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.
Significance 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.Aguilera Mochon, p. 8 As a result, the search for extraterrestrial liquid water is closely related with the search of extraterrestrial life.Aguilera Mochon, p. 12
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.Aguilera Mochon, p. 18
Liquid water in the Solar System
As of December 2015, the confirmed liquid water in the Solar System outside Earth is 25–50 times the volume of Earth's water (1.3 billion km3), i.e. about 3.25–6.5 × 1010 km3 (32.5 to 65 billion km3) and 3.25–6.5 × 1019 tons (32.5 to 65 billion billion tons) of water.
Mars
The Mars ocean theory suggests that nearly a third of the surface of Mars was once covered by water, though the water on Mars is no longer oceanic.
Water on Mars exists today almost exclusively as ice and underground, with a small amount present in the atmosphere as vapor. Some liquid water may occur transiently on the Martian surface today but only under certain conditions. No large standing bodies of liquid water exist because the atmospheric pressure at the surface averages just 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea level pressure—and because the global average temperature is far too low (210 K (−63 °C)), leading to either rapid evaporation or freezing. Features called recurring slope lineae are thought to be caused by flows of brine—hydrated salts.
In July 2018, scientists from the Italian Space Agency reported the detection of a subglacial lake on Mars, below the southern polar ice cap, and spanning horizontally, the first evidence for a stable body of liquid water on the planet. Because the temperature at the base of the polar cap is estimated at , scientists assume that the water may remain liquid due to the antifreeze effect of magnesium and calcium perchlorates. The ice layer covering the lake is composed of water ice with 10 to 20% admixed dust, and seasonally covered by a -thick layer of ice.
Europa Scientists' consensus is that a layer of liquid water exists beneath the surface of Europa, a moon of Jupiter and that heat from tidal flexing allows the subsurface ocean to remain liquid. It is estimated that the outer crust of solid ice is approximately thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about deep. This leads to a volume of Europa's oceans of 3 × 1018 m3 (3 billion km3), slightly more than twice the volume of Earth's oceans.
Enceladus Enceladus, a moon of Saturn, has shown geysers of water, confirmed by the Cassini spacecraft in 2005 and analyzed more deeply in 2008. Gravimetric data in 2010–2011 confirmed a subsurface ocean. While previously believed to be localized, most likely in a portion of the southern hemisphere, evidence revealed in 2015 now suggests the subsurface ocean is global in nature.
In addition to water, these geysers from vents near the south pole contained small amounts of salt, nitrogen, carbon dioxide, and volatile hydrocarbons. The melting of the ocean water and the geysers appear to be driven by tidal flux from Saturn.
Mimas Mimas, another moon of Saturn similar in size and orbit to Enceladus, was found by Cassini to have a "rocking" motion whose amplitude could only be explained by a large subsurface ocean.
Ganymede A subsurface saline ocean is theorized to exist on Ganymede, a moon of Jupiter, following observation by the Hubble Space Telescope in 2015. Patterns in auroral belts and rocking of the magnetic field suggest the presence of an ocean. It is estimated to be 100 km deep with the surface lying below a crust of 150 km of ice. As of 2015, the precise quantity of liquid water on Ganymede is highly uncertain (1–33 times as much as Earth).
Ceres Ceres appears to be differentiated into a rocky core and icy mantle, and may have a remnant internal ocean of liquid water under the layer of ice. The surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clay. In January 2014, emissions of water vapor were detected from several regions of Ceres. This was unexpected, because large bodies in the asteroid belt do not typically emit vapor, a hallmark of comets. Ceres also features a mountain called Ahuna Mons that is thought to be a cryovolcanic dome that facilitates the movement of high viscosity cryovolcanic magma consisting of water ice softened by its content of salts.
Ice giants The ice giant planets Uranus and Neptune are thought to have a supercritical water ocean beneath their clouds, which accounts for about two-thirds of their total mass,NASA Completes Study of Future 'Ice Giant' Mission Concepts . NASA TV. 20 June 2017.NASA, On to the Ice Giants. (PDF) Pre-Decadal study summary, presented at the European Geophysical Union, 24 April 2017. most likely surrounding small rocky cores, although a 2006 study by Wiktorowicz and Ingersall ruled out the possibility of such a water "ocean" existing on Neptune. This kind of planet is thought to be common in extrasolar planetary systems.
Pluto In June 2020, astronomers reported evidence that Pluto may have had a subsurface ocean, and consequently may have been habitable, when it was first formed.
Indicators, methods of detection and confirmation Most known extrasolar planetary systems appear to have very different compositions to the Solar System, though there is probably sample bias arising from the detection methods.
Spectroscopy
Liquid water has a distinct absorption spectroscopy signature compared to other states of water due to the state of its hydrogen bonds. Despite the confirmation of extraterrestrial water vapor and ice, however, the spectral signature of liquid water is yet to be confirmed outside of Earth. The signatures of surface water on terrestrial planets may be undetectable through thick atmospheres across the vast distances of space using current technology.
Seasonal flows on warm Martian slopes, though strongly suggestive of briny liquid water, have yet to indicate this in spectroscopic analysis.
Water vapor has been confirmed in numerous objects via spectroscopy, though it does not by itself confirm the presence of liquid water. However, when combined with other observations, the possibility might be inferred. For example, the density of GJ 1214 b would suggest that a large fraction of its mass is water and follow-up detection by the Hubble telescope of the presence of water vapor strongly suggests that exotic materials like 'hot ice' or 'superfluid water' may be present.
Magnetic fields For the Jovian moons Ganymede and Europa, the existence of a sub-ice ocean is inferred from the measurements of the magnetic field of Jupiter. Since conductors moving through a magnetic field produce a counter-electromotive field, the presence of the water below the surface was deduced from the change in magnetic field as the moon passed from the northern to southern magnetic hemisphere of Jupiter.
Geological indicators
Thomas Gold has posited that many Solar System bodies could potentially hold groundwater below the surface.
It is thought that liquid water may exist in the Martian subsurface. Research suggests that in the past there was liquid water flowing on the surface, creating large areas similar to Earth's oceans. However, the question remains as to where the water has gone. There are a number of direct and indirect proofs of water's presence either on or under the surface, e.g. stream beds, polar caps, spectroscopic measurement, eroded craters or minerals directly connected to the existence of liquid water (such as Goethite). In an article in the Journal of Geophysical Research, scientists studied Lake Vostok in Antarctica and discovered that it may have implications for liquid water still being on Mars. Through their research, scientists came to the conclusion that if Lake Vostok existed before the perennial glaciation began, that it is likely that the lake did not freeze all the way to the bottom. Due to this hypothesis, scientists say that if water had existed before the polar ice caps on Mars, it is likely that there is still liquid water below the ice caps that may even contain evidence of life.
"Chaos terrain", a common feature on Europa's surface, is interpreted by researchers studying images of Europa taken by NASA's Galileo spacecraft as regions where the subsurface ocean has melted through the icy crust.
Volcanic observation