Oceans are bodies of saline (salty) water that composes much of a planet's hydrosphere.


The Pacific ocean along the East Coast of  Australia. image: R.Conan-Davies

On Earth, the oceans are major conventional divisions of the World Ocean, which covers almost 71% of its surface. 

These are, in descending order by area, the: 

  • Pacific, 
  • Atlantic, 
  • Indian, 
  • Southern (Antarctic), and 
  • Arctic Oceans. 

Various ways to divide the World Ocean. image: wikipedia

Origin of the word ocean

The word ocean comes from Ancient Greek Ὠκεανός, transc. Okeanós, the sea of classical antiquity)

The word sea is often used interchangeably with "ocean" in American English but, strictly speaking, a sea is a body of saline water (generally a division of the world ocean) partly or fully enclosed by land.

Oceans cover much of the Earth

Saline water covers approximately 72% of the planet's surface (~3.6×108 km2) and is customarily divided into several principal oceans and smaller seas, with the ocean covering approximately 71% of 

Earth's surface and 90% of the earth's biosphere. The ocean contains 97% of Earth's water, and oceanographers have stated that less than 5% of the World Ocean has been explored. 

The total volume is approximately 1.35 billion cubic kilometres with an average depth of nearly 3,700 meters. 

Oceans have a significant biological and climate role

As it is the principal component of Earth's hydrosphere, the world ocean is integral to all known life, forms part of the carbon cycle, and influences climate and weather patterns. 

Thermo- meaning heat , haline meaning salty, circulation of the world’s oceans. image: wikipedia

It is the habitat of 230,000 known species, although much of the oceans depths remain unexplored, and over two million marine species are estimated to exist.  


Sharks are one of the top predators of the world’s oceans. image: R.Conan-Davies

The origin of Earth's oceans remains unknown; however, oceans are thought to have formed in the Hadean period and may have been the impetus for the emergence of life.

Oceanic divisions

Oceanic zones/divisions

1. Epipelagic zone: surface - 200 meters deep 

2. Mesopelagic zone: 200 - 1000m 

3. Bathypelagic zone: 1000m - 4000m 

4. Abyssopelagic zone: 4000m - 6000m 

5. Hadalpelagic zone (the trenches): 6000 m to the bottom of the ocean

note: The pelagic zone includes all open ocean regions.

Extraterrestrial oceans

There are also oceans on other planets or at other periods of time during the age of the solar system. Extraterrestrial oceans may be composed of water or other elements and compounds. 


Artists impression of what oceans on Mars may have looked like. image: wikipedia

The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, although there is evidence for the existence of oceans elsewhere in the Solar System. 

Early in their geologic histories, Mars and Venus are theorized to have had large water oceans. The Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, and a runaway greenhouse effect may have boiled away the global ocean of Venus.


An example of the  large hydrocarbon sea named Ligeia Mare on Saturn's moon Titan . image: NASA/JPL

Adapted from: Ocean. (2016, June 4). In Wikipedia, The Free Encyclopedia. Retrieved 09:42, June 8, 2016, from https://en.wikipedia.org/w/index.php?title=Ocean&oldid=723721274