Extraterrestrial liquid water

Extraterrestrial liquid water, the presence of water in its liquid state, is a subject of wide interest because it is commonly believed to be a prerequisite for extraterrestrial life.

Earth, with oceanic water covering 71% of its surface, is the only planet known to have stable bodies of liquid water on its surface^[4] and liquid water is essential to all life on Earth. Earth orbits within the Sun's habitable zone, a region in which liquid water can exist on the surface of a planet with sufficient atmospheric pressure.

There is growing evidence of liquids below the surface of several moons and dwarf planets elsewhere in the Solar System that may consist of liquid water, some are speculated to be large "oceans"; however, none have been confirmed. Oceans and water may be common in other planetary systems; for example, water vapour was found in 2007 in the proplanetary disc of a young star MWC 480, within 1 AU of the star.^[5]

Surface water was once thought to cover large areas of Venus and Mars and possible seasonal flows on warm Martian slopes have been observed since 2008.

Solar System

Obsolete theories

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.^{[lower-alpha 1]}

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.

Past surface water

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.

Astronomers believe that Venus had liquid water and perhaps oceans in its very early history. 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.^[6]

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.^[7] 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.^[8]

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.^[9][10]

Groundwater

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,^[11] creating large areas similar to Earth's oceans. However, the question remains as to where the water has gone.^[12] There are a number^[13] 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.^[14]

Thomas Gold has posited that many Solar System bodies could potentially hold groundwater farther below.

Asteroids

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet, Ceres, largest object in the asteroid belt.^[15] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.^[16] The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."^[16]

There is expectation of finding water-ice on Ceres, planned to be visited in February 2015.^[17]

The Dawn space probe has unexpectedly found evidence of water flows on the asteroid Vesta,^[18] and perhaps vast masses of water-ice underground.^[19]

The presence of water-ice on Vesta suggests it may be found on many of the asteroids.^[20]

Icy-moon and trans-Neptunian-object subsurface oceans

Subsurface oceans have been postulated for most of the icy moons of the outer planets, which are covered by a thick layer of water ice. In some cases it is thought that an ocean layer may have been present in the past, but has since cooled into solid ice.

Liquid water is thought to be present under the surface of several natural satellites, particularly the Galilean moons of Jupiter, such as Europa (liquid water underneath its icy surface due to tidal heating), and, with less certainty, Callisto and Ganymede.

Models of heat retention and heating via radioactive decay in smaller icy 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]

Geysers have been found on Enceladus. These contain water vapour and may mean liquid water deeper down. The water is either heated tidally or geothermally. It is known that Enceladus has liquid water, as there are active cryovolcanic mountains around its southern pole.^[22] It could also be just ice.^[23] In June 2009, evidence was put forward for salty subterranean oceans.^[24]

In December 2013, NASA announced, based on studies with the Hubble Space Telescope, that water vapor plumes were detected on Europa, moon of Jupiter, and were similar to the water vapor plumes detected on Enceladus, moon of Saturn.^[2]

It was believed after the Voyager observations that Titan might have seas or oceans of liquid hydrocarbons. The Cassini–Huygens space mission initially discovered only what appeared to be dry lakebeds and empty river channels, suggesting that Titan had lost what surface liquids it might have had. A more recent fly-by of Titan made by Cassini has produced radar images that strongly suggest hydrocarbon lakes near the polar regions where it is colder.^[25] Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.^[26][27]

Gas giants

Water in Jupiter's stratosphere appears to have its origins in comet impacts.^[28] The possibility of Uranus and Neptune possessing hot, highly compressed, supercritical water^{[lower-alpha 2]} under their thick atmospheres has been hypothesised. While their composition is still not fully understood, a 2006 study by Wiktorowicz et al. ruled out the possibility of such an water "ocean" existing on Neptune,^[29] though some studies have suggested that exotic oceans of liquid diamond are possible.^[30]

Water-ammonia layers on cool small bodies

A solution of water with ammonia would have an antifreeze effect, and enable liquid water to exist in small bodies that would otherwise be too cold.

The dwarf planet Ceres is believed to contain large amounts of water-ice,^[31] and might possess a tenuous atmosphere.^[32] It is too cold for liquid water, but an ocean of water plus ammonia has been suggested.^[33] More information should be available in 2015, when the Dawn Mission is due to become the first space probe to explore the planet.

Liquid water inside comets

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.^[34] 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

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).^[35] 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.^[36] But there is evidence that rocky planets hosting water may be commonplace throughout the Milky Way.^[37]

55 Cancri e

55 Cancri e orbits very close to 55 Cancri, but observations suggest that it is 30% supercritical water, a global ocean.^[38]

55 Cancri f

55 Cancri f is a large planet orbiting in the habitable zone of the star 55 Cancri A. Its composition is unknown but it is believed to be a gas giant. If it has rocky moons, these could have liquid water.^[39]

There is also a gap in the orbits of that system's five (known) planets which might contain something more Earth-like. If it exists, it cannot be detected by present methods, though these are constantly being improved.

AA Tauri

AA Tauri is a young star, less than a million years old and a typical example of a young star with a protoplanetary disk. Astronomers have recently found the spectral signatures of water vapor, plus three simple organic molecules – hydrogen cyanide, acetylene and carbon dioxide.^[40] Solid bodies condensing from the disk should have liquid water, if they are the right distance from the star.

Alpha Centauri B

The nearby star Alpha Centauri B may have a planet with oceans.^[41] This is speculative: the only confirmed planet in the system is Alpha Centauri Bb, which would be much too hot.

COROT-7b

COROT-7b is an exoplanet less than twice the size of Earth orbiting a Sun-like star very closely. It was discovered by the CoRoT space telescope and was announced to the public on 3 February 2009. The surface is predicted to be in the range of 1,000–1,500 degrees Celsius, but since its composition is not known it could be covered in molten lava or enshrouded in a thick water vapour cloud layer. The planet could also be made up of water and rock in almost equal amounts. If COROT-7b is water-rich it could be an ‘ocean planet’.^[42]

COROT-9b

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.^[43]

GD 61

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.^[44]

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

Gliese 581 c, d and g

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.^[45] 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.^[46] 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.^[47] 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.^[48]

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.^[49]

Gliese 667 C - three planets

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.^[51]

Further work published in June 2013 suggests that the system has six planets, and that three of them are in the habitable zone.^[52]

GJ 1214 b

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,^[53] 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'.^[54][55]

HD 28185 b

HD 28185 b was the first exoplanet to be detected in the habitable zone.^[56] 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.^[57]

HD 85512 b

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.^[58][59]

MOA-2007-BLG-192Lb

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.^[60]

Kepler-22b

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.^[61]

Kepler-62e and Kepler-62f

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.^[62]

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.^[63] Both are assumed to be rocky planets, but since the star is 1200 light-years away, it is hard to be sure.

Kepler-69c

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 ^[63]

Kepler (other results)

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.^[64] 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.^[65]

TW Hydrae

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.^[66]

See also

• Earth analog

• Extraterrestrial water vapor
• Gliese 163 c
• Gliese 581 d
• Gliese 581 g
• Gliese 667 Cc
• Habitable zone
• HD 85512 b
• Kepler 22 b
• List of nearest terrestrial exoplanet candidates
• Planetary habitability
• Super-Earth

References

Explanatory notes

Citations

External links

• The Extrasolar Planets Encyclopaedia
• Gliese 581: Extrasolar Planet Might Indeed Be Habitable
• Jupiter's Moon Europa: What Could Be Under The Ice?
• To Curious Aliens, Earth Would Stand Out As Living Planet
• Ocean-bearing Planets: Looking For Extraterrestrial Life In All The Right Places

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