Question

How and when water (seas and oceans) started existence on our earth?

We know that the Earth's ocean water probably rained down from the sky as the Earth cooled about 4 billion years ago. The real question is where did all that water come from? The answer is still unknown, however, scientists have come up with at least two possible ideas. The first theory is that the Earth always contained enough basic elements to make up water--hydrogen and oxygen--and that most of these elements were trapped separately in either hydrocarbons or iron oxides below the Earth's crust. The trapped water was later released as volcanic steam as the earth cooled. Once the steam entered the cooler atmosphere, it condensed into the rain that eventually filled the Earth's oceans. The second theory is that the earth collected the water over a long period of time as ice-bearing comets and meteorites struck its surface and atmosphere. The ice from many millions of small comets or meteorites over time may have condensed in the atmosphere to eventually rain down the Earth's water. Some evidence for this theory is the rate water loss from the Earth's natural geological processes. Without a constant influx of extraterrestrial water, some scientists say, the Earth would be dry as a bone. Further evidence for this theory lies in the (heavily disputed) rate of small comet bombardments in the upper atmosphere. Some measurements of "atmospheric holes" claim that one 20 to 40 ton comet vaporizes in the earth's atmosphere every three seconds, and that this rate would add about one inch of water to the Earth's surface every 20,000 years. One of several arguments against this theory is that if most or all the earth's oceans were derived from comets or meteorites, we should be seeing much higher levels of deuterium (heavy hydrogen) and xenon in the Earth's present day surface and atmosphere. While both theories remain in heated debate, scientists will readily admit the origin of the Earth's oceans is still very much unknown. Many suspect that the oceans may have resulted from some sort of combination of both theories. For more information on this debate, see the following Scientific American site that concerns your exact question: http://www.sciam.com/askexpert/environment/environment13.html Anna Lee Strachan, NASA Astrobiology Institute
December 11, 2001