Our Earth the blue planet . The reason for it’s beauty is water which covers 71% of its surface . But from where did the water came from which fills the oceans of Earth?Many of the hypothesises have been given to explain origin of earth’s water , although none of them is clearly accepted.
The Possible Source for origin of earth’s water can be – Extraplanetary sources Or internal sources.
The water may have come from the outer reaches of asteroid belt. Celestial bodies such as comets, water-rich- meteoroids(proto planets) may have collided to the Earth significantly over the years to fill oceans of earth. Previous measurement of the hydrogen isotopes’ concentrations in comets and trans-Neptunian objects correspond only slightly to water on Earth.
Measurements of the isotope ratios of deuterium to protium (D/H ratio) in the four comets Halley, Hyakutake, Hale-Bopp , and 67P/Churyumov–Gerasimenko, by researchers such as David Jewitt, is approximately double that of oceanic water.
So comets and trans-Neptunian objects must haven’t brought lots of water on earth.
Measurements of the ratio of the hydrogen isotopes deuterium and protium point to asteroids, since similar percentage impurities in carbon-rich chondrites were found in oceanic water.
Now the studies have shown that water with similar deuterium-to-hydrogen ratio was already available at the time of Earth’s formation, as evidenced in ancient “eucrites” meteorites originating from the asteroid Vesta.
According to Alessandro Morbidelli, the largest part of today’s water comes from protoplanets formed in the outer asteroid belt as the D/H proportions of water in carbon-rich chondrites point to a similar D/H ratio as oceanic water. Nevertheless, mechanisms have been proposed which suggest D/H-ratio of oceanic water may have increased significantly throughout Earth’s history. Such a proposal is consistent with the possibility that a significant amount of the water on Earth was already present during the planet’s early evolution.
Measurements of the chemical composition of Moon rocks suggest that Earth was born with its water already present. Investigating lunar samples carried to Earth by the Apollo 15 and 17 missions found a deuterium-to-hydrogen ratio that matched the isotopic ratio in carbonaceous chondrites. The ratio is also similar to that found in water on Earth. The findings suggest a common source of water for both objects. This supports a theory that Jupiter temporarily migrated into the inner Solar System, destabilizing the orbits of water-rich carbonaceous chondrites. As a result, some of the bodies could have fallen inwards and become part of the raw material for making Earth and its neighbors. The discovery of water vapor out-gassing from Ceres provides related information on water-ice content of the asteroid belt.
Gradual “dehydration melting” — leakage of water stored in hydrate
minerals of Earth’s rocks — could have formed a portion of its water.
Water may also have come from volcanism: water vapor in the atmosphere that originated in volcanic eruptions may have condensed to form rain, slowly filling Earth’s oceanic basins.
A sizeable quantity of water would have been in the material that formed Earth. Water molecules would have escaped Earth’s gravity more easily when it was less massive during its formation. Hydrogen and helium are expected to leak from the atmosphere continually, but the lack of denser noble gases in the modern atmosphere suggests that something disastrous happened to the early atmosphere.
Part of the young planet is theorized to have been disrupted by the impact which created the Moon, which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to melt and mix huge rock masses completely. However, a fair fraction of material should have been vaporized by this impact, creating a rock-vapor atmosphere around the young planet. The rock-vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy
carbon dioxide atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230°C because of the atmospheric pressure of the heavy CO2 atmosphere. As cooling continued, subduction and dissolving in ocean water removed most CO2 from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.
Study of zircons has found that liquid water must have existed as long ago as 4.404 ± 0.008 Ga , very soon after the formation of Earth.  This requires the presence of an atmosphere. The Cool Early Earth theory covers a range from about 4.4 Ga to 4.0 Ga.
In fact, recent studies of zircons (in the fall of 2008) found in Australian Hadean rock hold minerals that point to the existence of plate tectonics as early as 4 billion years ago. If this holds true, the previous beliefs about the Hadean period are far from correct. That is, rather than a hot, molten surface and atmosphere full of carbon dioxide, Earth’s surface would be very much like it is today. The action of plate tectonics traps vast amounts of carbon dioxide, thereby eliminating greenhouse effects, and leading to a much cooler surface temperature, and the formation of solid rock, and possibly even life.
Role of organisms
Some terrestrial water may have had a biochemical origin, during the Great Oxygenation Event , via redox reactions and photosynthesis .
In the early 1930s, Cornelis Van Niel discovered that sulfide-dependent chemoautotrophic bacteria ( purple sulfur bacteria ) fix carbon and synthesize water as a byproduct of a photosynthetic pathway using
hydrogen sulfide and carbon dioxide:
CO2 + 2H 2S → CH 2O + H 2O + 2S
Few modern organisms use this method of photosynthesis, making their water contribution negligible. But on the hydrogen-sulfide-rich and oxygen-poor early Earth, a small but significant portion of Earth’s water may have been synthesized biochemically through this pathway.