Extraterrestrial liquid water

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

Earth, with oceanic water covering 71% of its surface, is the only known planet with liquid water on its surface and is certainly the only one in the Solar System.^[1] There is growing evidence of sub-surface liquid water on several moons in our solar system orbiting the gas giant planets Jupiter, Saturn, Uranus and Neptune; however, none of these sub-surface moon water bodies has received final confirmation to date. Oceans and water may be common in other star systems and/or on their planets and other orbiting celestial bodies; for example, water vapour was found in 2007 in the proplanetary disc of a young star MWC 480, within 1AU of the star. ^[2]

Large bodies of water and extensive water systems were once thought to cover Venus and Mars. With increased telescopic resolution and enhanced observation techniques in modern times, these were ultimately disproven; however, the presence of water on Mars in the distant past remains a topic of contemporary debate.

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.^[3]

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

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

Past surface water

Further information: Mars Ocean Hypothesis

Assuming that the Giant impact hypothesis is correct, there were never real seas or oceans on the moon. 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.^[4]

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.^[5] 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.

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,^[6] creating large areas similar to Earth's oceans. However, the question remains as to where the water has gone.^[7] There are a number^[8] 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.^[9]

Thomas Gold has posited that many Solar System bodies could potentially hold groundwater farther down. [1]

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.^[10] Of particular interest in these cases are 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.^[10]

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.^[11] It could also be just ice.^[12] In June 2009, evidence was put forward for salty subterranean oceans.^[13]

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.^[14] Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.^[15][16]

Gas giants

Jupiter possesses a gaseous layer where, because of the Earthlike temperature and pressure, droplets may condense from the water vapor.^{[citation needed]}

Uranus and Neptune may possess large oceans of hot, highly compressed, supercritical water^[17] under their thick atmospheres, though their internal structure is not well understood at this time. It is agreed that they are different from the gas giants Jupiter and Saturn—some astronomers would class them separately as 'ice giants'.^[18]

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,^[19] and might possess a tenuous atmosphere.^[20] It is too cold for liquid water, but an ocean of water plus ammonia has been suggested.^[21] More information will be available in 2015, when the Dawn Mission visits it.

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.^[22] 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 of the 450+ detected extrasolar planetary systems look very different from ours, making our Solar System appear atypical, though there is probably a 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).^[23] 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.^[24] But there is evidence that rocky planets hosting water may be commonplace throughout the Milky Way.^[25]

55 Cancri e

55 Cancri e orbits very close to its sun, but observations suggest that it is 30% supercritical water, a global ocean.^[26]

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.^[27]

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.^[28] Solid bodies condensing from the disk should have liquid water, if they are the right distance from the star.

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’.^[29]

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.^[30]

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.^[31] 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.^[32] 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.^[33] 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.^[34]

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.^[35]

GJ 1214 b

GJ 1214 b is only 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 indicates that it is likely a mix of rock and water.^[36] However, the phase state of the water is unknown.^[37] Studying its atmosphere should be possible and will tell us more.^[38]

HD 28185 b

HD 28185 b was the first exoplanet to be detected in the habitable zone.^[39] 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.^[40]

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, maybe even life. ^{[41] [42]}

MOA-2007-BLG-192Lb

MOA-2007-BLG-192Lb is a small planet orbiting a small star. It is about three 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.^[43]

TW Hydrae

This very young star is in the process of forming a solar system. Scientists have now detected clouds of water vapour cold enough to form comets. 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.

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.^[44]

Kepler First Results

Among the 1,235 possible extra-solar 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.^[45] 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.^[46]

See also

Space portal

• Extraterrestrial water vapor

References

1. ^ "Earth". Nineplanets.org. http://nineplanets.org/earth.html. 
2. ^ Josh A. Eisner (2007). "Water vapour and hydrogen in the terrestrial-planet-forming region of a protoplanetary disk". Nature 447 (447): 562–564. arXiv:0706.1239. Bibcode 2007Natur.447..562E. doi:10.1038/nature05867. 
3. ^ '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.

4. ^ Did oceans on Venus harbour life?, issue 2626 of New Scientist magazine.
5. ^ Mars Probably Once Had A Huge Ocean
6. ^ "Science@NASA, The Case of the Missing Mars Water". http://science.nasa.gov/headlines/y2001/ast05jan_1.htm. Retrieved 2009-03-07. 
7. ^ "Water on Mars: Where is it All?". http://www.adlerplanetarium.org/cyberspace/planets/mars/water.html. Retrieved 2009-03-07. 
8. ^ "Water at Martian south pole". 17 March 2004. http://www.esa.int/SPECIALS/Mars_Express/SEMYKEX5WRD_0.html. Retrieved 29 September 2009. 
9. ^ "A numerical model for an alternative origin of Lake Vostok and its exobiological implications for Mars". http://www.agu.org/journals/je/v106/iE01/2000JE001254/2000JE001254.pdf. Retrieved 2009-04-08. 
10. ^ ^{a b} Hussmann, H.; Sohl, Frank; Spohn, Tilman (2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". Icarus 185 (1): 258–273. Bibcode 2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005. http://adsabs.harvard.edu/abs/2006Icar..185..258H.  edit
11. ^ Cassini Images of Enceladus Suggest Geysers Erupt Liquid Water at the Moon’s South Pole
12. ^ Saturn's Moon Enceladus Is Unlikely To Harbor Life
13. ^ Possible salty ocean hidden in depths of Saturn moon
14. ^ Saturn's Moon Titan -- Land Of Lakes And Seas
15. ^ Mysterious signal hints at subsurface ocean on Titan
16. ^ Saturn moon may have hidden ocean
17. ^ At pressures above one million atmospheres (for example, centre of Uranus is about 8 million atmospheres and 5000 Kelvin) water exists in a supercritical state with properties very different from water at one atmosphere (Water Phase Diagram)
18. ^ Giant Ice Planets
19. ^ Largest Asteroid Might Contain More Fresh Water than Earth
20. ^ Asteroid Ceres
21. ^ Dawn's destinations, Astronomy Now June 2007.
22. ^ http://www.sciencedaily.com/releases/2011/04/110405175018.htm
23. ^ Habitable planets may be common
24. ^ The hunt for habitable exomoons
25. ^ Water, water everywhere
26. ^ Astrophile: Supercritical water world does somersaults
27. ^ Astronomers discover new planet
28. ^ Organics and water found where new planets may grow
29. ^ CoRoT discovers most Earth-like exoplanet yet
30. ^ Sizing up a temperate exoplanet
31. ^ New Planet Could Harbor Water and Life
32. ^ Scientists might have picked right star, wrong world for hosting life
33. ^ Sibling worlds may be wettest and lightest known
34. ^ Exoplanet near Gliese 581 star 'could host life'
35. ^ "NASA and NSF-Funded Research Finds First Potentially Habitable Exoplanet". Release 10-237. NASA. 2010-09-29. http://www.nasa.gov/home/hqnews/2010/sep/HQ_10-237_Exoplanet_Findings.html. 
36. ^ The small planet with a thick coat
37. ^ Nearby Super-Earth May Be a Waterworld
38. ^ Alien planet could be ultimate water world
39. ^ Jones, B. W.; Sleep, P. N.; Underwood, D. R. (2006). "Habitability of Known Exoplanetary Systems Based on Measured Stellar Properties". The Astrophysical Journal 649: 1010. arXiv:astro-ph/0603200. Bibcode 2006ApJ...649.1010J. doi:10.1086/506557.  edit
40. ^ Barnes, J. W.; O’Brien, D. P. (2002). "Stability of Satellites around Close‐in Extrasolar Giant Planets". The Astrophysical Journal 575: 1087. arXiv:astro-ph/0205035. Bibcode 2002ApJ...575.1087B. doi:10.1086/341477.  edit
41. ^ Exoplanet Looks Potentially Lively
42. ^ 'Super-Earth,' 1 of 50 Newfound Alien Planets, Could Potentially Support Life
43. ^ Small Planet Discovered Orbiting Small Star
44. ^ Nearby Planet-Forming Disk Holds Water for Thousands of Oceans
45. ^ Kepler detects more than 1,200 possible planets
46. ^ NASA Finds Earth-Size Planet Candidates in Habitable Zone, Six Planet System

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