oceans.
Comets have been proposed as another possible source of water, but recent analysis of the 3 comets Halley, Hyakutake, and Hale-Bopp indicates these comets have a relatively high percentage of deuterium -- twice as much in the comets as in sea water, which poses a problem for any model involving comets as a source of early Earth's water.
More recently, it has been proposed that Earth has been continually bombarded by small house-sized comets, enough to fill the oceans with water over the lifetime of the Earth, but this hypothesis remains in sharp controversy.
Note: The water "trapped" in these meteorites is released upon intense heating (something that was very much present in the early earth ... due to impact heating, intense pressure, and radioactivity). This would account for the ample supply of water emitted by present day volcanoes. It should be noted that approximately 25% of this water emitted by volcanoes is believed to be "recycled" when subducting oceanic crust carries ocean water with it. It then finds its way back to the surface through volcanic vents.
According to Michael Drake, director of the University of Arizona's Lunar and Planetary laboratory, most of earth's water came from geochemical processes during the formation of the planet 4.6 billion years ago. His model suggests that water molecules clung to dust particles which were the first steps in planet building. These dust particles coalesced to form bigger and bigger objects which eventually collected to form the earth. This trapped lots of water inside the earth and it has been trying to get out ever since.
The Martian rovers, Spirit and Opportunity, have already concluded that Mars once had a lot of surface water in the past. Exactly where it went is still a mystery.
Chemical analysis of the atmosphere of Venus suggests that it, too, once had surface water.
Interesting topics:
Evidence of water on Jupiter's moon - Europa
Evidence of water flowing on Mars in its past - learn all about Mars here.
Astronomers find water
(vapor) in
deep space using the European Space Agency's Infrared Astronomy
Satellite ISO in 1996.
ŠJim Mihal 2004, 2006 - all rights reserved