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Planet Ocean

Oceans cover almost three quarters (71%) of the surface of the Earth, and nearly half of the world's marine waters are over 3,000 meters deep. The area of the oceans is 361 million sq. km.

Geologically, an ocean is an area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the Earth's mantle where there are no continents. From this point of view, there are three "oceans" today: the World Ocean, and the Black and Caspian Seas that were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is very nearly its own "ocean", being connected to the World Ocean through the Strait of Gibraltar, and indeed several times over the last few million years movement of the African Continent has closed the strait off entirely, making the Mediterranean a fourth "ocean". (The Black Sea is connected to the Mediterranean through the Bosporus, but this is in effect a natural canal cut through continental rock some 7,000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.)

The area of the World Ocean is 361 million square kilometers, its volume is over 1,340 million cubic kilometers, and its average depth is 3,711 meters. Nearly half of the world's marine waters are over 3,000 meters deep. The vast abyssal plains of the deep ocean cover about 40% of the Earth's surface. This does not include seas not connected to the World Ocean, such as the Caspian Sea.
The total mass of the hydrosphere is about 1.4x1021 kilograms, which is about 0.023% of the Earth's total mass.

Extraterrestrial oceans
Earth is the only known planet with liquid water on its surface and is certainly the only one in our own solar system. However, liquid water is thought to be present under the surface of several natural satellites, particularly the Galilean moons of Europa, and, with less certainty, Callisto and Ganymede. Geysers have been found on Enceladus. Other icy moons may have once had internal oceans that have now frozen, such as Triton. The planets Uranus and Neptune may also possess large oceans of liquid water under their thick atmospheres, though their internal structure is not well understood at this time.
There is currently much debate over whether Mars once had an ocean of water in its northern hemisphere, and over what happened to it if it did; recent findings by the Mars Exploration Rover mission indicate it had some long-term standing water in at least one location, but its extent is not known.
Liquid hydrocarbons were thought to be present on the surface of Titan, though it may be more accurate to describe them as "lakes" rather than an "ocean". The Cassini-Huygens space mission initially discovered only what appeared to be dry lakebeds and empty river channels, suggesting that Titan had lost what surface liquids it might have had. A more recent fly-by of Titan made by Cassini has produced radar images that strongly suggest hydrocarbon lakes near the polar regions where it is colder. Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.

Picture taken in Port Douglas over the great barrier reef with a fisheye.

This picture has been published in: Troubled Waters “Ocean Deserts” are Expanding, Disrupting Habitats and Suffocating Marine Life By Jessica A. Knoblauch, in The Environmental Magazine, Sept-Oct Vol XIX, number 5, pp. 16-18 (2008)

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Additional Photos by Michel Detay (mdetay) Gold Star Critiquer/Gold Note Writer [C: 487 W: 1 N: 1045] (4929)
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