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- Also see: Exotic life
Despite its diversity, all life on Earth displays a remarkable similarity at biochemical level: all the main molecules used in organic processes are built on skeletons of carbon and hydrogen; ions and molecules are dissolved and trasported in water-based fluids; most organisms breathe oxygen to release energy from food.
It's still a matter of debate if organic processes can be supported by different biochemical systems. If they can, though, and therefore much wider conditions are suitable to life, potential life-bearing planets become far more common, and their flora and fauna could be much more alien.
Non-Carbon Basis of BiochemistryEdit
Carbon is perhaps the most important element in our chemical makeup, to the point that organic molecules are defined by the presence of carbon atoms in their structure. It has a unique capacity of bonding with many different kinds of atoms, and forming long, stable polymeric chains.
The element most similar to carbon, being right under it in the periodic table, is silicon (Si). It has similar bonding property, but less versatility, and, being heavier, has more difficulty forming double bonds. Silicon's closest analogue to hydrocarbons, silanes (SinH2n+2), are highly reactive in water and don't form long chains. Moreover, most of the complex molecules found in space contain carbon but lack silicon. Also, while ten times less abundant than carbon in the cosmos, it's a thousand times more abundant in Earth's crust, and yet terran life never integrated it in its biochemical processes. Finally, the product of its oxidation, silicon dioxide (SiO2), is a non-soluble solid until more than 1600°C, whereas carbon dioxide is a gas. However, silicon dioxide would not be produced at all if silicon life breathed in a reducing atmosphere, containing gases such as hydrogen (H2), hydrogen sulphide (H2S), carbon monoxide (CO) or nitrogen: if silicon-based life exists at all, it will be in an atmosphere without oxygen, and probably a reducing one.
Silicon's best chance to form complex, stable molecules is to combine with oxygen in alternate chains (...Si-O-Si-O-Si...), creating silicones, stable even in the presence of oxygen and water; just under 300-350°C, silicones can form long polymeric structures where carbon cannot, and some silicon-carbon combination last until 500°C. On the other hand, William Bains argues that silicon could form stable and complex molecules with many other elements (carbon, germanium, nitrogen, oxygen, sulfur, phosphorus, halogens and metals) in very cold, hydrogen-less solvents such as liquid nitrogen.
Since silicon rarely forms double bonds, and never forms triple bonds like carbon does, silicon-based biomolecules would have to store energy in a different way, possibly exploiting silicon's properties as a semiconductor.
Similar results can be obtained with other carbon-group elements: namely, germanium (Ge), tin (Sn) and lead (Pb). The Xenology chapters linked at the bottom of the page gives some examples of germanium-, tin- and lead-based organic polymers.
Boron (B) is another possibility: it's even more versatile than carbon, and while its simplest compunds, boranes (BxHy), are extremely reactive in the presence of oxygen, they'd be more stable in a reducing atmosphere. A combination of alternating boron and nitrogen closely resembles carbon, and can form analogues of hydrocarbons such as ethane (C2H6), methane (CH4) and benzene (C6H6).
Less convincing substitutes for carbon can be metal oxides based on magnesium (Mg), aluminium (Al), iron (Fe) or titanium (Ti), which can form stable nanotubes and diamond-like crystals at high temperatures. Both phosphorus (P) and sulfur (S) are able to form long linear chains, but not branched ones, and they need a reducing atmosphere to be stable. Nitrogen (N) can also form long chains in solvents like ammonia (NH3) or hydrogen cyanide (HCN); it always tends to revert to a simple biatomic form, but it can be more stable when combined with carbon, phosphorus, boron or sulfur.
It's probably a safe bet to say that life will need a liquid to transport ions and molecules around the body. For terran life, this liquid (called a solvent) is always water. Water does have a number of advantages over other possible solvents:
- Water has a large liquidity range, which means that can be found in a liquid form in many different conditions (but hydrogen fluoride and formic acid have a similar liquidity range, and that of ethanol and sulfuric acid is even wider).
- Water is extremely common in the universe: in fact, it's the most common chemical compound (but sulfuric acid, methane and nitrogen are already known to exist in large amounts on other worlds).
- Ice is less dense than liquid water. That allows ice to float and insulate the water below, preventing a whole ocean to freeze.[note 1]
- Water is a polar solvent: that allows it to carry dissolved salts. Moreover, it can dissolve proteins, sugars, DNA and oxygen (but the same is true for other solvents, such as ammonia and formamide).
- Water is transparent to visible light, permitting the development of submarine photosynthetical ecologies.
- Water has a high specific heat and heat of vaporization, which allows it to absorb large quantities of heat, therefore regulating temperature differences.
- Water is amphoteric: it can work both as an acid and as a base, and is therefore able to participate in a large array of reactions, as a solvent, a reactant or a product.
Besides being abundant, a good solvent should at least have a wide liquidity range and dissolve many different compounds, and preferably it should have a high heat of fusion and vaporization (which would make it more stable when temperature changes), a high dielectric constant (making it a good insulant) and a low viscosity (so that it opposes little resistance to movement). It would likely be a thalassogen, that is, a substance that forms a planet's oceans.
Polar solvents are composed by molecules that have a slightly different electrical charge on different atoms: for example, the oxygen in a water molecule is slightly more negative than the hydrogen atoms. Such solvents are able to dissolve polar molecules and ionic compounds, but not non-polar molecules such as hydrocarbons.
The most likely alternative to water is ammonia (NH3), especially at low temperatures where water is solid. It's able to dissolve most organic compounds, lightweight metals, iodine, sulfur, selenium and phosphorous. It's not as good an insulant as water and it's easier to boil, but also harder to freeze (therefore, it has a high heat of fusion) and it's much less viscous. Its liquidity range is not as wide at 1 atm pressure (-78°C to -33°C), but at a pressure of 60 atm (life on Venus) it boils at 98°C, making it wider than water's range.
Alcohols, sugars, fatty acids and amino acid can all be easily solved into ammonia, especially replacing their -OH groups with -NH2. For example, the analogue of the alcohol ethanol (C2H5OH) would be C2H5NH2. It's also possible to replicate water's acids-and-bases chemistry: NH4+ would replace H3O+ as a (weaker) acid and NH2- would replace OH- as a (stronger) base. In an ammonia-based metabolism, energy would be stored in double bonds C=N.
Sulfuric acid (H2SO4) is another likely solvent that could support a variety of chemical reactions. It has a very wide liquidity range (10 to 338°C) and is exists in a large amount in the clouds of Venus. In this case, H2SO4 would work as acid and HSO4- as base; the balance would be strongly tilted in favour of the acid, but, even on Earth, extremophile organisms are known to live in extremely acidic conditions. In this case, the C=C double bond would be the best way to store energy.
Other possibilities include:
- formamide (HCONH2), produced by the reaction of hydrogen cyanide with water, as good a solvent as water is (in particular, it can dissolve aminoacids, proteins and nucleic acids), but not as reactive;
- hydrogen cyanide (HCN), with a wide liquidity range, low viscosity, high dielectric constant and specific heat, but also a tendency to polymerize that could interfere with biochemical processes;
- hydrogen fluoride (HF), which has the same liquidity range as water and dissolves polar molecules, just as water does. Paraffin waxes are stable in a HF environment, and the fluorine it contains can be breathed as an oxidant (see "Alternatives to oxygen").
Non-polar solvents are either composed of molecules where all the atoms have the same charge, such as nitrogen, or where the atoms are arranged in such a way that both the positive and the negative charges are located in the same point, such as methane. These solvents cannot dissolve proteins or other polar molecules as water does, so the biochemistry in a non-polar solvent would be radically different from ours.
Liquid hydrocarbons, such as methane (CH4) and ethane (C2H6), already known to be present on Titan's surface, lack polarity, so they couldn't dissolve proteins; hower, they could carry non-polar polymers, such as polylipids. Being less reactive and having weaker hydrogen bonds, they'd permit hydrocarbon-life to use hydrogen bonds of its own, especially in a low temperature biochemistry. Lifeforms living in hydrocarbon lakes could gain energy making ethylene (C2H4) and acetylene (C2H2) react with hydrogen, reducing them to methane.
Cryosolvents are composed by simple non-polar molecules that can exist as a liquid only at extremely low temperatures. Hydrogen (H2) is by far the most common molecule in the Solar System and in the Universe; although its liquidity range at low pressures is only 6 degrees wide, it can be greatly expanded as a supercritical fluid in the atmosphere of gas giants (at 230°C on Jupiter, too high for most organic molecules, but only 30°C on Saturn and -110°C on Uranus and Neptune).
Liquid nitrogen (N2) is another cryosolvent that might be the main solvent on Triton. Besides other non-polar compounds, it could dissolve complex silicon molecules such as polysilanes, perhaps allowing for a silicon-based biochemistry.
Properties of SolventsEdit
This table recaps the main characteristics of some possible solvents, in order of liquidity range. The values most similar to water are written in bold characters; names in italic denote solvents that are also likely thalassogens; NP marks non-polar solvents.
|Solvent||Fusion and evaporation points (°C)||Liquidity range (°C)||Heat of fusion (kcal/mole)||Heat of vaporization (kcal/mole)||Dielectric constant||Viscosity|
|SulfurNP (S2)||113 to 445||332||0.413||23.2||3.48||1|
|Sulfuric acid (H2SO4)||10 to 338||328||2.56||12.0||100||48.4|
|Glycerol (C3H8O3)||19 to 290||271||4.42||18.2||42.5||>100|
|Ethanol (C2H5OH)||-114 to 78||193||1.20||10.9||24.3||1.08|
|Formamide (HCONH2)||3 to 193||190||2.02||15.0||111||3.31|
|Methanol (CH3OH)||-98 to 65||163||0.759||8.42||32.6||0.544|
|ChloroformNP (CHCl3)||-64 to 61||125||2.10||7.50||5.61||0.70|
|Hydrogen fluoride (HF)||-83 to 20||103||1.09||7.23||83.6||0.256|
|Acetic acid (CH3COOH)||17 to 118||102||2.76||5.81||9.7||1.16|
|Water (H2O)||0 to 100||100||1.46||9.72||81.1||0.959|
|Formic acid (HCOOH)||9 to 101||92.3||3.04||4.77||58.5||1.80|
|Methylamine (CH3NH2)||-92 to -6||86.0||3.47||6.47||11.4||0.236|
|BenzeneNP (C6H6)||6 to 80||74.0||2.36||8.08||2.27||0.652|
|Formaldehyde (HCHO)||-92 to -21||71.0||?||5.92||?||?|
|Chlorine (Cl2)||-101 to -34||66.9||1.53||4.78||2.0||4.9|
|Sulfur dioxide (SO2)||-72 to -10||62.7||1.97||5.96||13.8||0.429|
|Ammonia (NH3)||-78 to -33||44.4||1.84||5.64||22.0||0.265|
|Hydrogen cyanide (HCN)||-13 to 26||39.0||2.01||6.03||123||0.201|
|Hydrogen chloride (HCl)||-115 to -85||29.9||0.476||3.86||12.0||0.51|
|Hydrogen sulfide (H2S)||-85 to -60||24.8||0.568||4.46||10.2||0.432|
|MethaneNP (CH4)||-182 to -161||21.0||0.22||2.13||1.7||0.0124|
|NitrogenNP (N2)||-210 to -196||14.3||0.086||1.33||1.001||0.0192|
|HydrogenNP (H2)||-259 to -253||6.3||0.133||0.215||1||0.00966|
Alternatives to Other ElementsEdit
Besides the basic carbon-oxygen-hydrogen combination, terran lifeforms use many other chemical elements in their organic processes, for example phosphorus (part of DNA structure and phospholipids), sulfur (in some amino acids) and calcium (strengthens tissues such as shells, bones and teeth).
There are at least some examples of this elements being replaced on Earth. Discovered in 2001 in Lake Mono (California), the bacterium GFAJ-1 was believed to replace phosphates in its metabolism with arsenates; while biologists do not think this anymore, arsenic (As) could have taken part in the biochemistry of early organisms before being outcompeted by far more common phosphorus; today, some marine algae incorporate arsenic in organic compounds, like arsenosugars and arsenobetaine. Heavier (and therefore less likely) elements similar to phosphorus are antimony (Sb) and bismuth (Bi).
Sulfur is already replaced in many organisms, including animals, by selenium (Se), in form of selenoproteins, while some fungi produce amino acids in which sulfur is replaced by tellurium (Te). The only other natural element in sulfur's same chemical group is polonium (Po).
Calcium minerals (sulphates and carbonates) impregnate animal tissues forming bones and shells, but glass sponges and many diatoms use silica (silicon dioxide), which can form hard, glass-like crystals. Other possible substitutes are metals such as iron or copper (Cu).
Alternatives to OxygenEdit
Plants, animals, fungi and most protozoans extract from glucose the chemical energy they need to live by oxidizing with oxygen, thus breaking it up and freeing the energy of its atomic bonds, producing less energetic water and carbon dioxide as waste products.
This, however, is not the rule for all organisms. A large number of bacteria "breathe" different chemicals: they have to be oxidizing agents, that is, chemicals that take electrons away from another reactant:
|Type of respiration||Oxidizing agent||Product||Example|
|metal reduction||Fe(III) (ferric iron), Mn(IV) (manganese), Co(III) (cobalt), U(VI) (uranium)||Fe(II), Mn(II), Co(II), U (IV)||Geobacter, Desulfuromonadales, etc.|
|denitrification||NO3− (nitrate)||NO2– (nitrite)||Escherichia coli, Paracoccus|
|sulfate respiration||SO42− (sulfate)||HS− (sulfide)||Escherichia coli|
|sulfur respiration||S (sulfur)||HS− (sulfide)||Desulfuromonadales|
|methanogenesis||CO2 (carbon dioxide)||CH4 (methane)||Methanothrix|
|acetogenesis||CO2 (carbon dioxide)||CH3CO−|
|dehalorespiration||halocarbons: organic compounds with halogens (see below)||halide ions, ex. Cl− or F−|
, and remaining organic compound
Other common oxidizing agents include ozone (O3), hydrogen peroxide (H2O2), halogens such as fluorine (F2) and chlorine (Cl2), nitric acid (HNO3), nitrous oxide (N2O), etc.
It's important to remember that the more reactive is a substance (at the top there are halogens, oxygen and the sodium group, see here the strongest shades of red and yellow), the less likely is to find it in nature in a pure form, because it will tend to form compounds. Oxygen is still abundant in Earth's atmosphere only because it's replenished by photosynthesis; an atmosphere rich in halogens or strong acids requires them to be continuously produced by organisms.
A great part of organic chemistry is occupied by hydrocarbons, compounds entirely made up by carbon and hydrogen, which can form polymeric chains and combine with oxygen to form carbohydrates. In a strongly oxidant, hydrogen-poor environment, where life still uses carbon as basis of its biochemistry, hydrocarbons may be replaced by oxocarbons. Carbon suboxide (C2O3) is an oxocarbon that can spontaneously polymerize in long chains.
Even with a biochemistry very similar to our own, there are still many possibilities for organic molecules: for example, chirality, the orientation in space of molecules. Sugars and amino acids are asymmetrical, and the two specular forms (enantiomers) have the same chemical properties, as long as all reactants in a reaction change their chirality. Terran life uses L ("left-handed") amino acids and D ("right-handed") sugars. Alien organisms with D amino acids could digest L sugars in the same way we do with D sugars, which they couldn't digest.
Amino acids can also have alpha and beta forms: in the first case, the amino group (-NH2) appears near the end of the carbon structure; in the second case, at the front. Terran biochemistry uses only alpha amino acids, but the difference probably depends only from the way water interacts with the molecules during formation. As in the chirality case, it's likely just a random event during the origin of life that stuck with every organism afterwards.
Alternative Nucleic AcidsEdit
- Also see: Genetics
DNA keeps genetic information thanks to its modular structure: two long polymeric chains made up from deoxyribose (C5H10O4, a sugar with five carbon atoms) and phosphate groups, each holding a large number of interchangeable nucleotides and connected to the other, containing the complementar nucleotides.
A number of similar polymers with different structures are known: TNA replaces deoxyribose with threose (C4H8O4), a sugar that has only four carbon atoms; HNA uses hexitol (C6H8(OH)6), which has six. GNA doesn't use sugars at all, but glycol (C2H6O2); PNA, even more different, has a compound similar to an amino acid linked to phosphate groups linked by peptide bonds. None of this polymers exists in nature, but they've all been produced in a lab. PNA could even combine with DNA to form a triple helix structure; it's thought to be a possible precursor to RNA in early life.
Alternative Photosynthetic PigmentsEdit
To absorb light, photosynthetic organisms use pigments that reflect light of a specific wavelength, and therefore colour (and this is the visible colour of the organism), and absorb the rest. On Earth, they absorb mostly visible light, thanks to a variety of pigments.
The most important are porphyrins, simple ring-shaped molecules, extremely stable, with a single metal atom in their centre. All green plants use chlorophyll, a green porphyrin with a magnesium atom, with two forms: chlorophyll a absorbs better purple and orange light, while chlorophyll b absorbs mostly blue and yellow light. Some bacteria use zinc- and copper-based porphyrins, and there is the possibility of porphyrins with nickel, cobalt or manganese atoms.
A different but still common kind of photosynthetic pigments is carotenoids, which in turn comprehend carotene (a hydrocarbon) and xanthophyll (made up from carbon, hydrogen and oxygen). They both absorb the more energetic blue-violet part of the light spectrum, so carotene appears orange and xanthophyll yellow; they're present in many vegetables, and xanthophyll is the chemical that makes an egg's yolk yellow.
Phaeophytin is derived from chlorophyll that lost its magnesium atom when treated with an acid; it appears dark blue and it's present in many algae and bacteria; most bacteria and some algae also use phycobiliproteins, which include blue phycocyanin and orange-red phycoerythrin. Halobacteria use another protein, bacteriorhodopsin, to absorb green light (it looks purple) directly to move protons with solar energy, without it taking part in true photosynthesis.
As a simpler alternative, some metal oxides, such as zinc oxide (white), titanium oxide (white) and tungsten oxide (light yellow) appear to be able to capture and store energy when exposed to light and take part in oxidation-reduction in a similar way to chlorophyll; silicon and germanium, instead, could convert light in electric energy as they do in photovoltaic cells, splitting water molecules through electrolysis (see here).
Here there is a collection of graphs that shows the most common wavelengths produced by different stars, in different environments. Whatever these wavelengths are, the plants likely won't be of that colour, because that is the wavelength they will want to absorb, rather than reflect. The peak of light in each situation is summarized in the table below; wavelengths are measured in nanometres (see here for detail on star classes and definition of Ms, Ls and AU). The table assumes a planet with an Earth-like atmosphere.
|High atmosphere||Surface||Underwater (5 cm deep)||Underwater (60 cm deep)|
|Mature M star: 0.2 Ms, 0.0044 Ls, 0.07 AU||988 nm (infrared)||1044 nm (infrared)||800 nm (infrared)||700-800 nm (red-infrared)|
|Young M star: 0.5 Ms, 0.023 Ls, 0.16 AU||1004 nm (infrared)||1045 nm (infrared)||800 and 1100 nm (infrared)||700-800 nm (red-infrared)|
|G star: 1.0 Ms, 1.0 Ls, 1.0 AU||583 nm (yellow)||685 nm (red)||700 nm (red)||600 nm (yellow)|
|F star: 1.4 Ms, 3.6 Ls, 1.69 AU||451 nm (blue)||451 nm (blue)||450 nm (blue)||450 nm (blue)|
Compare to the table the values of peak absorption of the most common photosynthetic pigments, which will have to be as close as possible to the value in the table:
- Pheomelanin: 330 nm (ultraviolet) - colour: reddish brown
- Eumelanin: 340 nm (ultraviolet) - colour: black or dark brown
- Clorophyll a: 430 nm (blue); 680 nm (red) - colour: green
- Chlorophyll b: 450 nm (blue); 660 nm (red) - colour: green-yellow
- Carotenoids: 480-520 nm (light blue to green) - colour: orange or yellow
- Phycoerythrin: 500-580 nm (green to orange) - colour: orange-red
- Bacteriorhopsin: 570 nm (green) - colour: purple
- Phycocyanin: 620 nm (red) - colour: blue
- Xanthophyll: - colour: yellow
Alternative Blood PigmentsEdit
- Also see: here.
All terran organisms having more than a few cells need a fluid transporting molecules around the body, that is, blood. While a liter of seawater at body temperature cannot carry more than 5 cm3 O2, adequate pigments can have a carrying capacity fifty times higher, by binding oxygen to metallic atoms. Most animal groups use hemoglobin, a porphyrin very similar to chlorophyll with an iron atom at the centre; it can simply bind to oxygen in the respiratory system, thanks to its high pressure, and then release it in the tissues.
Since iron oxide is red, hemoglobin (and the cells that contain it) colour blood red; it's been suggested that red has such a connotation of danger and emotion to humans because it's the colour of blood. However, different pigments are found throughout the animal kingdom that use different metal, ad therefore have a different colour: for example, hemocyanin, the second most common blood pigment, contains copper, and makes blood blue.
|Pigment||Metal||Colour (without oxygen)||Colour (with oxygen)||Notes||Examples|
|Hemoglobin||Iron||Dark red||Bright red||Needs to be carried by cells (red blood cells)||Most animals|
|Chlorocruorin||Iron||Green||Red||25% as efficient as hemoglobin||Polychaeta|
|Hemerythrin||Iron||Colourless||Bright pink or violet||Needs to be carried by cells; immune to CO poisoning||Some marine worms and brachiopods|
|Hemocyanin||Copper||Colourless||Blue||25% as efficient as hemoglobin; immune to CO poisoning||Many mollusks and arthropods|
|Vanadium chromagen||Vanadium||Apple-green, orange or blue||Needs to be carried by cells (vanadocytes); appears to freely bind and release oxygen in acidic solutions; insoluble in water||Tunicates (dubious)|
|Pinnaglobin||Manganese||Brown||Still largely unknown||Pinna|
|Coboglobin||Cobalt||Amber yellow||Light pink or colourless||Probably needs to be carried by cells; when exposed to light, it degrades in a few hours; immune to CO poisoning||(hypothetical)|
|Vaska's complex||Iridium||Brilliant yellow||Dark orange||Insoluble in polar solvents (water, ammonia, alcohols); very vulnerable to light; can also bind hydrogen||(hypothetical)|
In Earth-like conditions, hemoglobin is still the most efficient oxygen carrier known, though it could be outcompeted by others if other metals are more abundant or the organisms that develop them happen to have other evolutionary advantages. As discussed here (also see the last picture here), anyway, at lower concentrations of oxygen (px[note 2] < 0.02 atm) hemocyanin is actually more efficient than hemoglobin. Chlorocruorin and hemerythrin are never really competitive, while Vaska's complex is more efficient than several forms of hemoglobin at px < 0.02 atm and px > 0.2 atm. Coboglobin is roughly at par with hemoglobin, and slightly more efficient with px < 0.05 atm. Still, in favourable condition (high atmospheric pressure, high concentration of oxygen, high temperature) a very efficient transport of oxygen might be unnecessary, so minor pigments could thrive anyway. In any case, iron and copper are favoured over other metals simply by virtue of being more abundant.
Abundance of ElementsEdit
The relative abundance of different elements is very important to determine the chance of their involvement in a biochemistry. As a reference, here are the fifteen elements most common in the human body (that is, those that make up at least the 0.0001% of it) and in other systems, as measured by number of atoms:
|Z||Element||Universe||Earth's crust||Seawater||Human body||Biological role|
|8||Oxygen||0.08%||60%||33.1%||24%||organic molecules, respiration|
|7||Nitrogen||0.009%||0.0029%||<0.0001%||1.2%||amino acids, nucleic acids|
|15||Phosphorus||<0.0001%||0.07%||<0.0001%||0.22%||nucleic acids, ATP|
|16||Sulfur||0.002%||0.027%||0.0179%||0.039%||some amino acids, e.g. cysteine|
|9||Fluorine||<0.0001%||0.059%||<0.0001%||0.0012%||fluorapatite (tooth enamel)|
- Hypotethical types of biochemistry (Wikipedia)
- Exotic Biochemistries and Alien Blood (Xenology) (also see further pages)
- Alternative Biochemistries WIP
- Alternative forms of life and Blood varieties (Encyclopedia of Science)
- ↑ On the other hand, the expansion of water when freezing can also damage cells through the formation of large ice crystals: life in another solvent could hybernate better when the temperature becomes too low for them.
- ↑ px = partial pressure, the product of the atmospheric pressure and the concentration by volume of the gas. For example, if the atmosphere has a pressure of 5 atm and it has a concentration of 30% (0.30) of oxygen, the oxygen partial pressure is 0.3x5=1.5 atm.
- ↑ 1.0 1.1 William Bains, "Many Chemistries Could Be Used to Build Living Systems", Astrobiology, 2004. <http://www.agentincite.com/wp-content/uploads/2012/08/Baines_astrobio04.pdf>
- ↑ 2.0 2.1 National Research Council, "The Limits of Organic Life in Planetary Systems", 2007, page 72 and following. <http://ecoversity.org/archives/LOLPS.pdf>
- ↑ Markus Schmidt, "Xenobiology: A new form of life as the ultimate biosafety tool", National Institutes of Health, April 2010. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2909387/>
- ↑ Peter E. Nielsen, "Triple Helix: Designing a New Molecule of Life", Scientific American, December 2008. <http://www.scientificamerican.com/article/triple-helix-designing-a-new-molecule/>
- ↑ 5.0 5.1 5.2 5.3 WebElements, Abundances in the universe. <http://www.webelements.com/periodicity/abundance_universe_a/>