Extraterrestrial liquid water

Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and is thus surmised to be essential for extraterrestrial life.

Although many celestial bodies in the Solar System have a hydrosphere, Earth is the only celestial body known to have stable bodies of liquid water on its surface, with oceanic water covering 71% of its surface, which is essential to life on Earth. The presence of liquid water is maintained by Earth's atmospheric pressure and stable orbit in the Sun's circumstellar habitable zone; however, the origin of Earth's water remains uncertain.

The main methods currently used for confirmation are absorption spectroscopy and geochemistry. These techniques have proven effective for atmospheric water vapor 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 the 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 Mars and Venus. Water is thought to exist as liquid beneath the surface of some 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. Some are speculated to be large extraterrestrial "oceans". 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. In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies.

Water is a fundamental element for the biochemistry of all known living beings. With some areas on Earth such as deserts being drier than others, their local lifeforms are adapted to make efficient use of the scarce available water.

No known lifeform can live completely without water.

It is also one of the simplest molecules, composed of one oxygen and two hydrogen atoms, and can be found in all celestial bodies of the solar system. It is only useful for life in a liquid state, and extraterrestrial water is commonly found as water vapor or ice. Although life eventually adapted to live on land, the first forms of life on Earth appeared in liquid water. As a result, the search for extraterrestrial liquid water is closely related with the search of extraterrestrial life.

Liquid water also has several properties that are beneficial for lifeforms. For example, unlike most other liquids, it becomes less dense when it solidifies rather than denser. As a result, if a body of water gets cold enough, the ice floats and eventually creates an ice layer, trapping liquid water and its ecosystems below. Without this property, lakes and oceans would become ice in their full size, along with any creatures living in them.