Origin of water on Earth

Water covers about 70% of the Earth's surface.

The question of the origin of water on Earth, or more accurately put, the question of why there is clearly more water on the Earth than on the other planets of the Solar System, has not been clarified. There are various popular theories as to how the world's oceans were formed over the past 4.6 billion years. Some of the most likely contributing factors to the origin of the Earth's oceans are as follows:

• The cooling of the primordial Earth to the point where the outgassed volatile components were held in an atmosphere of sufficient pressure for the stabilization and retention of liquid water.
• Comets, trans-Neptunian objects or water-rich asteroids (protoplanets) from the outer reaches of the asteroid belt colliding with a pre-historic Earth may have brought water to the world's oceans. Measurements of the ratio of the hydrogen isotopes deuterium and protium point to asteroids, since similar percentage impurities in carbon-rich chondrites were found to 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 mineralization and photosynthesis (guttation, transpiration).
• Gradual leakage of water stored in hydrous minerals of the Earth's rocks.
• Photolysis: radiation can break down chemical bonds on the surface.

Water in the development of the Earth

When the earth was at the planetesimal stage, there was probably already water present. This water and other lightweight, fluid constituents such as carbon dioxide (CO₂), methane (CH₄) and nitrogen (N₂) originated mostly from eruptions or outgassings of the primal earth and formed a young, water-vapour free primal earth atmosphere. These were, according to present models and simulations, carried away by the solar wind that at the time of the formation of the Earth was much stronger than today, and so escaped the Earth until it had about 40% its current radius (and gravity could retain the atmosphere). Later through volcanism came the creation of a newer atmosphere, which may also have contained water-vapour released from the earth's interior. With the development of a solid earth's crust and further cooling down, the water vapour condensed and hence formed the first oceans.

The large amount of Arlene ' and gardenea alarconm hsahgduhasgfjhasgfjhagdhasdwater that is present on the Earth in comparison to other earth-like bodies cannot be alone explained by that released from the earth's interior. The planetesimals formed in a period of the early Solar system, when there was relatively little water around. The closer to the sun one was, the higher the temperature and the less water present. First, outside the solar "snow line," which lay roughly where the Asteroid belt is today, water could be found in considerable abundance. Carbonaceous chondrites, which it is generally agreed formed in the outer reaches of the asteroid belt, indicate a water content of sometimes more than 10% of their weight, whereas common chondrites or enstatite chondrites from the nearer regions of the asteroid belt comprise less than 0.1% of their weight in water.. Moreover it can be supposed that during the accretion of the planetesimals into planets and the loss of the primitive atmosphere would result in the larger proportion of the originally present water being lost. Hence it is in many cases assumed that the majority of the water present on the Earth today came from the outer regions of the Solar System.

Extraterrestrial sources

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 Halley, Hyakutake and Hale-Bopp by researchers like David Jewitt, as according to this research the ratio of deuterium to protium (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 Kuiper Belt. According to A. Morbidelli the largest part of today's water comes from protoplanets formed in the outer asteroid belt that plunged toward 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.

The large amount of Earth's water may be explained, if the object which struck Earth over 4 billion years ago and created Earth's moon was a large icy body, similar to Europa (Jupiter's 6th moon). The huge volume of water on Earth could be accounted for by this cosmic collision.

Role of organisms

In the primordial seas hydrogen sulfide and in the primitive atmosphere present carbon dioxide was used by sulfide-dependent chemoautotrophic bacteria (prokaryotes) with the supply of light energy for the creation of organic compounds, whereby water and sulfur resulted:

$\mathrm {4\ H_{2}S+CO_{2}\rightarrow CH_{4}+2\ H_{2}O+4\ S}$

The greatest proportion of today's water may have been synthesized biochemically through mineralisation and photosynthesis (guttation, transpiration).

See also

• Evolution of water on Mars and Earth

References

• Jörn Müller, Harald Lesch (2003): Woher kommt das Wasser der Erde? - Urgaswolke oder Meteoriten. Chemie in unserer Zeit 37(4), pg. 242 – 246, ISSN 0009-2851
• Parts of this article were translated from the original article from the German Wikipedia, on 4/3/06

External links

• Dr. C's Oceans Online website (archived copy)
• UniverseToday.com