Origin of the Hydrosphere in Structure of Biomolecules

ORIGIN OF HYDROSPHERE

The hydrosphere was formed 4 billion years ago from the condensation of water vapor, resulting in oceans of water in which sedimentation occured. The Earth is thought to have HYPERLINK "http://www.britannica.com/science/accretion" accreted from a cloud of ionized particles around the Sun. This gaseous matter condensed into small particles that coalesced to form a protoplanet, which in turn grew by the gravitational attraction of more particulates. At an early stage the Earth thus did not have water or water vapour at its surface. Once the planet’s surface had cooled sufficiently, water contained in the minerals of the accreted material and released at depth could escape to the surface and, instead of being lost to space, cooled and condensed to form the initial hydrosphere.

A large, cool Earth most certainly served as a better trap for water than a small, hot body because the lower the temperature, the less likelihood for water vapour to escape, and the larger the Earth, the stronger its gravitational attraction for water vapour. It is likely that the hydrosphere attained its present volume early in the Earth’s history, and since that time there have been only small losses and gains. Gains would be from continuous HYPERLINK "http://www.britannica.com/science/degassing" degassing of the Earth; the present degassing rate of juvenile water has been determined as being only 0.3 cubic kilometre per year. Water loss in the upper atmosphere is by photodissociation, the breakup of water vapour molecules into hydrogen and oxygen due to the energy of ultraviolet light. The hydrogen is lost to space and the oxygen remains behind. Only about 4.8 × 10−4 cubic kilometre of water vapour is presently destroyed each year by photodissociation. The very cold temperatures of the upper atmosphere result in a cold trap at an altitude of about 15 kilometres, where most of the water vapour condenses and returns to lower altitudes, thereby escaping photodissociation. Since the early formation of the hydrosphere, the amount of water vapour in the atmosphere has been regulated by the temperature of the Earth’s surface—hence its radiation balance.

The early hydrosphere

The gases released from the Earth during its early history, including water vapour, have been called HYPERLINK "http://www.britannica.com/science/excess-volatile" excess HYPERLINK "http://www.britannica.com/science/volatile-component" volatiles because their masses cannot be accounted for simply by rock weathering. These volatiles are thought to have formed the early atmosphere of the Earth. At an initial crustal temperature of about 600° C, almost all of these compounds, including H2O, would have been in the atmosphere. Below 100° C all of the water would have condensed, and the acid gases reacted with the original igneous crustal minerals to form sediments and an initial hydrosphere that was dominated by a salty ocean.

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