Hypothetical types of biochemistry
Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time.[2] The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons it may be chemically similar, though it is also possible that there are organisms with quite different chemistries[3] – for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water.
The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in
The element
Overview of hypothetical types of biochemistry
Type | Basis | Brief description | Remarks |
---|---|---|---|
Alternative-chirality biomolecules | Alternative biochemistry | Mirror image biochemistry | Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules using D amino acids or L sugars are possible, though they would be incompatible with organisms using the opposing chirality molecules. Gram-positive bacteria incorporate D alanine into their Peptidoglycan layer, created through the actions of Racemases[4]
|
Ammonia biochemistry | Non-water solvents | Ammonia-based life | Ammonia is liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J. B. S. Haldane raised the topic at a symposium about life's origin.
|
Arsenic biochemistry | Alternative biochemistry | Arsenic-based life | Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. |
Borane biochemistry (Organoboron chemistry) | Alternative biochemistry | Boranes-based life | Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. Boron, however, is exceedingly rare in the universe in comparison to its neighbours carbon, nitrogen, and oxygen. On the other hand, structures containing alternating boron and nitrogen atoms share some properties with hydrocarbons. |
Cosmic necklace-based biology | Nonplanetary life | Non-chemical life | In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky hypothesized that life composed of magnetic semipoles connected by cosmic strings could evolve inside stars.[5] |
Dusty plasma-based biology | Nonplanetary life | Non-chemical life | In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space.[6] |
Extremophiles | Alternative environment | Life in variable environments | It would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it. |
Heteropoly acid biochemistry | Alternative biochemistry | Heteropoly acid-based life | Various metals can form complex structures with oxygen, such as heteropoly acids .
|
Hydrogen fluoride biochemistry | Non-water solvents | Hydrogen fluoride-based life | Hydrogen fluoride has been considered as a possible solvent for life by scientists such as Peter Sneath. |
Hydrogen sulfide biochemistry | Non-water solvents | Hydrogen sulfide-based life | Hydrogen sulfide is a chemical analog of water, but is less polar and a weaker inorganic solvent. |
Methane biochemistry (Azotosome) | Non-water solvents | Methane-based life | Methane (CH4) is relatively abundant in the solar system and the universe, and is known to exist in liquid form on Titan, the largest moon of Saturn. Though highly unlikely, it is considered to be possible for Titan to harbor life. If so, it will most likely be methane-based life. |
Non-green photosynthesizers | Other speculations | Alternate plant life | Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than Earth. In particular, retinal is capable of, and has been observed to, perform photosynthesis.[7] Bacteria capable of photosynthesis are known as microbial rhodopsins. A plant or creature that uses retinal photosynthesis is always purple. |
Shadow biosphere | Alternative environment | A hidden life biosphere on Earth | A shadow biosphere is a hypothetical processes than currently known life. |
Silicon biochemistry ( Organosilicon )
|
Alternative biochemistry | Silicon-based life | Like carbon, silicon can create molecules that are sufficiently large to carry biological information; however, the scope of possible silicon chemistry is far more limited than that of carbon. |
Silicon dioxide biochemistry | Non-water solvents | Silicon dioxide-based life | Gerald Feinberg and Robert Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium. |
Sulfur biochemistry | Alternative biochemistry | Sulfur-based life | The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones. |
Alternative nucleic acids | Alternative biochemistry | Different genetic storage | peptide bonds.[8] In comparison, Hachimoji DNA changes the base pairs instead of the backbone. These new base pairs are P (2-Aminoimidazo[1,2a][1,3,5]triazin-4(1H)-one), Z (6-Amino-5-nitropyridin-2-one), B (Isoguanine), and S (rS = Isocytosine for RNA, dS = 1-Methylcytosine for DNA).[9][10]
|
Shadow biosphere
A shadow biosphere is a hypothetical
Alternative-chirality biomolecules
Perhaps the least unusual alternative biochemistry would be one with differing
It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternative stereochemistry, molecules that are overwhelmingly found in one enantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal) organisms such as in comparisons between members of Archaea and other domains,[citation needed] making it an open topic whether an alternative stereochemistry is truly novel.
Non-carbon-based biochemistries
On Earth, all known living things have a carbon-based structure and system. 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 such atoms to form all the necessary structures. However, as Carl Sagan argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe.[14] Sagan used the term "carbon chauvinism" for such an assumption.[15] He regarded silicon and germanium as conceivable alternatives to carbon[15] (other plausible elements include but are not limited to palladium and titanium); but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos).[16] Norman Horowitz devised the experiments to determine whether life might exist on Mars that were carried out by the Viking Lander of 1976, the first U.S. mission to successfully land a probe on the surface of Mars. Horowitz argued that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets.[17] He considered that there was only a remote possibility that non-carbon life forms could exist with genetic information systems capable of self-replication and the ability to evolve and adapt.
Silicon biochemistry
The silicon atom has been much discussed as the basis for an alternative biochemical system, because silicon has many
However, silicon has several drawbacks as a carbon alternative. Carbon is ten times more cosmically abundant than silicon, and its chemistry appears naturally more complex.[19] By 1998, astronomers had identified 84 carbon-containing molecules in the interstellar medium, but only 8 containing silicon, of which half also included carbon.[20] Even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (silicon is roughly 925 times more abundant in Earth's crust than carbon), terrestrial life bases itself on carbon. It may eschew silicon because silicon compounds are less varied, unstable in the presence of water, or block the flow of heat.[19]
Relative to carbon, silicon has a much larger
All known silicon macromolecules are artificial polymers, and so "monotonous compared with the combinatorial universe of organic macromolecules".[18][21] Even so, some Earth life uses biogenic silica: diatoms' silicate skeletons. A. G. Cairns-Smith hypothesized that silicate minerals in water played a crucial role in abiogenesis, in that biogenic carbon compounds formed around their crystal structures.[24][25] Although not observed in nature, carbon–silicon bonds have been added to biochemistry under directed evolution (artificial selection): a cytochrome c protein from Rhodothermus marinus has been engineered to catalyze new carbon–silicon bonds between hydrosilanes and diazo compounds.[26]
Other exotic element-based biochemistries
- Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing atmosphere. However, boron's low cosmic abundance makes it less likely as a base for life than carbon.
- Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds;[heteropoly acids are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond-like crystals (such as cubic zirconia). Titanium, aluminium, magnesium, and iron are all more abundant in the Earth's crust than carbon. Metal-oxide-based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon-based life would be unlikely. The Cronin group at Glasgow University reported self-assembly of tungsten polyoxometalates into cell-like spheres.[27] By modifying their metal oxide content, the spheres can acquire holes that act as porous membrane, selectively allowing chemicals in and out of the sphere according to size.[27]
- Sulfur is also able to form long-chain molecules, but suffers from the same high-reactivity problems as phosphorus and silanes. The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones. (The biological use of sulfur as an electron acceptor is widespread and can be traced back 3.5 billion years on Earth, thus predating the use of molecular oxygen.[28] Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)
Arsenic as an alternative to phosphorus
can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy.It has been speculated that the earliest life forms on Earth may have used arsenic biochemistry in place of phosphorus in the structure of their DNA.[31] A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic is poorly suited for this function.[32]
The authors of a 2010
Non-water solvents
In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. This has led to discussions about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist
Water as a solvent limits the forms biochemistry can take. For example, Steven Benner, proposes the
Carl Sagan once described himself as both a
Some of the properties of water that are important for life processes include:
- A complexity which leads to a large number of permutations of possible reaction paths including acid–base chemistry, H+ cations, OH− anions, hydrogen bonding, van der Waals bonding, dipole–dipole and other polar interactions, aqueous solvent cages, and hydrolysis. This complexity offers a large number of pathways for evolution to produce life, many other solvents[which?] have dramatically fewer possible reactions, which severely limits evolution.
- Thermodynamic stability: the free energy of formation of liquid water is low enough (−237.24 kJ/mol) that water undergoes few reactions. Other solvents are highly reactive, particularly with oxygen.
- Water does not combust in oxygen because it is already the combustion product of hydrogen with oxygen. Most alternative solvents are not stable in an oxygen-rich atmosphere, so it is highly unlikely that those liquids could support aerobic life.
- A large temperature range over which it is liquid.
- High solubility of oxygen and carbon dioxide at room temperature supporting the evolution of aerobic aquatic plant and animal life.
- A high heat capacity (leading to higher environmental temperature stability).
- Water is a room-temperature liquid leading to a large population of quantum transition states required to overcome reaction barriers. Cryogenic liquids (such as liquid methane) have exponentially lower transition state populations which are needed for life based on chemical reactions. This leads to chemical reaction rates which may be so slow as to preclude the development of any life based on chemical reactions.[citation needed]
- Spectroscopic transparency allowing solar radiation to penetrate several meters into the liquid (or solid), greatly aiding the evolution of aquatic life.
- A large heat of vaporizationleading to stable lakes and oceans.
- The ability to dissolve a wide variety of compounds.
- The solid (ice) has lower density than the liquid, so ice floats on the liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds), then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life.
Water as a compound is cosmically abundant, although much of it is in the form of vapor or ice. Subsurface liquid water is considered likely or possible on several of the outer moons: Enceladus (where geysers have been observed), Europa, Titan, and Ganymede. Earth and Titan are the only worlds currently known to have stable bodies of liquid on their surfaces.
Not all properties of water are necessarily advantageous for life, however.[50] For instance, water ice has a high albedo,[50] meaning that it reflects a significant quantity of light and heat from the Sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.[50]
There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Because boiling points vary with the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid-phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus, with 92 bars (91 atm) of pressure, it can indeed exist in liquid form over a wide temperature range.
Ammonia
The ammonia molecule (NH3), like the water molecule, is abundant in the universe, being a compound of hydrogen (the simplest and most common element) with another very common element, nitrogen.[51] The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J. B. S. Haldane raised the topic at a symposium about life's origin.[52]
Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water.
Ammonia, like water, can either accept or donate an H+ ion. When ammonia accepts an H+, it forms the
However, ammonia has some problems as a basis for life. The
A
Another set of conditions where ammonia is liquid at Earth-like temperatures would involve it being at a much higher pressure. For example, at 60
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
Methane and other hydrocarbons
There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia.[57][58][59] Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell.[60] However, water is also more chemically reactive and can break down large organic molecules through hydrolysis.[57] A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way.[57] Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules.[50] Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules.[57] Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry.[57]
Astrobiologist
Azotosome
A hypothetical
Hydrogen fluoride
Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. At atmospheric pressure, its melting point is 189.15 K (−84.00 °C), and its boiling point is 292.69 K (19.54 °C); the difference between the two is a little more than 100 K. HF also makes hydrogen bonds with its neighbor molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath[69] and Carl Sagan.[49]
HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such as paraffin waxes, are stable with it.[49] Like water and ammonia, liquid hydrogen fluoride supports an acid–base chemistry. Using a solvent system definition of acidity and basicity, nitric acid functions as a base when it is added to liquid HF.[70]
However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.[71]
Hydrogen sulfide
Hydrogen sulfide is the closest chemical analog to water,[72] but is less polar and is a weaker inorganic solvent.[73] Hydrogen sulfide is quite plentiful on Jupiter's moon Io and may be in liquid form a short distance below the surface; astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there.[74] On a planet with hydrogen sulfide oceans, the source of the hydrogen sulfide could come from volcanoes, in which case it could be mixed in with a bit of hydrogen fluoride, which could help dissolve minerals. Hydrogen sulfide life might use a mixture of carbon monoxide and carbon dioxide as their carbon source. They might produce and live on sulfur monoxide, which is analogous to oxygen (O2). Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the small temperature range where it is liquid, though that, like that of hydrogen cyanide and ammonia, increases with increasing pressure.
Silicon dioxide and silicates
Silicon dioxide, also known as silica and quartz, is very abundant in the universe and has a large temperature range where it is liquid. However, its melting point is 1,600 to 1,725 °C (2,912 to 3,137 °F), so it would be impossible to make organic compounds in that temperature, because all of them would decompose. Silicates are similar to silicon dioxide and some have lower melting points than silica. Feinberg and Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium.[75]
Other solvents or cosolvents
Other solvents sometimes proposed:
- Supercritical fluids: supercritical carbon dioxide and supercritical hydrogen.[76]
- Simple hydrogen compounds: hydrogen chloride.[77]
- More complex compounds: sulfuric acid,[43] formamide,[44] methanol.[77]
- Very-low-temperature fluids: liquid nitrogen[45] and hydrogen.[45]
- High-temperature liquids: sodium chloride.[78]
Sulfuric acid in liquid form is strongly polar. It remains liquid at higher temperatures than water, its liquid range being 10 °C to 337 °C at a pressure of 1 atm, although above 300 °C it slowly decomposes. Sulfuric acid is known to be abundant in the clouds of Venus, in the form of aerosol droplets. In a biochemistry that used sulfuric acid as a solvent, the alkene group (C=C), with two carbon atoms joined by a double bond, could function analogously to the carbonyl group (C=O) in water-based biochemistry.[43]
A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent.[79] A 61.2% (by mass) mix of water and hydrogen peroxide has a freezing point of −56.5 °C and tends to
Supercritical carbon dioxide has been proposed as a candidate for alternative biochemistry due to its ability to selectively dissolve organic compounds and assist the functioning of enzymes and because "super-Earth"- or "super-Venus"-type planets with dense high-pressure atmospheres may be common.[76]
Other speculations
Non-green photosynthesizers
Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than Earth.[82][83] These studies indicate that blue plants would be unlikely; however yellow or red plants may be relatively common.[83]
Variable environments
Many Earth plants and animals undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having a spore or hibernation state that can be sustained for years or even millennia between more active life stages.[84] Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.
For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state,[84] whereas desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods.[85] Either type of frog would appear biochemically inactive (i.e. not living) during dormant periods to anyone lacking a sensitive means of detecting low levels of metabolism.
Alanine world and hypothetical alternatives
The genetic code may have evolved during the transition from the RNA world to a protein world.[86] The Alanine World Hypothesis postulates that the evolution of the genetic code (the so-called GC phase[87]) started with only four basic amino acids: alanine, glycine, proline and ornithine (now arginine).[88] The evolution of the genetic code ended with 20 proteinogenic amino acids. From a chemical point of view, most of them are Alanine-derivatives particularly suitable for the construction of α-helices and β-sheets – basic secondary structural elements of modern proteins. Direct evidence of this is an experimental procedure in molecular biology known as alanine scanning.
A hypothetical "Proline World" would create a possible alternative life with the genetic code based on the proline chemical scaffold as the protein backbone. Similarly, a "Glycine World" and "Ornithine World" are also conceivable, but nature has chosen none of them.[89] Evolution of life with Proline, Glycine, or Ornithine as the basic structure for protein-like polymers (foldamers) would lead to parallel biological worlds. They would have morphologically radically different body plans and genetics from the living organisms of the known biosphere.[90]
Nonplanetary life
Dusty plasma-based
In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space.[91][92] Computer models showed that, when the dust became charged, the particles could self-organize into microscopic helical structures, and the authors offer "a rough sketch of a possible model of...helical grain structure reproduction".
Cosmic necklace-based
In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky of the City University of New York hypothesized that cosmic necklace-based life composed of magnetic monopoles connected by cosmic strings could evolve inside stars.[5] This would be achieved by a stretching of cosmic strings due to the star's intense gravity, thus allowing it to take on more complex forms and potentially form structures similar to the RNA and DNA structures found within carbon-based life. As such, it is theoretically possible that such beings could eventually become intelligent and construct a civilization using the power generated by the star's nuclear fusion. Because such use would use up part of the star's energy output, the luminosity would also fall. For this reason, it is thought that such life might exist inside stars observed to be cooling faster or dimmer than current cosmological models predict.
Life on a neutron star
Frank Drake suggested in 1973 that intelligent life could inhabit neutron stars.[93] Physical models in 1973 implied that Drake's creatures would be microscopic.[citation needed]
Scientists who have published on this topic
Scientists who have considered possible alternatives to carbon-water biochemistry include:
- J. B. S. Haldane (1892–1964), a geneticist noted for his work on abiogenesis.[52]
- V. Axel Firsoff (1910–1981), British astronomer.[94]
- Isaac Asimov (1920–1992), biochemist and science fiction writer.[51]
- Fred Hoyle (1915–2001), astronomer and science fiction writer.
- Norman Horowitz (1915–2005) Caltech geneticist who devised the first experiments carried out to detect life on Mars.[17]
- George C. Pimentel (1922–1989), American chemist, University of California, Berkeley.[95]
- Peter Sneath (1923–2011), microbiologist, author of the book Planets and Life.[69]
- Gerald Feinberg (1933–1992), physicist and Robert Shapiro (1935–2011), chemist, co-authors of the book Life Beyond Earth.[96][97]
- SETIproponent.
- Jonathan Lunine (born 1959), American planetary scientist and physicist.
- Robert Freitas (born 1952), specialist in nano-technology and nano-medicine.[98][99]
See also
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{{cite web}}
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- ^ Ellison, Doug (2007-08-24). "Europlanet : Life's a bleach". Planetary.org. Archived from the original on 2010-09-25. Retrieved 2007-08-25.
- ^ "NASA – NASA Predicts Non-Green Plants on Other Planets". Nasa.gov. 2008-02-23. Retrieved 2010-05-29.
- ^ S2CID 9172251.
- ^ a b "Christmas in Yellowstone". Pbs.org. Retrieved 2010-05-29.
- JSTOR 1933854.
- PMID 27926995.
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- ^ "Physicists Discover Inorganic Dust With Lifelike Qualities". Science Daily. 2007-08-15.
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{{cite journal}}
: CS1 maint: multiple names: authors list (link - ^ Drake, F.D. (December 1973). "Life on a Neutron Star: An Interview with Frank Drake" (PDF). Astronomy: 5–8. Archived (PDF) from the original on 2021-03-15.
- ^ V. Axel Firsoff (January 1962). "An Ammonia-Based Life". Discovery. 23: 36–42. cited in Darling, David. "ammonia-based life". Archived from the original on 2012-10-18. Retrieved 2012-10-01.
- ^ a b Shklovskii, I.S.; Carl Sagan (1977). Intelligent Life in the Universe. Picador. p. 229.
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- ^ A detailed review of this book is: John Gribbin (2 Oct 1980). "Life beyond Earth". New Scientist: xvii.
- ^ Freitas, Robert A. (1979). Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization. Sacramento, CA: Xenology Research Institute.
- ^ This work is acknowledged the partial basis of the article Darling, David. "ammonia-based life". Archived from the original on 2012-10-18. Retrieved 2012-10-01.
- ^ Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; The Limits of Organic Life in Planetary Systems; The National Academies Press, 2007.
- ^ Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; The Limits of Organic Life in Planetary Systems; The National Academies Press, 2007; page 5
Further reading
- Bains, William (2004). "Many Chemistries Could Be Used to Build Living Systems". Astrobiology. 4 (2): 137–167. S2CID 27477952.