Origin Of Water On Earth

[anbe__shivam]

The [b]origin of water on Earth[/b], or the reason why there is clearly more [url="http://en.wikipedia.org/wiki/Water"]water[/url] on the Earth than on the other planets of the [url="http://en.wikipedia.org/wiki/Solar_System"]Solar System[/url], has not been clarified. There are several acknowledged theories as to how the world's [url="http://en.wikipedia.org/wiki/Ocean"]oceans[/url] were formed over the past [url="http://en.wikipedia.org/wiki/History_of_Earth"]4.6 billion years[/url].

[u][b]Origins:[/b][/u]

Some of the most likely contributory factors to the origin of the [url="http://en.wikipedia.org/wiki/Earth"]Earth[/url]'s oceans are as follows:[list]
[*]The cooling of the [url="http://en.wikipedia.org/wiki/Hadean"]primordial[/url] Earth to the point where the [url="http://en.wikipedia.org/wiki/Outgassing"]outgassed[/url] volatile components were held in an [url="http://en.wikipedia.org/wiki/Earth%27s_atmosphere"]atmosphere[/url] of sufficient [url="http://en.wikipedia.org/wiki/Atmospheric_pressure"]pressure[/url] for the stabilization and retention of liquid water.
[*][url="http://en.wikipedia.org/wiki/Comet"]Comets[/url], [url="http://en.wikipedia.org/wiki/Trans-Neptunian_object"]trans-Neptunian objects[/url] or water-rich [url="http://en.wikipedia.org/wiki/Meteorite"]meteorites[/url] ([url="http://en.wikipedia.org/wiki/Protoplanet"]protoplanets[/url]) from the outer reaches of the main [url="http://en.wikipedia.org/wiki/Asteroid_belt"]asteroid belt[/url] colliding with the Earth may have brought water to the world's oceans. Measurements of the ratio of the [url="http://en.wikipedia.org/wiki/Hydrogen"]hydrogen[/url] [url="http://en.wikipedia.org/wiki/Isotope"]isotopes[/url] [url="http://en.wikipedia.org/wiki/Deuterium"]deuterium[/url] and [url="http://en.wikipedia.org/wiki/Hydrogen-1"]protium[/url] point to [url="http://en.wikipedia.org/wiki/Asteroid"]asteroids[/url], since similar percentage impurities in carbon-rich [url="http://en.wikipedia.org/wiki/Chondrites"]chondrites[/url] were found in oceanic water, whereas previous measurement of the isotopes' concentrations in comets and trans-Neptunian objects correspond only slightly to water on the earth.
[*]Biochemically through [url="http://en.wikipedia.org/wiki/Mineralization_(geology)"]mineralization[/url] and [url="http://en.wikipedia.org/wiki/Photosynthesis"]photosynthesis[/url].
[*]Gradual leakage of water stored in [url="http://en.wikipedia.org/wiki/Hydrate"]hydrous[/url] [url="http://en.wikipedia.org/wiki/Mineral"]minerals[/url] of the Earth's [url="http://en.wikipedia.org/wiki/Rock_(geology)"]rocks[/url].
[*][url="http://en.wikipedia.org/wiki/Photodissociation"]Photolysis[/url]: [url="http://en.wikipedia.org/wiki/Radiation"]radiation[/url] can break down chemical bonds on the surface.
[/list]

[u][b]Water in the development of the Earth:[/b][/u]

A sizeable quantity of water would have been in the material which formed the Earth.[sup][url="http://en.wikipedia.org/wiki/Origin_of_water_on_Earth#cite_note-0"][size=2][1][/size][/url][/sup] Water molecules would have escaped Earth's gravity more easily when it was less [url="http://en.wikipedia.org/wiki/Mass"]massive[/url] during its formation. Hydrogen and helium are expected to continually leak from the atmosphere, but the lack of denser [url="http://en.wikipedia.org/wiki/Noble_gas"]noble gases[/url] 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 [url="http://en.wikipedia.org/wiki/Giant_impact_hypothesis"]impact which created the Moon[/url], which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses.[sup][url="http://en.wikipedia.org/wiki/Origin_of_water_on_Earth#cite_note-1"][size=2][2][/size][/url][/sup] 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 [url="http://en.wikipedia.org/wiki/Carbon_dioxide"]carbon dioxide[/url] atmosphere with hydrogen and [url="http://en.wikipedia.org/wiki/Water_vapor"]water vapor[/url]. Liquid water oceans existed despite the surface temperature of 230°C because of the atmospheric pressure of the heavy CO[sub][size=2]2[/size][/sub] atmosphere. As cooling continued, [url="http://en.wikipedia.org/wiki/Subduction"]subduction[/url] and dissolving in ocean water removed most CO[sub][size=2]2[/size][/sub] from the atmosphere but levels oscillated wildly as new surface and [url="http://en.wikipedia.org/wiki/Mantle_(geology)"]mantle[/url] cycles appeared.[sup][url="http://en.wikipedia.org/wiki/Origin_of_water_on_Earth#cite_note-2"][size=2][3][/size][/url][/sup]
Study of [url="http://en.wikipedia.org/wiki/Zircon"]zircons[/url] has found that liquid water must have existed as long ago as 4.4 [url="http://en.wikipedia.org/wiki/Ga_(unit)"]Ga[/url], very soon after the formation of the Earth.[sup][url="http://en.wikipedia.org/wiki/Origin_of_water_on_Earth#cite_note-3"][size=2][4][/size][/url][/sup][sup][url="http://en.wikipedia.org/wiki/Origin_of_water_on_Earth#cite_note-4"][size=2][5][/size][/url][/sup][sup][url="http://en.wikipedia.org/wiki/Origin_of_water_on_Earth#cite_note-5"][size=2][6][/size][/url][/sup] This requires the presence of an atmosphere. The [url="http://en.wikipedia.org/wiki/Cool_Early_Earth"]Cool Early Earth[/url] 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 [url="http://en.wikipedia.org/wiki/Plate_tectonics"]plate tectonics[/url] as early as 4 billion years ago. If this holds true, the previous beliefs about the [url="http://en.wikipedia.org/wiki/Hadean"]Hadean period[/url] are far from correct. That is, rather than a hot, molten surface and atmosphere full of carbon dioxide, the Earth's surface would be very much like it is today. The action of plate tectonics traps vast amounts of carbon dioxide, thereby eliminating the greenhouse effects and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life.

[u][b]Extraterrestrial sources:[/b][/u]


That the Earth's water originated purely from comets is implausible, as a result of measurements of the isotope ratios of hydrogen in the three comets [url="http://en.wikipedia.org/wiki/Halley%27s_Comet"]Halley[/url], [url="http://en.wikipedia.org/wiki/Hyakutake"]Hyakutake[/url] and [url="http://en.wikipedia.org/wiki/Hale-Bopp"]Hale-Bopp[/url] by researchers like [url="http://en.wikipedia.org/wiki/David_Jewitt"]David Jewitt[/url], as according to this research the ratio of [url="http://en.wikipedia.org/wiki/Deuterium"]deuterium[/url] to [url="http://en.wikipedia.org/wiki/Hydrogen-1"]protium[/url] (D/H ratio) of the comets is approximately double that of oceanic water. What is however unclear is whether these comets are representative of those from the [url="http://en.wikipedia.org/wiki/Kuiper_Belt"]Kuiper Belt[/url]. According to A. Morbidelli [sup][url="http://en.wikipedia.org/wiki/Origin_of_water_on_Earth#cite_note-7"][size=2][8][/size][/url][/sup] the largest part of today's water comes from protoplanets formed in the outer asteroid belt that plunged towards the Earth, as indicated by the D/H proportions in carbon-rich chondrites. The water in carbon-rich chondrites point to a similar D/H ratio as oceanic water. Nevertheless, mechanisms have been proposed[sup][url="http://en.wikipedia.org/wiki/Origin_of_water_on_Earth#cite_note-8"][size=2][9][/size][/url][/sup] to suggest that the 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.

[u][b]Role of organisms:[/b][/u]


In the early 1930s [url="http://en.wikipedia.org/wiki/Cornelis_Van_Niel"]Cornelis Van Niel[/url] discovered that sulfide-dependent chemoautotrophic bacteria ([url="http://en.wikipedia.org/wiki/Purple_sulfur_bacteria"]purple sulfur bacteria[/url]) fix carbon and synthesize water as a byproduct of a photosynthetic pathway using [url="http://en.wikipedia.org/wiki/Hydrogen_sulfide"]hydrogen sulfide[/url] and carbon dioxide[sup][url="http://en.wikipedia.org/wiki/Origin_of_water_on_Earth#cite_note-9"][size=2][10][/size][/url][/sup] :
[img]http://upload.wikimedia.org/math/2/0/a/20a739e1642dde225c73e2bc2b083768.png[/img]
Few organisms utilize this method of photosynthesis today, making their water contribution negligible. But on the [url="http://en.wikipedia.org/wiki/Hydrogen_sulfide"]hydrogen sulfide[/url]-rich and oxygen-poor early planet, a small but significant portion of the earth's water may have been synthesized biochemically through this pathway.

[b][color=#FF0000]SOURCE: WIKI[/color][/b]

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