::Extraterrestrial liquid water


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Ocean Worlds in the Solar System<ref name="NASA-20150407" />
Warm season flows in Palikir Crater (inside Newton crater) on Mars. While there is intriguing but inconclusive evidence suggestive of extraterrestrial liquid water, it has so far eluded direct confirmation.

Extraterrestrial liquid water (from the Latin words: extra ["outside of, beyond"] and terrestris ["of or belonging to Earth"]) is water in its liquid state that is found beyond Earth. It is a subject of wide interest because it is commonly thought to be one of the key prerequisites for extraterrestrial life.<ref name="NASA-20150407">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

With oceanic water covering 71% of its surface, Earth is the only planet known to have stable bodies of liquid water on its surface,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and liquid water is essential to all known life forms on earth. The presence of water on the surface of Earth is a product of its atmospheric pressure and a stable orbit in the Sun's circumstellar habitable zone, though the origin of Earth's water remains unknown.

The main methods currently used for confirmation are absorption spectroscopy and geochemistry. These techniques have proven effective for atmospheric water vapour and ice. However, using current methods of astronomical spectroscopy it is substantially more difficult to detect liquid water on terrestrial planets, especially in the case of subsurface water. Due to this, astronomers, astrobiologists and planetary scientists use habitable zone, gravitational and tidal theory, models of planetary differentiation and radiometry to determine potential for liquid water. Water observed in volcanic activity can provide more compelling indirect evidence, as can fluvial features and the presence of antifreeze agents, such as salts or ammonia.

Using such methods, many scientists infer that liquid water once covered large areas of Venus and Mars. Water is thought to exist as liquid beneath the surface of planetary bodies, similar to groundwater on Earth. Water vapour is sometimes considered conclusive evidence for the presence of liquid water, although atmospheric water vapour may be found to exist in many places where liquid water does not. Similar indirect evidence, however, supports the existence of liquids below the surface of several moons and dwarf planets elsewhere in the Solar System.<ref name="NASA-20150407" /> Some are speculated to be large extraterrestrial "oceans".<ref name="NASA-20150407" /> Liquid water is thought to be common in other planetary systems, despite the lack of conclusive evidence, and there is a growing list of extrasolar candidates for liquid water.

Extraterrestrial liquid water sections
Intro   Liquid water in the Solar System    Methods of detection and confirmation    History    Evidence of past surface water    Liquid water inside comets    Extrasolar habitable zone candidates for water    See also    References    External links   

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