Extraterrestrial liquid water

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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 our own solar system. Oceans and water may be common in other star systems and or/on their planets and other orbiting celestial bodies: for example, water vapour has recently been found at the right spot in a protoplanetary disc.^[1]

Large bodies of water and extensive water systems were once thought to cover the Moon, 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.

Contents 1 Solar System 1.1 Obsolete theories 1.2 Past surface water 1.3 Groundwater 1.4 Icy moon subsurface oceans 1.5 Gas giants 1.6 Water-ammonia layers on cool small bodies 2 Beyond the Solar System 2.1 55 Cancri f 2.2 AA Tauri 2.3 Gliese 436 b 2.4 Gliese 581 c & d 2.5 HD 28185 b 2.6 HD 113766 2.7 HD 189733 b 2.8 HD 209458b 2.9 MOA-2007-BLG-192Lb 3 References 4 External links

[edit] Solar System

[edit] 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 expresses some doubt about the lunar 'seas' in his Dialogue Concerning the Two Chief World Systems. '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.

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

Mars was never supposed to have oceans. Its dryness was long recognised, and gave credibility to the spurious Martian canals.

[edit] Past surface water

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.^[2]

It was thought it might have dried up from something more Earth-like. The initial discovery of a cratered surface made this seem unlikely. But further evidence has change 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. ^[3] 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.

[edit] Groundwater

It is widely thought that liquid water may exist in the Martian subsurface. Thomas Gold, however, has posited that many Solar System bodies could potentially hold groundwater farther down.[1]

[edit] Icy moon subsurface oceans

Subsurface oceans have been postulated for most of the icy moons of the Outer planets, which are covered by a 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 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.

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.^[4] It could also be just ice.^[5]

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.^[6] Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.^{[7] [8]}

Neptune's moon Triton may have once had internal oceans that have now frozen. This could also be true of other icy moons.

[edit] Gas giants

Jupiter possesses a gaseous layer where, because of the Earthlike temperature and pressure, droplets may condense from the water vapor.

The "ice giant" planets Uranus and Neptune may possess large oceans of liquid water under their thick atmospheres, though their internal structure is not well understood at this time. It is agreed that they are different from Jupiter and Saturn.

[edit] 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.^[9] It might have a tenuous atmosphere.^[10] It is too cold for liquid water, but an ocean of water plus ammonia has been suggested.^[11] More information will be available in 2015, when the Dawn Mission visits it.

[edit] Beyond the Solar System

Our solar system may not be typical. Most of the 200+ solar systems look very different from ours, though there is probably a bias arising from the detection methods. The hope is for Earth-sized planets in the habitable zone,^[12] (which is also sometimes called the Goldilocks zone). Planets with oceans could include Earth-sized moons of giant planets, though it remains speculative whether such 'moons' really exist.

[edit] 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 had rocky moons, these might have liquid water.^[13]

There is also a gap in the orbits of the five known planets which might contain something more Earth-like. If it existed, it could not be detected by present methods. But these are constantly being improved.

[edit] 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.^[14]

[edit] Gliese 436 b

Gliese 436 b is believed to have 'hot ice'. It is not cool enough for liquid water: but if water molecules exist there, they are likely to be found also on planets at a suitable temperature.^[15]

The detection of such a hot ice world has important consequences. It shows for the first time that planets similar to the "ice giants" Uranus and Neptune in our own Solar System exist at close distances from their star... Many of the planets of similar mass detected around other stars by the astronomers may therefore also be composed mainly of water. Some of them will have cooler temperatures, allowing the water on the surface to be liquid. Such planets covered by a single huge ocean have been dubbed "ocean planets" by the specialists.^[16]

[edit] Gliese 581 c & d

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^[17]. Since it does not transit its sun, there is no way to know if there is any water there.

More recent work suggests that it would probably be too hot for liquid water. But Gliese 581 d might be warm enough for oceans if a greenhouse effect was operating. ^[18] Gliese 581 d is eight times the mass of the Earth and might have a thick atmosphere.

[edit] HD 28185 b

HD 28185 b was the first exoplanet to be detected in the habitable zone. 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.^[19]

[edit] HD 113766

The disk around a star called HD 113766 may be forming an Earth-like planet that will have oceans in the future, and perhaps life also. It is still a very young star, unlikely to have these features for hundreds of millions of years. ^[20]

[edit] HD 189733 b

In July 2007, evidence was published that HD 189733 b had water vapour in its atmosphere.^[21] Like HD 209458b, it would be too hot for liquid water.

[edit] HD 209458b

HD 209458b may have water vapour in its atmosphere - this is currently being disputed. It is not cool enough for liquid water.

[edit] MOA-2007-BLG-192Lb

MOA-2007-BLG-192Lb is a small planet orbiting a small star. It is about three Earth masses, currently the smallest extrasolar planet orbiting a normal star:

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]

[edit] References

[edit] 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