Origin of water on Earth
The origin of water on Earth, or the reason that there is clearly more water on the Earth than on the other planets of the Solar System, has not been clarified. There are several acknowledged theories as to how the world's oceans were formed over the past 4.6 billion years.
|This section does not cite any references or sources. (August 2013)|
Some of the most likely contributory factors to the origin of the Earth's oceans are as follows:
- The cooling down of the primordial world 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 meteoroids (protoplanets) from the outer reaches of the main asteroid belt colliding with the 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 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 mineralization and photosynthesis.
- 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
A sizeable quantity of water would have been in the material which formed the 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 continually leak from the atmosphere, 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 completely melt and mix huge rock masses. 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 4.4 Ga, very soon after the formation of the 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, 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.
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 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 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.
Role of organisms
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:
Few organisms utilize this method of photosynthesis today, making their water contribution negligible. But on the hydrogen sulfide-rich and oxygen-poor early planet, a small but significant portion of the earth's water may have been synthesized biochemically through this pathway.
Seeds of Life in Asteroids
Researchers have proposed a new and improved theory to explain how biomolecules were once able to form inside of asteroids. These researchers at Renesselaer Polytechnic Institute theory aim to explain the ancient heating of the asteroid belt. The asteroid belt is located between Mars and Jupiter. Although it is now cold and dry in the past years scientists have concluded that at one point it had wet and warm conditions that would cater to the formation of biomolecules. Biomolecules were found inside of meteorites, which were originated, in the asteroid belt. To figure this out two researchers Wayne Roberge and Ray Menzel examined two theories about how asteroids could have been heated in the young solar system. The first theory involved the same radioactive process that is used to describe how the Earth is heated. The second theory involved the interaction between plasma and the magnetic field. Some of today’s models of radioactive heating do help with making predictions that temperatures in the asteroid belt are inconsistent with observations. This piece of information motivated Roberge and Mendel to take further look into the second theory. This theory discusses the early assessment of the sun and the suggestion that an object moving through the magnetic field of the solar system will experience an electric field. This electric field will then push electrical currents through the asteroid. The asteroid will than be heated such as an electrical currents heating the wires in a toaster. To go deeper in their research Mendel and Roberge had to calculate the electric field everywhere. They calculated the electric field in the interior of the asteroid. By doing this research with the electric field they concluded that the asteroids would have experienced the solar wind, plasma conditions and multi-fluid magneto-hydrodynamics. In multi-fluid magneto-hydrodynamics plasma is very weakly ionized and neutral particles behave distinctly from the charged particles. The neutral particles will interact with charged particles by friction. This creates a problem of treating the dynamics of the neutral gas and allowing for the presence of the small number of charged particles interacting with the magnetic field. Although this theory is promising it raises many questions that will have to be further examined. Roberge and Mendel feel as though they have discovered new information and look forward to more exploration of this theory.
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