Alternative biochemistry
From Freepedia
Alternative biochemistry collectively refers to an assortment of astrobiology theories and hypotheses in which life is based on chemical systems other than those used by currently known forms of life. Proponents of such theories sometimes use the expression carbon chauvinism to disparage the assumption that carbon molecules are necessarily the basis for all life. Up to this point, however, no non-carbon based life-form has been discovered.
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Silicon biochemistry
The most common other proposed basis is silicon, since silicon has many similar chemical properties to carbon. Silicon has a number of handicaps as a carbon analogue, however. Because silicon atoms are much bigger, they have difficulty forming double or triple bonds. Silanes (hydrogen-silicon compounds analogous to the alkane hydrocarbons) are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating Si-O-Si bonds (known collectively as silicones) instead of Si-Si bonds are much more stable; ordinary sand is one such example. However, silicon dioxide (the analogue of carbon dioxide) is a non-soluble solid at the temperature range where liquid water is possible making it difficult for silicon to be introduced into water-based biochemical systems even if the necessary range of biochemical molecules could be constructed out of it.
In general, complex long-chain silicone molecules are still more unstable than their carbon counterparts.
Finally, of the varieties of molecules identified in interstellar space as of 1998, 84 are based on carbon and 8 are based on silicon. Moreover, of the eight Si-based compounds, four also include carbon within them. This suggests a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation upon which to build silicon-based biologies. The cosmic abundance of carbon to silicon is roughly 10 to 1.
It is possible that silicon compounds may be biologically useful under certain exotic environmental conditions, however, either in conjunction with carbon or in a role less directly analogous to carbon. A simple real-world example is the silicate skeletal structure of diatoms.
Nitrogen/Phosphorus biochemistry
Nitrogen and phosphorus also offer possibilities as the basis for biochemical molecules. Phosphorus can form long chain molecules on its own like carbon, and so potentially could be built up into complex macromolecules, but phosphorus is fairly reactive. In combination with nitrogen, however, it can form much more stable phosphorus-nitrogen (P-N) bonds; compounds containing these can form a wide range of molecules, including rings.
Earth's atmosphere is approximately 80% nitrogen, but this would probably not be much use to a P-N lifeform since molecular nitrogen (N2) is very inert and energetically expensive to "fix" (certain Earth plants such as legumes can fix nitrogen using symbiotic anaerobic bacteria contained within their root nodules). A nitrogen dioxide (NO2) or ammonia (NH3) atmosphere would be more useful (Nitrogen actually forms a number of oxides, such as NO, N2O, N2O4, and all would be present in a nitrogen dioxide-rich atmosphere).
In a nitrogen dioxide atmosphere, phosphorus-nitrogen-based plant analogues could absorb nitrogen dioxide from the atmosphere and phosphorus from the ground. The nitrogen dioxide would be reduced, P-N sugar analogues being produced in the process, and waste oxygen would be released into the atmosphere. P-N animal analogues would consume the plants, use atmospheric oxygen to metabolize the P-N sugar analogues, exhaling nitrogen dioxide and depositing phosphorus (or phosphorus rich material) as solid waste.
In an ammonia atmosphere, P-N plants would absorb ammonia from the atmosphere and phosphorus from the ground, then oxidize the ammonia to produce P-N sugars and release hydrogen waste. P-N animals are now the reducers, breathing in hydrogen and converting the P-N sugars to ammonia and phosphorus. This is the opposite pattern of oxidation and reduction from a nitrogen dioxide world, and indeed from the known biochemistry of Earth; it would be analogous to Earth's atmospheric carbon supply being in the form of methane instead of carbon dioxide. Debate continues as several aspects of a P-N cycle biology would be energy deficient.
Still, nitrogen and phosphorus are not likely to be found in the ratios and quantity required in the real universe. Carbon, being preferentially formed during nuclear fusion, is more abundant and is more likely to end up in a preferred location.
Other exotic biochemical elements
Chlorine is sometimes proposed as a biological alternative to oxygen, either in carbon-based biologies or hypothetical non-carbon-based ones. Chlorine is much less abundant than oxygen in the universe, however, and so it is unlikely that a planet will be able to form which has a large enough concentration of chlorine available on its surface to form the basis of a biochemistry. Chlorine will instead likely be bound up in the form of salts and other inert compounds.
Sulfur is also able to form long-chain molecules, but suffers from the same high reactivity problems that phosphorus and silanes do. While the biological use of sulfur as an alternative to carbon is theoretical, strains of sulfur-reducing bacteria have been discovered in exotic locations on earth. These bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide. Examples of this type of metabolism are green sulfur bacteria and purple sulfur bacteria.
Non-water solvents
In addition to carbon compounds all currently known terrestrial life also requires water as a solvent. It is sometimes assumed that water is the only suitable chemical to fill this role. Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity useful for temperature regulation, a large heat of vaporization, and the ability to dissolve a wide variety of compounds. There are other chemicals with similar properties that have sometimes been proposed as alternatives.
Ammonia is perhaps the most commonly proposed alternative. Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has some chemical similarities with water. Ammonia can dissolve most organic molecules at least as well as water does, and in addition it is capable of dissolving many elemental metals. Given this set of chemical properties it has been theorized that ammonia-based life forms might be possible.
However, ammonia does have some problems as a basis for life. The heat of vaporization of ammonia is half that of water and its surface tension three times smaller. This means that hydrogen bonds between ammonia molecules will always be much weaker than those in water, so ammonia is less able to concentrate non-polar molecules through a hydrophobic effect. For this reason, mainstream science questions how well ammonia could hold prebiotic molecules together in order to allow the emergence of a self-reproducing system. Ammonia is also combustible and oxidizable and could not exist sustainable in a biosphere that oxidizes it, it would however, be stable in a reducing environment.
A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual for terrestrial life. Terrestial life usually exists within the melting point and boiling point of water at normal pressure, between 0°C (273 K) and 100°C (373 K); at normal pressure ammonia's melting and boiling points are between −78°C (195 K) and −33°C (240 K). Problems with biospheres at extremely cooled temperatures are that biochemical reactions are slowed down tremendously as well as some biochemicals may precipitate out of solution due to high melting points. Ammonia could be a liquid at normal temperatures but at much higher pressures; for example, at 60 atm ammonia melts at −77°C (196 K) and boils at 98°C (371 K).
Ammonia and ammonia-water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based "habitability zone". Such conditions could exist, for example, under the surface of the Saturn's largest moon Titan [1].
Others sometimes proposed include methanol, hydrogen sulfide and hydrogen chloride. The latter two suffer from a relatively low cosmic abundance of sulfur and chlorine, which tend to be bound up in solid minerals. A mixture of hydrocarbons, such as the methane/ethane seas that were once speculated to be present on the surface of Titan, could act as a solvent over a wide range of temperatures but would lack polarity. Isaac Asimov, the biochemist and science-fiction writer, suggested that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane or even liquid hydrogen.[2]
Artificial life
It is possible in principle to construct a robot or a system of robots that is capable of replicating itself from raw ores and natural energy sources without any external direction or assistance (a "clanking replicator"). Such a machine system could be considered alive, in that it is capable of evolution through mutational errors in its inherited design patterns, but is in no way required to be composed of carbon-based compounds. The most detailed proposition for machine life made so far considered self-replicating lunar factories, which were composed primarily of refined metal and cast basalt since the Earth's moon is extremely carbon-poor.
Related to macroscopic machine life is the concept of self-replicating nanotechnology, sometimes referred to as "grey goo" when it is operating without programmed limitations. Nanotechnology, like larger scale machines, could potentially be made of non-carbon-containing materials (including any of the other elements already mentioned earlier). Both diamondoid and carbon nanotubes are also commonly proposed materials for use in nanomachines, forms of carbon not used by life as it is currently known, and furthermore it is often proposed that nanotechnological devices will operate without the water environment that life as it is currently known requires. Any of the other life-bases mentioned previously could also serve as the basis for an artificial life form.
These beings almost assuredly could not have evolved without the help of carbon-based (or other) beings, since macroscopic machines would need to be designed and originally programmed, while the incredible scarcity of naturally occurring nanotech materials would preclude any sort of evolution of nanomachines.
After being created, these machines could potentially out-compete or destroy their creators if robustly enough designed. They would in a sense inherit the world or civilization of their creators, and be indistinguishable to most outsiders from native beings. Such an occurrence resembles the Intelligent Design form of creationism--intelligent life has been designed by an intelligent creator.
Scientifically, the relevance of this possibility is that high intelligence in a transition species could be the substrate for the development of an "impractical" form of life. Afterwards, the new form of life might continue to evolve by natural means. This could be considered as an argument for carbon chauvinism, or at least for teaching it to any artificial life forms that human beings may create.
In fiction
In the realm of science fiction there have occasionally been forms of life proposed that, while often highly speculative and unsupported by rigorous theoretical examination, are nevertheless interesting and in some cases even somewhat plausible.
One of the major sentient species in Terry Pratchett's Discworld universe is Trolls. Their being mineral-based has various interesting effects on their physiology and culture. Other silicon-based lifeforms are said to exist there but few appeared in the books. In the Star Wars universe, at least two life forms are based on Silicon, and they live in space: the Mynocks and the Space slugs.
Fred Hoyle's classic novel The Black Cloud features a life form consisting of a vast cloud of interstellar dust, the individual particles of which interact via electromagnetic signalling analogous to how the individual cells of multicellular Earth life interact. On a somewhat less science fictional level, life in interstellar dust has been proposed as part of the panspermia hypothesis. The low temperatures and densities of interstellar clouds would seem to imply that life processes would operate much more slowly there than on Earth.
In Forward's Rocheworld series a relatively Earthlike biochemistry is proposed that uses a mixture of water and ammonia as its solvent.
Robert L. Forward's Camelot 30K describes an ecosystem existing on the surface of Kuiper belt objects that is based on a fluorocarbon chemistry with OF2 as the principal solvent instead of H2O. The organisms in this ecology keep themselves warm by secreting a pellet of uranium-235 inside themselves and then moderating its nuclear fission using a boron-rich carapace around it. Kuiper belt objects are known to be rich in organic compounds such as tholins, so some form of life existing on their surfaces is not entirely implausible - though perhaps not going so far as to develop natural internal nuclear reactors as Forward's have. Fluorine is also of low cosmic abundance, so its use in this manner is also not likely. Gregory Benford's Heart Of The Comet features a comet with a conventional carbon-and-water-based ecosystem that becomes active near perihelion when the Sun warms it.
In Dragon's Egg and Starquake, Robert Forward proposes life on the surface of a neutron star utilizing "nuclear chemistry" in the degenerate matter crust. Since such life utilized strong nuclear forces instead of electromagnetic interactions, it was posited to function millions of times faster than Earth life.
David Brin's Sundiver is an example of science fiction proposing a form of life existing within the plasma atmosphere of a star using complex self-sustaining magnetic fields. Similar sorts of plasmoid life have sometimes been proposed to exist in other places, such as planetary ionospheres or interstellar space, but usually only by fringe theorists (see ball lightning for some additional discussion). Gregory Benford had a form of plasma-based life exist in the accretion disk of a primordial black hole in his novel Eater.
Stephen Baxter has imagined perhaps some of the most unusual exotic lifeforms in his Xeelee series of novels and stories, including supersymmetric photino-based life that congregate in the gravity wells of stars, and the Qax, who thrive in any form of convection cells, from swamp gas to the atmospheres of gas giants.
In his novel Diaspora, Greg Egan posits the existence of entire virtual universes implemented on Turing Machines encoded by Wang Tiles in gargantuan polysaccharide 'carpets.'
