Earth: the Origins of Water - Essay Example

Kasting weighs the various theories regarding the source of the Earth’s water. These theories are concerned with the bombardment of the Earth by “icy clumps of material” (30), or planetesimals. This collision results in the formation of steam in the atmosphere. After some steam is ejected into the sky, and some are split by ultraviolet radiation, sufficient steam remains in the atmosphere to condense and form the oceans when the Earth cools. The origin of these planetesimals is subject to different viewpoints. They are variously argued to be asteroids, comets and meteorites.

Plate tectonics is responsible for the creation and present configuration of the ocean basins, which serve as containers to hold water. This process agitates the Earth’s mantle, separating it into the lighter granitic rock which floats on top to form the continents and the heavier basalt which sinks to form the ocean basins.

The Earth’s orbit is another important factor for the presence of water. To one side of the Earth, Venus is so close to the sun, that water evaporates skyward, and is split by the sun’s ultraviolet rays into hydrogen which is lost into the sky. On the other side, Mars is too far from the sun and remains frozen due to runaway glaciation. Unlike its neighbors, the Earth’s orbit lies in the ‘habitable zone’, with regard to its distance from the sun, thus permitting the presence of liquid water.

The Earth’s stratified atmosphere, due to the significant presence of carbon dioxide and methane, which absorb infrared radiation, results in the greenhouse effect, keeping the planet warm even during the age of the early, faint sun. This prevents the loss of water and hydrogen into the sky.

Volcanism is the last factor that contributes to the presence of water on Earth, due to the carbonate-silicate cycle. Volcanism gives the Earth a “built-in (geochemical) thermostat” (33), which regulates the Earth’s climate and maintains the surface temperature in a range that permits liquid water. Volcanoes spew carbon dioxide into the atmosphere. This gas is absorbed by silicate minerals, which erode and sink into the oceans as solid calcium carbonate. Plate tectonics drive these deposits of carbonate to the Earth’s mantle, from where it is again ejected into the air as carbon dioxide by volcanism.

Kasting gives a very lucid picture of the factors contributing to the presence of water on Earth. However, although he gives a comprehensive account of the theories regarding the origin of water, no conclusion is drawn. Kasting does not unequivocally weigh in on the side of anyone's theory. He concludes by admitting the possibility of there being other earth-like planets, with oceans, orbiting other stars.