Extraterrestrial liquid water

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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 believed to be a prerequisite for extraterrestrial life.

With oceanic water covering 71% of its surface, Earth is the only planet known to have stable bodies of liquid water on its surface,[1] and liquid water is essential to all known life forms. 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, similarly to groundwater on Earth. Water vapour is sometimes considered a smoking gun for the presence of liquid water, but atmospheric water vapour is 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.

Methods of detection and confirmation[edit]

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[edit]

Absorption spectrum of liquid water
Liquid water has not been detected in spectroscopic analysis of suspected seasonal Martian flows.

The most conclusive method for detection and confirmation of extraterrestrial liquid water is currently absorption spectroscopy. Liquid water has a distinct spectral signature to other states of water due to the state of its Hydrogen bonds. Despite the confirmation of extraterrestrial water vapor and ice, the spectral signature of liquid water is yet to be confirmed. 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 followup detection by the Hubble telescope of the presence if water vapor strongly suggests that exotic materials like 'hot ice' or 'superfluid water' may be present.[2][3]

Geological indicators[edit]

Further information: Groundwater on Mars

Thomas Gold has posited that many Solar System bodies could potentially hold groundwater below the surface.[4]

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,[5] creating large areas similar to Earth's oceans. However, the question remains as to where the water has gone.[6] There are a number[7] 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.[8]

Volcanic observation[edit]

A possible mechanism for cryovolcanism on bodies like Enceladus

Geysers have been found on Enceladus, a moon of Saturn, and Europa, moon of Jupiter.[9] These contain water vapour and could be indicators of liquid water deeper down.[10] It could also be just ice.[11] In June 2009, evidence was put forward for salty subterranean oceans on Enceladus.[12] On April 3, 2014, NASA reported that evidence for a large underground ocean of liquid water on Enceladus, moon of planet Saturn, had been found by the Cassini spacecraft. According to the scientists, evidence of an underground ocean suggests that Enceladus is one of the most likely places in the solar system to "host microbial life".[13][14]

Gravitational evidence[edit]

Scientists' consensus is that a layer of liquid water exists beneath Europa's surface, and that heat energy from tidal flexing allows the subsurface ocean to remain liquid.[15][16] The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons). Galileo imaging team members argue for the existence of a subsurface ocean from analysis of Voyager and Galileo images.[16] The most dramatic example is "chaos terrain", a common feature on Europa's surface that some interpret as a region where the subsurface ocean has melted through the icy crust.

Scientists used gravitational measurements from the Cassini spacecraft to confirm a water ocean under the crust of Enceladus. [13][14] Such tidal models have been used as theories for water layers in other Solar System moons.

Density calculation[edit]

Artists conception of the subsurface water ocean confirmed on Enceladus in 2014 as calculated using gravitational measurements and density predictions.[13][14]

Planetary scientists can use calculations of density to determine the composition of planets and their potential to possess liquid water, though the method is not highly accurate as the combination of many compounds and states can produce similar densities.

Initial analysis of 55 Cancri e's low density indicated that it consisted 30% supercritical fluid which Diana Valencia of the Massachusetts Institute of Technology proposed could be in the form of salty supercritical water,[17] though follow-up analysis of its transit failed to detect traces of either water or hydrogen.[18]

Scientists used low frequency radio signal from the Cassini probe to predict the existence of a layer of liquid water and ammonia beneath the surface of Saturn's moon Titan that are consistent with calculations of the moon's density.[19][20]

Models of radioactive decay[edit]

Models of heat retention and heating via radioactive decay in smaller icy Soldar System bodies suggest that Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus may have oceans underneath solid icy crusts approximately 100 km thick.[21] Of particular interest in these cases is the fact that the models predict that the liquid layers are in direct contact with the rocky core, which allows efficient mixing of minerals and salts into the water. This is in contrast with the oceans that may be inside larger icy satellites like Ganymede, Callisto, or Titan, where layers of high-pressure phases of ice are thought to underlie the liquid water layer.[21]

Models of radioactive decay suggest that MOA-2007-BLG-192Lb, a small planet orbiting a small star could be as warm as the Earth and completely covered by a very deep ocean.[22]

Internal differentiation models[edit]

Diagram showing a possible internal structure of Ceres
Two models for the composition of Europa predict a large subsurface ocean of liquid water. Similar models have been proposed for other celestial bodies in the Solar System

Models of Solar System objects indicate the presence of liquid water in their internal differentiation.

Some models of the dwarf planet Ceres, largest object in the asteroid belt indicate the possibility of a wet interior layer. Water vapor detected to be emitted by the dwarf planet[23][24] may be an indicator, thought sublimation of surface ice.

A global layer of liquid water thick enough to decouple the crust from the mantle is believed to be present on Titan, Europa and, with less certainty, Callisto, Ganymede[21] and Triton.[25][26] Other icy moons may also have internal oceans, or have once had internal oceans that have now frozen.[21]

Habitable zone[edit]

Artist's impression of Upsilon Andromedae d, portrayed as a class II planet with water vapor clouds, as seen from a hypothetical large moon with surface liquid water

A planet's orbit in the habitable zone is a popular method used to predict its potential for surface water at its surface. Habitable zone theory has put forward several extrasolar candidates for liquid water, though they are highly speculative as a planet's orbit around a star alone does not guarantee that a planet it has liquid water. In addition to its orbit, a planetary mass object must have the potential for sufficient atmospheric pressure to support liquid water and a sufficient supply of hydrogen and oxygen at or near its surface.

The Gliese 581 system contains multiple planets that may be candidates for surface water, including Gliese 581 c,[27] Gliese 581 d might be warm enough for oceans if a greenhouse effect was operating.,[28] Gliese 581 e[29] and the unconfirmed planet Gliese 581 g.[30]

Gliese 667 C has three of them are in the habitable zone[31] including Gliese 667 Cc is believed to have surface temperatures similar to Earth and a strong chance of liquid water.[32]

Kepler-22b one of the first 54 candidates found by the Kepler telescope and reported is 2.4 times the size of the Earth, with an estimated temperature of 22 °C. It is described as having the potential for surface water, though its composition is currently unknown.[33]

History[edit]

Lunar maria are vast basaltic plains on the Moon that were thought to be bodies of water by early astronomers, who referred to them as "seas". Galileo expressed some doubt about the lunar 'seas' in his Dialogue Concerning the Two Chief World Systems.[a]

Before space probes were landed, the idea of oceans on Venus was credible science, but the planet was discovered to be much too hot.

Telescopic observations from the time of Galileo onward have shown that Mars has no features resembling watery oceans.[citation needed] Mars' dryness was long recognized, and gave credibility to the spurious Martian canals.

Evidence of past surface water[edit]

An artist's impression of ancient Mars and its hypothesized oceans based on geological data

Assuming that the Giant impact hypothesis is correct, there were never real seas or oceans on the Moon, only perhaps a little moisture (liquid or ice) in some places, when the Moon had a thin atmosphere created by degassing of volcanoes or impacts of icy bodies.

The Dawn space probe found possible evidence of past water flow on the asteroid Vesta,[34] leading to speculation of underground reservoirs of water-ice.[35]

Astronomers believe that Venus had liquid water and perhaps oceans in its very early history[citation needed]. Given that Venus has been completely resurfaced by its own active geology, the idea of a primeval ocean is hard to test. Rock samples may one day give the answer.[36]

It was once thought that Mars might have dried up from something more Earth-like. The initial discovery of a cratered surface made this seem unlikely, but further evidence has changed this view. Liquid water may have existed on the surface of Mars in the distant past, and several basins on Mars have been proposed as dry sea beds.[37] The largest is Vastitas Borealis; others include Hellas Planitia and Argyre Planitia.

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. The Opportunity Mars rover photographed bright veins of a mineral leading to conclusive confirmation of deposition by liquid water.[38]

On December 9, 2013, NASA reported that the planet Mars had a large freshwater lake (which could have been a hospitable environment for microbial life) based on evidence from the Curiosity rover studying Aeolis Palus near Mount Sharp in Gale Crater.[39][40]

Further information: Mars Ocean Hypothesis

Liquid water inside comets[edit]

Comets contain large proportions of water ice, but are generally thought to be completely frozen due to their small size and large distance from the Sun. However, studies on dust collected from comet Wild-2 show evidence for liquid water inside the comet at some point in the past.[41] It is yet unclear what source of heat may have caused melting of some of the comet's water ice.

Extrasolar habitable zone candidates for water[edit]

Artist's illustration of the signatures of water in exoplanet atmospheres by Hubble.[42]

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.

The goal of current searches is to find Earth-sized planets in the habitable zone of their planetary systems (also sometimes called the Goldilocks zone).[43] Planets with oceans could include Earth-sized moons of giant planets, though it remains speculative whether such 'moons' really exist. The Kepler telescope might be sensitive enough to detect them.[44] But there is evidence that rocky planets hosting water may be commonplace throughout the Milky Way.[45]

COROT-9b[edit]

COROT-9b has been called a temperate exoplanet as its cloudtop temperature ranges from −20 degrees to 160 degrees Celsius. It is the size of Jupiter but a similar distance as Mercury is from our Sun. There are other similar planets cases known, but this planet can be studied in detail because it transits its star. Although it is mostly made of hydrogen and helium it may contain up to 20 Earth masses of other elements, including water and rock at high temperatures and pressures.[46]

GD 61[edit]

GD 61 is a white dwarf star, with an asteroid which has given the first direct evidence of a water-rich rocky planetary body outside the Solar System. The asteroid may be part of the debris from what might once have been a rocky planet with either ice or oceans.[47]

Previous detections of water vapor have been in giant planets. This find confirms that rocky planets with water exist outside our own solar system.

Gliese 581 c, d and g[edit]

Gliese 581 c, a world five times the size of the Earth, was originally reported to be the right distance from its sun for liquid water to exist on the planet's surface.[27] Since it does not transit its sun, there is no way to know if there is any water there.

Later work suggests that Gliese 581 c would probably be too hot for liquid water. It was then suggested that Gliese 581 d might be warm enough for oceans if a greenhouse effect was operating.[28] Gliese 581 d is eight times the mass of the Earth and might have a thick atmosphere.

Gliese 581 d looks an even better candidate. The orbital period was originally estimated at 83 days and has now been revised to 66 days.[48] This was announced along with another new world, Gliese 581 e, which is next to twice the mass of Earth but too close to its sun for liquid water. In May 2011, a new study suggested that the planet might have a thick atmosphere, oceans and even life.[29]

The unconfirmed planet Gliese 581 g is another good candidate. This planet is estimated to be between three to four times as massive as the earth, and as such it is too small to be a gas giant. The orbital period is estimated at 37 days, which places its orbit right in the middle of the habitable zone of the star Gliese 581.[30]

Gliese 667 C - three planets[edit]

Artist's impression of Gliese 667 Cc. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system.[49]

Gliese 667 Cc was originally described as one of two 'super-Earth' planets around Gliese 667 C, a dim red star that is part of a triple star system. The stars of this system have a concentration of heavy elements only 25% that of our Sun's. Such elements are the building blocks of terrestrial planets so it was thought to be unusual for such star systems to have an abundance of low mass planets.[50] It seems that habitable planets can form in a greater variety of environments than previously believed.

Gliese 667 Cc, in a tight 28-day orbit of a dim red star, must receive 90% of the light that Earth receives, but most of its incoming light is in the infrared, a higher percentage of this incoming energy should be absorbed by the planet. The planet is expected to absorb about the same amount of energy from its star that Earth absorbs from the Sun, which would allow surface temperatures similar to Earth and perhaps liquid water.[32]

Further work published in June 2013 suggests that the system has six planets, and that three of them are in the habitable zone.[31]

Gliese 832 c[edit]

Gliese 832 c is a 'super-Earth at least five times as massive as the Earth. It is in close 36-day orbit round a red dwarf star just 16 light-years from Earth. The planet receives about as much stellar energy as Earth does, despite orbiting much closer to its star, since red dwarf stars are much dimmer and cooler. It is reckoned as one of the top three most Earth-like planets so far found.[51]

It was announced in June 2014. Gliese 832 was already known to have a planet, Gliese 832 b, a Jupiter-like planet in a much more distant orbit.

GJ 1214 b[edit]

GJ 1214 b was the second exoplanet (after CoRoT-7b) to have an established mass and radius less than those of the giant Solar System planets. It is three times the size of Earth and about 6.5 times as massive. Its low density indicated that it is likely a mix of rock and water,[52] and follow-up observations using the Hubble telescope now seem to confirm that a large fraction of its mass is water, so it is a large waterworld. The high temperatures and pressures would form exotic materials like 'hot ice' or 'superfluid water'.[2][3]

HD 28185 b[edit]

HD 28185 b was the first exoplanet to be detected in the habitable zone.[53] The planet has only been detected indirectly, but is believed to be a gas giant, with no solid surface. Some scientists have argued that it could have moons large and stable enough to have oceans.[54]

HD 85512 b[edit]

HD 85512 b was discovered in August 2011. It is larger than Earth, but small enough to be probably a rocky world. It is on the borders of its star's habitable zone and might have liquid water, and is a potential candidate for a life-supporting world.[55][56]

MOA-2007-BLG-192Lb[edit]

MOA-2007-BLG-192Lb is a small planet orbiting a small star. It is about 3 Earth masses, currently the second smallest detected extrasolar planet orbiting a normal star, after Gliese 581 e.

The planet orbits its host star or brown dwarf with an orbital radius similar to that of Venus. But the host is likely to be between 3,000 and 1 million times fainter than the Sun, so the top of the planet's atmosphere is likely to be colder than Pluto. However, the planet is likely to maintain a massive atmosphere that would allow warmer temperatures at lower altitudes. It is even possible that interior heating by radioactive decays would be sufficient to make the surface as warm as the Earth, but theory suggests that the surface may be completely covered by a very deep ocean.[22]

Kapteyn b[edit]

Kapteyn b is one of two known planets of Kapteyn's Star, which is 13 light-years away and 11 billion years old. It is a super-Earth planet and believed to be at the right temperature for liquid water.[57] Kapteyn c is further out and too cold.

Kepler-22b[edit]

Kepler-22b is a planet 2.4 times the size of the Earth, with an estimated temperature of 22 °C. It was one of 54 candidates found by the Kepler telescope and reported in February as potentially habitable. It is the first of these to be formally confirmed using other telescopes. Its composition is currently unknown.[33]

Kepler-62e and Kepler-62f[edit]

The star Kepler-62 has five planets, two of which are the right distance from the star to have liquid water and potentially sustain life.[58]

Kepler-62f is only 40 percent larger than Earth, making it the exoplanet closest to the size of our planet known in the habitable zone of another star. Kepler-62e orbits on the inner edge of the habitable zone and is roughly 60 percent larger than Earth.[59] Both are assumed to be rocky planets, but since the star is 1200 light-years away, it is hard to be sure.

Kepler-69c[edit]

This large rocky planets is one of two known to be orbiting the star Kepler 69, which is similar to our sun. It is believed to be in the star's habitable zone.

It is 70% more massive than the Earth and has a 242-day orbit, similar to that of Venus in our own solar system.

NASA announced its discovery on 18 April 2013, along with the two Earth-like planets of Kepler 62 [59]

Kepler-186f[edit]

Kepler-186f is only 10% larger than Earth, and orbits the red dwarf star Kepler-186 within the habitable zone. It was announced on 17 April 2014 and described as the most Earth-like planet so far discovered.[60]

The star is about 500 light-years away from the Earth. It has four other known planets, all of them much closer to the star and too hot for liquid water.

Kepler (other results)[edit]

Among the 1,235 possible extrasolar planet candidates detected by NASA's planet-hunting Kepler space telescope during its first four months of operation, 54 are orbiting in the parent star's habitable 'Goldilocks' zone where liquid water could exist.[61] Five of these are near Earth-size, and the remaining 49 habitable zone candidates range from twice the size of Earth to larger than Jupiter.[62]

TW Hydrae[edit]

TW Hydrae is a very young star is in the process of forming a planetary system. Scientists have now detected clouds of water vapour cold enough to form comets.[citation needed] This could eventually deliver oceans to dry planets, which is believed by most scientists to have happened on the early Earth and other rocky planets.[citation needed]

Water vapour has previously been detected in planet-forming disks, but too warm to form comets. This cloud is cool enough and is estimated to contain thousands of Earth-oceans' worth of water.[63]

See also[edit]

References[edit]

Explanatory notes

  1. ^ 'Salviati', who normally gives Galileo's own opinions, says:

    I say then that if there were in nature only one way for two surfaces to be illuminated by the sun so that one appears lighter than the other, and that this were by having one made of land and the other of water, it would be necessary to say that the moon's surface was partly terrene and partly aqueous. But because there are more ways known to us that could produce the same effect, and perhaps others that we do not know of, I shall not make bold to affirm one rather than another to exist on the moon...

    What is clearly seen in the moon is that the darker parts are all plains, with few rocks and ridges in them, though there are some. The brighter remainder is all fill of rocks, mountains, round ridges, and other shapes, and in particular there are great ranges of mountains around the spots...

    I think that the material of the lunar globe is not land and water, and this alone is enough to prevent generations and alterations similar to ours.

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