Hydrosphere
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A hydrosphere (Greek hydro means "water") in physical geography describes the collective mass of water found on, under, and over the surface of a planet.
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[edit] Earth's hydrosphere
The Earth's hydrosphere consists of water in its liquid, solid, and gaseous forms: the ocean (which comprises the bulk of the hydrosphere), other surface waters including inland seas, lakes, and rivers; rain; underground water; ice (as in glaciers and snow); and atmospheric water vapor (as in clouds). The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1.35 × 1018 tonnes, or about 1/4400 of the total mass of the Earth (ranges reported: 1.347 × 1021 to 1.4 × 1021 kg.[1] )
The abundance of water on Earth is a unique feature that distinguishes our "Blue Planet" from others in the solar system. Approximately 70.8 percent (97% of it being sea water and 3% fresh water[2]) of the Earth is covered by water and only 29.2 percent is landmass. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make Earth a water planet.
Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, Earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (Cyanobacteria) had colonized the oceans, the greenhouse effect failed, and Earth's oceans may have completely frozen over for 10 to 100 million years in what is called a snowball Earth event.
On other planets, such as Venus, gaseous water is destroyed (cracked) by solar ultraviolet radiation, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. This is one hypothesis explaining why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.
In the Earth's atmosphere, a tenuous layer of ozone within the stratosphere absorbs most of this energetic ultraviolet radiation high in the atmosphere, reducing the cracking effect. The ozone, too, can only be produced in an atmosphere with a large amount of free diatomic oxygen, and so also is dependent on the biosphere (plants). The magnetosphere also shields the ionosphere from direct scouring by the solar wind.
Finally, volcanism continuously emits water vapor from the interior. Earth's plate tectonics recycle carbon and water as limestone rocks are subducted into the mantle and volcanically released as gaseous carbon dioxide and steam. It is estimated that the minerals in the mantle may contain as much as 10 times the water as in all of the current oceans, though most of this trapped water will never be released.
The water cycle describes the methods of transport for water in the hydrosphere. This cycle includes water beneath the Earth's surface and in rocks (lithosphere), the water in plants and animals (biosphere), the water covering the surface of the planet in liquid and solid forms, and the water in the atmosphere in the form of water vapor, clouds, and precipitation. Movement of water within the hydrosphere is described by the hydrologic cycle. It is easy to see this motion in rivers and streams, but it is harder to tell that there is this motion in lakes and ponds.
The water in the oceans moves as it is of different temperature and salinity on different locations. Surface waters are also moved by winds, giving rise to surface ocean currents. Warm water is lighter or less dense than cold water which is more dense or heavier and salty water is also more dense than fresh water. The combination of the water's temperature and salinity determines whether it rises to the surface, sinks to the bottom, or stays at some intermediate depth.
[edit] History
[edit] Formation
There are several theories regarding the formation of the hydrosphere on the Earth. The planet contains proportionately more surface water than comparable bodies in the inner solar system. Outgassing of water from the interior of the Earth is not sufficient to explain the quantity of water.
A hypothesis that has gained popularity among scientists is that the early Earth was subjected to a period of bombardment by Comets and water-rich Asteroids. Much of the water on the surface today is thought to have originated from the outer parts of the solar system, such as from trans-Neptunian objects.
[edit] Ice ages
During the history of the Earth there have been a series of periods in which a significant portion of the hydrosphere was locked up in the form of glacial ice. It has even been hypothesized that during the Cryogenian period this sea ice extended all the way to the equator. (See Snowball Earth).
In all there are currently believed to have been four major ice ages during the Earth's history. The current ice age began about 40 million years ago, and gained in intensity during the Pleistocene. The most recent withdrawal of the ice sheets occurred only 10,000 years ago.
[edit] Life
All currently recognized forms of life rely on an active hydrosphere. The water cycle in the Earth's hydrosphere allows for the purification of salt water into fresh water. Evaporation and wetland swamps serve to remove a large portion of atmospheric pollutants from the atmosphere (ie. acid rain). Through this process the water cycle purifies the gaseous atmosphere. Although most life on the planet exists in the salt water oceans, humans are particularly interested in the hydrosphere because it provides the fresh water we depend upon.
The search for life in other celestial bodies in our solar system is focused on first locating water. The hydrosphere's of other planetary bodies is also the focus of research to find places that humans can inhabit without having to transport all their water with them.
[edit] Extinction
Scientists estimate that in approximately 4 × 109 years the Sun will have exhausted the supply of Hydrogen in its core and will evolve into a supergiant. The outer atmosphere will expand significantly and the planet Earth will lie within the photosphere. During this process the surface temperature will rise far above the boiling point of water, and all water on the Earth's surface will evaporate.
[edit] Other hydrospheres
A thick hydrosphere is thought to exist around the Jovian moon Europa. The outer layer of this hydrosphere is almost entirely frozen, but current models predict that there is an ocean up to 100 km in depth underneath the ice. This ocean remains in a liquid form due to tidal flexing of the moon in its orbit around Jupiter.
It has been suggested that the moon Ganymede may also possess a sub-surface ocean. However the ice covering is expected to be thicker on Ganymede than on Europa.
[edit] See also
[edit] References
- ^ The Physics Factbook, http://hypertextbook.com/facts/1998/AvijeetDut.shtml
- ^ http://earthobservatory.nasa.gov/Library/Water/
