All life on Earth depends on water. Humans use water for many purposes like drinking, irrigation, fisheries, industrial processes, transportation, and waste disposal. Water is also an essential part of the geological cycle. Rain water converts the granitic rocks of the continents to clay, sand and solutes, and transports them to the ocean where they become the raw material of future continents. Approximately 80 percent of the water on the Earth is in the oceans, 19 percent is in the pores of rocks beneath the Earth’s surface, 1 percent in the form of ice, 0.002 percent is in the streams and lakes, and only about 0.0008 percent in the atmosphere.
Considering the central role of water in human affairs, it is remarkable how little we know about it.
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:
1. The cooling of the primoridal 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.
Today, the air we breathe is stable mixture of 79 percent nitrogen, 20 percent oxygen, about 1 percent argon (or inert gas), and trace gases like carbon dioxide and water vapour. But our planet’s original atmosphere, several billion year ago, was far different. Earth’s very earliest atmosphere probably was swept into space by the solar wind, a vast stream of particles emitted by the Sun. as Earth slowly cooled, a more enduring atmosphere formed. The molten surface solidified into a crust, and gases that had been dissolved in the molten rock were gradually released, a process called outgassing. Outgassing continues today from hundreds of active volcanoes worldwide, thus, geologists hypothesize the Earth’s original atmosphere was made up of gases similar to those released in volcanic emissions today: water vapor, carbondioxide, nitrogen, and several trace gases.
As the planet continued to cool, the water vapor condensed to form clouds, and great rain commenced. At first, the water evaporated in the hot air before reaching the ground, or quickly boiled away upon contacting the surface, just like water sprayed on a hot grill. This accelerated the cooling of Earth’s crust. When the surface had cooled below water’s boiling point (100 degree c or 212 degree F), torrential rains slowly filled the low areas, forming the oceans.
2. 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. 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. According to this research the ratio of deuterium to protium (D/H ratio) of the comets is approximately double that of oceanic water.The Earth is believed to have formed hot and dry, meaning that its current water content must have been delivered after the planet cooled. Possible candidates for supplying this water are colliding comets and asteroids. Because of their large ice comet water has shown that comet water is significantly different from typical ocean water on Earth.
Asteroidal ice may give a better match to Earth's water, but until now, any ice that the asteroids may have once contained was thought to either be long gone or so deeply buried inside large asteroids as to be inaccessible for further analysis.
3. Gradual leakage of water stored in hydrous minerals (actinolite, borax, epsomite, serpentine, tremolite, gypsum etc.) of the Earth’s crust. The heating or metamorphism of minerals containing water results in the extraction of water. These are the water that have been trapped inside rocks for millions or billions of years. Loss of volatile constituents, H2O, CO2, and the like, are the dominant processes which occur when rocks change their pressure-temperature environment and undergo prograde metamorphism through tectonic processes.
4. Magma represents a fiery-liquid silicate melt, containing various elements, oxides and volatile components (fluorine, chlorine, water, carbon dioxide, etc.). Magma can be solidified in the depth of the Earth’s crust under the cover of the overlying rocks and at the surface or near the surface of the Earth. In the former case the process of solidification of magma is slow; it takes the whole of magma enough time to be crystallized. When there is a rapid uplift of the magma on to Earth’s surface its temperature becomes lower, the pressure drops down to normal, and volatile components are separated including water. Release of water to the atmosphere from the cooling of the magma is happening from millions of years.
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