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Revision as of 09:06, 14 September 2006 by 212.135.1.185 (talk)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)Alternative biochemistry is the biochemistry of alien lifes forms that differ radically from those on earth. Ifuk ut includes the use of atoms other that carbon to construct primary cellular structures and the use of solvents besides water. Speculation about extraterrestrial life based on alternative biochemistries is common in science fiction.
Atoms other than carbon
Scientists have speculated about the pros and cons of using atoms other than carbon to form the molecular structures necessary for life. But no one has proposed a theory employing an such atoms to form all the molecular machinery necessary for life. Since we are in fact carbon-based beings excluding the possibility of all other elements may be considered carbon chauvinism since we have never encountered any life that has evolved outside the earth’s environment.
Silicon biochemistry
The most common proposed basis for an alternative biochemical system is the silicon atom, since silicon has many chemical properties similar to carbon and is in the same periodic table group.
But silicon has a number of handicaps as a carbon analogue. Because silicon atoms are much bigger, having a larger mass and atomic radius, they have difficulty forming double or triple covalent bonds, which are important for a biochemical system. Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulphuric-acid-rich environment, as is found in some extraterrestrial locations. In general, however, complex long-chain silicone molecules are still more unstable than their carbon counterparts.
Another obstacle is that silicon dioxide (a common ingredient of many sands), the analogue of carbon dioxide, is a non-soluble solid at the temperature range where water is liquid, 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.
Finally, of the varieties of molecules identified in the interstellar medium as of 1998, 84 are based on carbon and 8 are based on silicon. Moreover, of those 8 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.
The Earth, as well as other terrestrial planets, is exceptionally silicon-rich and carbon-poor. However, terrestrial life is carbon-based. Rare carbon proved to be much more successful as a life base than abundant silicon.
It is possible, however, that silicon compounds may be biologically useful under certain exotic environmental conditions, either in conjunction with or in a role less directly analogous to carbon. A simple real-world example is the silicate skeletal structure of diatoms. See biogenic silica.
Nitrogen and 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. But in combination with nitrogen, it can form much more stable covalent bonds between phosphorus and nitrogen; 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 nearly inert and energetically expensive to "fix" due to its triple bond. Certain Earth plants such as legumes can fix nitrogen using symbiotic anaerobic bacteria contained in their root nodules. A nitrogen dioxide (NO2) or ammonia (NH3) atmosphere would be more useful. Nitrogen also forms a number of oxides, such as nitrogen monoxide, dinitrogen oxide, and dinitrogen tetraoxide, and all would be present in a nitrogen-dioxide-rich atmosphere.
In a nitrogen dioxide atmosphere, phosphorus-nitrogen based (P-N) plant analogues could absorb nitrogen dioxide from the atmosphere and phosphorus from the ground. The nitrogen dioxide would be reduced, with analogues to sugar being produced in the process, and waste oxygen would be released into the atmosphere. Animals based on phosphorus and nitrogen would consume the plants, use atmospheric oxygen to metabolize the 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 phosphorus-nitrogen 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. But chlorine is much less abundant than oxygen in the universe, 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. The biological use of sulfur as an alternative to carbon is purely theoretical, but 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
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 hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be three times smaller, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. For these reasons, 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 sustainably 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. Terrestrial 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). Such extremely cooled temperatures create problems, as they slow biochemical reactions tremendously and may cause biochemical precipitation 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 Saturn's largest moon Titan.
Other solvents
Other solvents sometimes proposed include methanol, uranium hexafluoride, 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 exist 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 liquid hydrogen.
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.
Novels and short stories
Perhaps the most extreme example in science fiction is James White's Sector General: a series of novels and short stories about multienvironment hospital for the strangest lifeforms imaginable, some of them breathing methane, chlorine, water and sometimes also oxygen. Some of the species metabolise directly hard radiation and their environment doesn't differ much from the atmosphere of a star, while others live in near absolute zero temperatures. All of the life forms are classified according to their metabolism, internal and external features, and more extreme abilities (telepathy, empathy, hive mind, etc) with four letter codes. Humans from Earth share the DBDG specification with small furry beings called Nidians.
One of the major sentient species in Terry Pratchett's Discworld universe are Trolls. They are mineral-based and this has various interesting effects on their physiology and culture. Trolls eat rocks, which suggests that their biochemistry is similar to that of plants. A heterotrophic silicon-based lifeform could no more eat rock than a carbon-based lifeform could eat coal. However, if they were photosynthetic, like plants, they could utilise silicon dioxide, which makes up the vast majority of most rock, in the same manner that plants utilise carbon dioxide, to create the silicon/glucose analogue from which they could derive nourishment.
Pratchett has also written the science fiction novel The Dark Side of the Sun which features a range of extraordinary lifeforms, including a telepathic body of water, creatures called "Sundogs", which are capable of interstellar travel from birth, and a sentient planet: effectively a giant silicon-based computer.
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 terrestrial life interact. Outside of science-fiction, 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.
Similarly, Arthur C. Clarke's "Crusade" revolves around a planetwide silicon-based lifeform located in deep intergalactic space, processing its thoughts very slowly by human standards, that sends probes to look for similar life in nearby galaxies. It concludes that it needs to make planets more habitable for similar lifeforms, and sends out other probes to foment supernovae in order to do so. Clarke implies that this is what accounts for most supernovae having occurred in the same region of space and warns that the effort will eventually reach Earth.
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 have Forward's. Fluorine is also of low cosmic abundance, so its use in this manner is unlikely.
In Forward's Rocheworld series, a relatively Earth-like biochemistry is proposed that uses a mixture of water and ammonia as its solvent. In Dragon's Egg and Starquake, Forward proposes life on the surface of a neutron star utilizing "nuclear chemistry" in the degenerate matter crust. Since such life utilised strong nuclear forces instead of electromagnetic interactions, it was posited that life might function millions of times faster than typical on Earth.
Gregory Benford's Heart of the Comet features a comet with a conventional carbon-and-water-based ecosystem that becomes active near the perihelion when the Sun warms it. 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.
The suggestion that life could even occur within the plasma of a star has been picked up by other science fiction writers, as in David Brin's Uplift Saga and in Stanislaw Lem's "Solaris". Any place in which reactions occur–even an incredible environment as a star–presents a possible medium for some chain of events that could produce a system able to replicate itself.
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.' The sentient ocean that covers much of the surface of Solaris in Stanislaw Lem's eponymous novel also seems, from much of the fictional research quoted and discussed in the book, to based on some element other than carbon.
In Metroid Prime: Hunters, Spire is a rock-like, silicon based alien. He is the last Diamont (presumably a play on the word diamond, which is composed of carbon). In the Star Control series, the Chenjesu, are intelligent, peaceful silicon-based lifeforms that were the backbone of the Alliance of Free Stars. In the game of Xenosaga, artificial life forms known as Raelians have been created using silicon-based chemistry. They resemble humans in every aspect, however they are considered to be "below" humans in the social ladder. A more humorous example comes from the Hitchhiker's Guide to the Galaxy, where the Hooloovoo are a hyperintelligent shade of the colour blue.
Star Trek
The best-known example of a non–carbon-based lifeform in science fiction is the Horta in the original Star Trek episode "The Devil in the Dark". A highly intelligent silicon-based creature made almost of pure rock, they tunnel through it as easily as humans move through air. The entire species dies out every 50,000 years save for one who tends all the eggs, which take the form of silicon nodules scattered throughout the caverns and tunnels of its home planet, Janus VI. The inadvertent destruction of many of these eggs by a human mining colony led the mother Horta to respond by murdering the colonists and sabotaging their equipment; it was only through a Vulcan mind meld that the race's benevolence and intelligence were discovered and peaceful relations established.
Star Trek would later offer other corporeal lifeforms with an alternative biochemistry. The Tholians of "The Tholian Web" are depicted and described, in that episode and later in the Star Trek: Enterprise episode "In a Mirror, Darkly" as being primarily of mineral-based composition and thriving only in superheated conditions. Another episode from TOS's third season, "The Savage Curtain", depicted another rock creature called an Excalbian, which is believed in fanon to also have been silicon-based.
Later on, in Star Trek: The Next Generation, the Crystalline Entity appeared in two episodes. This was an enormous spacefaring crystal lattice that had taken thousands of lives in its quest for energy. It may have been unaware of this, however, but it was destroyed before communications could be established at a level sufficient to ascertain it. Intelligent crystals that formed a "microbrain" and described the humans they encountered as "ugly bags of mostly water", were also found in the TNG episode "Home Soil".
"The Disease", an episode of Star Trek:Voyager featured some artificially-engineered silicon-based parasites, and an Enterprise episode, "Observer Effect", also presented a lethal silicon-based virus. In another Voyager episode, "Hope and Fear", a xenon-based lifeform was mentioned. In the Enterprise episode The Communicator, an alien species is encountered whose blood chemistry, while not explicitly stated, is sufficiently different to terrestrial organisms that it is not red and iron is toxic to it.
Other film and television
In "Firewalker", a second-season episode of The X-Files, a silicon-based plant that infects humans parasitically through its spores is discovered living deep in a volcano. A key plot point in the comedy Evolution involves nitrogen-based life forms, and using selenium-based shampoo to poison them (with the bonus of a product placement for Head & Shoulders).
See also
- Astrobiology
- Carbon-based life
- Extraterrestrial life
- Non-cellular life
- Alternative biology
- Iron-sulfur world theory
References
- Gillette, Stephen. World-Building. Writer's Digest Books.
- Lazio, Joseph. "F.10 Why do we assume that other beings must be based on carbon? Why couldn't organisms be based on other substances?". ET Life (Astronomy Frequently Asked Questions). Retrieved 2006-07-21.
- As of 2006-03-02, the page appears unavailable.
- "Aliens". Atomic Rockets. Retrieved 2006-03-03.
- W. Bains (2004). "Many Chemistries Could Be Used to Build Living Systems". Astrobiology. 4: 137–167.