Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. 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; it is also possible that there are organisms with quite different chemistries—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 also a common subject in science fiction.
The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.
False-color Cassini radar mosaic of Titan’s north polar region; the blue areas are lakes of liquid hydrocarbons.
“The existence of lakes of liquid hydrocarbons on Titan opens up the possibility for solvents and energy sources that are alternatives to those in our biosphere and that might support novel life forms altogether different from those on Earth.”—NASA Astrobiology Roadmap 2008
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 may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. Amino acids whose chirality is opposite to the norm are found on Earth, and these substances are generally thought to result from decay of organisms of normal chirality. However, physicist Paul Davies speculates that some of them might be products of “anti-chiral” life.
也许最不寻常的替代生物化学将是具有不同手性的生物分子。 在已知的地球生命中，氨基酸几乎普遍为L型，而糖则为D型。 使用D氨基酸或L糖的分子是可能的； 然而，具有这种手性的分子将与使用相反手性分子的生物不相容。 在地球上发现了手性与标准相反的氨基酸，通常认为这些物质是由正常手性的生物体腐烂产生的。 但是，物理学家保罗·戴维斯（Paul Davies）推测其中一些可能是“反手性”生命的产物。
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, making it an open topic whether an alternative stereochemistry is truly novel.
但是，这样的生物化学是否真的是外来的却值得怀疑。 尽管这肯定是另一种立体化学，但在绝大多数生物体中的一种对映体中绝大多数都发现了分子，但经常可以在不同（通常为基础）生物体的另一种对映体中找到，例如古细菌成员与其他域之间的比较， [需要的引用]使替代立体化学是否真正新颖成为一个公开的话题。
A shadow biosphere is a hypothetical microbialbiosphere of Earth that uses radically different biochemical and molecular processes than currently known life. Although life on Earth is relatively well-studied, the shadow biosphere may still remain unnoticed because the exploration of the microbial world targets primarily the biochemistry of the macro-organisms.
阴影 生物圈是地球上一个假想的微生物生物圈，它使用的生物化学和分子过程与目前已知的生命完全不同。  尽管对地球上的生命进行了相对深入的研究，但由于对微生物世界的探索主要针对宏观生物的生物化学，因此阴影生物圈可能仍未被关注。
碳基替代生物化学Carbon-based alternate biochemistries
砷替代磷Arsenic as an alternative to phosphorus
See also: GFAJ-1
Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy.
化学上与磷相似的砷，虽然对地球上大多数生命形式都是有毒的，但已被纳入某些生物的生物化学中。 一些海藻将砷掺入复杂的有机分子中，例如砷糖和砷甜菜碱。 真菌和细菌会产生挥发性的甲基化砷化合物。 在微生物中已经观察到砷还原和亚砷氧化（Chrysiogenes arsenatis）。 另外，一些原核生物可以在厌氧生长过程中将砷酸盐用作末端电子受体，而一些原核生物可以利用砷酸盐作为电子供体来产生能量。
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. 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.
The authors of a 2010 geomicrobiology study, supported in part by NASA, have postulated that a bacterium, named GFAJ-1, collected in the sediments of Mono Lake in eastern California, can employ such ‘arsenic DNA’ when cultured without phosphorus. They proposed that the bacterium may employ high levels of poly-β-hydroxybutyrate or other means to reduce the effective concentration of water and stabilize its arsenate esters.
2010年地球微生物学研究的作者在一定程度上得到了美国国家航空航天局（NASA）的支持，它假设一种收集在加利福尼亚东部莫诺湖沉积物中的名为GFAJ-1的细菌在无磷培养时可以利用这种“砷DNA”。[11 ]  他们建议细菌可以使用高含量的聚-β-羟基丁酸酯或其他手段来降低水的有效浓度并稳定其砷酸酯。
This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls. Science writer Carl Zimmer contacted several scientists for an assessment: “I reached out to a dozen experts … Almost unanimously, they think the NASA scientists have failed to make their case”. Other authors were unable to reproduce their results and showed that the study had issues with phosphate contamination, suggesting that the low amounts present could sustain extremophile lifeforms. Alternatively, it was suggested that GFAJ-1 cells grow by recycling phosphate from degraded ribosomes, rather than by replacing it with arsenate.
这项主张在发表后几乎立即就因缺乏适当的控制而受到严厉批评。  科学作家卡尔·齐默（Carl Zimmer）联系了几位科学家进行评估：“我接触了十几位专家……几乎所有人，他们都认为美国宇航局的科学家没有提出他们的论点。”  其他作者无法重现他们的结果，并表明该研究存在磷酸盐污染问题，表明存在的低含量可维持极端微生物的生命形式。 另外，有人建议，GFAJ-1细胞的生长是通过回收降解核糖体中的磷酸盐，而不是用砷酸盐代替。
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. Sagan used the term “carbon chauvinism” for such an assumption. He regarded silicon and germanium as conceivable alternatives to carbon; (other plausible elements include but aren’t 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.
在地球上，所有已知的生物都具有碳基结构和系统。 科学家已经推测出使用碳以外的原子来形成生命所必需的分子结构的利弊，但没有人提出使用这种原子来形成所有必需结构的理论。 但是，正如卡尔·萨根（Carl Sagan）所论证的那样，很难确定适用于地球上所有生命的陈述是否会适用于整个宇宙中的所有生命。 萨根（Sagan）使用“碳沙文主义”一词来表示这一假设。 他认为硅和锗可以作为碳的替代品； （其他可能的元素包括但不限于钯和钛），但另一方面，他指出碳在化学上似乎更通用，并且碳在宇宙中更丰富。
See also: Organosilicon
Structure of the silicone polydimethylsiloxane (PDMS)
Marine diatoms—carbon-based organisms that extract silicon from sea water, in the form of its oxide (silica) and incorporate it into their cell walls
The silicon atom has been much discussed as the basis for an alternative biochemical system, because silicon has many chemical properties similar to those of carbon and is in the same group of the periodic table, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.
However, silicon has several drawbacks as an alternative to carbon. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms as is necessary for the chemical versatility required for metabolism, and yet this precise inability is what makes silicon less susceptible to bond with all sorts of impurities from which carbon, in comparison, is not shielded. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms.
然而，硅具有替代碳的若干缺点。 硅与碳不同，它缺乏与新陈代谢所必需的化学多功能性所必需的，与各种类型的原子形成化学键的能力，但是这种精确的无能使硅较不易与碳， 相比之下，没有被屏蔽。 用碳形成有机官能团的元素包括氢，氧，氮，磷，硫和诸如铁，镁和锌的金属。 另一方面，硅与极少数其他类型的原子相互作用。
Moreover, where it does interact with other atoms, silicon creates molecules that have been described as “monotonous compared with the combinatorial universe of organic macromolecules”. This is because silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds (the double-bonded carbon is part of the carbonyl group, a fundamental motif of carbon-based bio-organic chemistry).
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 sulfuric-acid-rich environment, as is found in some extraterrestrial locations.
硅烷是氢和硅的化学化合物，类似于烷烃，与水的反应性强，长链硅烷会自发分解。 结合有交替的硅和氧原子的聚合物而不是硅之间的直接键合的分子（统称为硅酮）要稳定得多。 有人建议，在某些富含地球硫酸的环境中，有机硅基化学物质比同等碳氢化合物更稳定，这在一些地球外环境中也可以发现。
Of the varieties of molecules identified in the interstellar medium as of 1998, 84 are based on carbon, while only 8 are based on silicon. Moreover, of those 8 compounds, 4 also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation on which to build silicon-based biologies, at least under the conditions prevalent on the surface of planets.
截至1998年，在星际介质中识别出的各种分子中，有84种基于碳，而只有8种基于硅。 此外，在这8种化合物中，其中4种还包含碳。 碳对硅的宇宙丰度大约为10比1。这可能表明整个宇宙中存在更多种类的复杂碳化合物，至少在表面普遍存在的条件下， 行星提供了更少的基础来构建基于硅的生物。
Also, even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (the relative abundance of silicon to carbon in Earth’s crust is roughly 925:1), terrestrial life is carbon-based. The fact that carbon is used instead of silicon may be evidence that silicon is poorly suited for biochemistry on Earth-like planets. Reasons for which may be that silicon is less versatile than carbon in forming compounds, that the compounds formed by silicon are unstable, and that it blocks the flow of heat.
同样，即使地球和其他陆地行星异常富含硅且碳贫乏（地壳中硅与碳的相对丰度约为925：1），陆地生命也是基于碳的。 用碳代替硅的事实可能证明硅不太适合于类地行星上的生物化学。 原因可能是硅在形成化合物时比碳的通用性差，硅形成的化合物不稳定，并且阻碍了热流。
Even so, biogenic silica is used by some Earth life, such as the silicate skeletal structure of diatoms. According to the clay hypothesis of A. G. Cairns-Smith, silicate minerals in water played a crucial role in abiogenesis: they replicated their crystal structures, interacted with carbon compounds, and were the precursors of carbon-based life.
即便如此，地球某些生命仍使用生物二氧化硅，例如硅藻的硅酸盐骨架结构。 根据凯恩斯·史密斯（A. G. Cairns-Smith）的粘土假说，水中的硅酸盐矿物在生物发生中起着至关重要的作用：它们复制其晶体结构，与碳化合物相互作用，并且是碳基生命的先驱。 
Although not observed in nature, carbon–silicon bonds have been added to biochemistry by using directed evolution (artificial selection). A heme containing cytochrome c protein from Rhodothermus marinus has been engineered using directed evolution to catalyze the formation of new carbon–silicon bonds between hydrosilanes and diazo compounds.
尽管在自然界中没有观察到，但碳-硅键已通过定向进化（人工选择）添加到生物化学中。 利用定向进化技术设计了一种来自红景天（Rhodothermus marinus）的含有血红素的细胞色素c蛋白，以催化氢硅烷与重氮化合物之间新的碳-硅键的形成。
Silicon compounds may possibly be biologically useful under temperatures or pressures different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon. Polysilanols, the silicon compounds corresponding to sugars, are soluble in liquid nitrogen, suggesting that they could play a role in very-low-temperature biochemistry.
硅化合物在与地球行星表面不同的温度或压力下，可能与碳结合或作用比碳更直接地在生物学上有用。 聚硅醇类（相当于糖类的硅化合物）可溶于液氮，表明它们可能在极低温生物化学中发挥作用。 
In cinematic and literary science fiction, at a moment when man-made machines cross from nonliving to living, it is often posited,[by whom?] this new form would be the first example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as computers (or computer-guided robots) and filed under “silicon-based life”, even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for “wet life”.
在电影和文学科幻小说中，当人造机器从无生命过渡到有生命的那一刻，它常常被摆放，[由谁来]这种新形式将成为无碳生活的第一个例子。 自从1960年代后期微处理器问世以来，这些机器通常被归类为计算机（或计算机引导的机器人），属于“基于硅的生命”，尽管这些处理器的硅支持矩阵对 它们作为碳的运行是为了“湿生活”。
其他基于外来元素的生物化学Other exotic element-based biochemistries
See also: Organoboron chemistry
- Boranes are dangerously explosive in Earth’s atmosphere, but would be more stable in a reducing environment. 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; the 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. 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.
- 各种金属与氧气一起可以形成非常复杂且热稳定的结构，可以与有机化合物相媲美； [需要引证]杂多酸就是其中一个。一些金属氧化物在形成纳米管结构和类金刚石晶体（如立方氧化锆）的能力上也与碳相似。地壳中的钛，铝，镁和铁比碳更丰富。因此，在某些条件下，包括基于碳氧化物的生命不可能发生的情况下，基于金属氧化物的生命可能成为可能。格拉斯哥大学的Cronin研究小组报告说，多金属氧酸钨能自组装成细胞状球体。通过改变其金属氧化物的含量，这些球可以获取充当多孔膜的孔，从而根据尺寸选择性地允许化学物质进出该球。
- 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. Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)
- 硫也能够形成长链分子，但也遭受与磷和硅烷相同的高反应性问题。硫作为生物替代碳的生物学用途纯属假设，尤其是因为硫通常仅形成线性链而不是支链。 （硫作为一种电子受体的生物用途已广泛存在，可追溯到地球上35亿年，因此早于分子氧的使用。还原硫细菌可以利用元素硫代替氧气，从而将硫还原为氢硫化物。）
Carl Sagan speculated that alien life might use ammonia, hydrocarbons or hydrogen fluoride instead of water.
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 Steven Benner, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia, sulfuric acid, formamide, hydrocarbons, and (at temperatures much lower than Earth’s) liquid nitrogen, or hydrogen in the form of a supercritical fluid.
除碳化合物外，所有目前已知的陆地生命也需要水作为溶剂。 这引发了关于水是否是唯一能够发挥这种作用的液体的讨论。 生物化学家史蒂文·本纳和由约翰·A·巴罗斯（John A. Baross）主持的天体生物学委员会在最近的科学文献中已经认真对待了一种观点，即地外生命形式可能基于水以外的其他溶剂的观点。 Baross委员会讨论的溶剂包括氨，硫酸，甲酰胺，碳氢化合物和（在比地球温度低得多的温度下）液态氮或超临界流体形式的氢。 
Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist; however, on another occasion he said that he was a carbon chauvinist but “not that much of a water chauvinist”. He speculated on hydrocarbons,:11 hydrofluoric acid, and ammonia as possible alternatives to water.
卡尔·萨根曾经将自己描述为既有碳素主义者，又有水质主义者； 然而，在另一场合，他说自己是一名碳素主义者，但“不是那么多的水质主义者”。 他推测碳氢化合物：11氢氟酸和氨 可能替代水。
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.
- 复杂性导致许多可能的反应路径发生排列，包括酸碱化学，H +阳离子，OH-阴离子，氢键，范德华键，偶极-偶极和其他极性相互作用，水性溶剂笼和水解。 这种复杂性为进化产生生命提供了许多途径，许多其他溶剂[可能]发生的反应也大大减少，从而严重限制了进化。
- 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.
- 热力学稳定性，液态水形成的自由能足够低（-237.24 kJ / mol），使水几乎不发生反应，其他溶剂则具有很高的反应性，尤其是与氧气反应。
- 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.
- 水不会在氧气中燃烧，因为它已经是氢气与氧气的燃烧产物。 大多数替代溶剂在富氧气氛中不稳定，因此这些液体极不可能支持需氧寿命。
- 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.
- 水是一种室温液体，会导致需要克服反应障碍的大量量子跃迁态。 低温液体（例如液态甲烷）的指数迁移率较低，这是基于化学反应生命所必需的。 这会导致化学反应速率过慢，以至于无法阻止任何基于化学反应的生命的发展。[需要引证]
- 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 vaporization leading 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.
- 固体（冰）的密度低于液体，因此冰漂浮在液体上。 这就是水体冻结但不冻结固体（自下而上）的原因。 如果冰比液态水致密（几乎所有其他化合物都如此），那么大的液态物质将缓慢冻结成固体，这不利于生命的形成。
Water as a compound is cosmically abundant, although much of it is in the form of vapour 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.
尽管作为水的化合物大部分以蒸气或冰的形式存在，但宇宙中的水却非常丰富。 人们认为在以下几个外卫星上可能存在地下液态水：土卫二（已观察到间歇泉），欧罗巴，泰坦和木卫三。 地球和土卫六是目前已知仅有的在其表面上具有稳定液体的世界。
Not all properties of water are necessarily advantageous for life, however. For instance, water ice has a high albedo, 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.
然而，并非水的所有特性都一定对生命有利。 例如，水冰的反照率很高，意味着它反射了来自太阳的大量光和热。 在冰河时期，由于反射冰在水面上堆积，全球冷却的影响增加。
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.
在成功的生物圈中，某些特性使某些化合物和元素比其他化合物和元素更适合作为溶剂。 溶剂必须能够在行星物体通常会遇到的一定温度范围内以液体平衡状态存在。 因为沸点随压力而变化，所以问题往往不是预期的溶剂是否保持液态，而是在什么压力下。 例如，氰化氢在1个大气压下的液相温度范围很窄，但是在金星的压力下，压力为92巴（91个大气压），它确实可以在很宽的温度范围内以液态形式存在。
艺术家对具有氨基生命的行星的外观的看法Artist’s conception of how a planet with ammonia-based life might look
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. 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.
像水分子一样，氨分子（NH3）在宇宙中也很丰富，是氢（最简单和最常见的元素）与另一个非常常见的元素氮的化合物。 液氨可能是生命的替代溶剂，这一想法至少可以追溯到1954年，当时J. B. S. Haldane在关于生命起源的座谈会上提出了这个话题。
Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has 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. Haldane made the point that various common water-related organic compounds have ammonia-related analogs; for instance the ammonia-related amine group (−NH2) is analogous to the water-related hydroxyl group (−OH).
在氨溶液中可能发生许多化学反应，并且液氨与水具有化学相似性。  氨至少可以像水一样溶解大多数有机分子，此外，它还可以溶解许多元素金属。 霍尔丹指出，各种常见的与水有关的有机化合物都有与氨有关的类似物。 例如，与氨有关的胺基（-NH2）与与水有关的羟基（-OH）类似。
Ammonia, like water, can either accept or donate an H+ ion. When ammonia accepts an H+, it forms the ammonium cation (NH4+), analogous to hydronium (H3O+). When it donates an H+ ion, it forms the amide anion (NH2−), analogous to the hydroxide anion (OH−). Compared to water, however, ammonia is more inclined to accept an H+ ion, and less inclined to donate one; it is a stronger nucleophile. Ammonia added to water functions as Arrhenius base: it increases the concentration of the anion hydroxide. Conversely, using a solvent system definition of acidity and basicity, water added to liquid ammonia functions as an acid, because it increases the concentration of the cation ammonium. The carbonyl group (C=O), which is much used in terrestrial biochemistry, would not be stable in ammonia solution, but the analogous imine group (C=NH) could be used instead.
氨像水一样，可以接受或捐赠H +离子。 当氨接受H +时，它会形成铵离子（NH4 +），类似于水合氢（H3O +）。 当它提供一个H +离子时，它会形成一个酰胺阴离子（NH2-），类似于氢氧根阴离子（OH-）。 但是，与水相比，氨更倾向于接受H +离子，而更不愿意捐赠一个。 它是一个更强的亲核试剂。 加到水中的氨起阿累尼乌斯碱的作用：它增加了阴离子氢氧化物的浓度。 相反，使用溶剂系统定义的酸度和碱度，添加到液氨中的水起酸的作用，因为它会增加阳离子铵的浓度。 在陆地生物化学中大量使用的羰基（C = O）在氨溶液中不稳定，但可以使用类似的亚胺基（C = NH）。
However, ammonia has 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 a third, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. Gerald Feinberg and Robert Shapiro have questioned whether ammonia could hold prebiotic molecules together well enough to allow the emergence of a self-reproducing system. Ammonia is also flammable in oxygen and could not exist sustainably in an environment suitable for aerobic metabolism.
然而，氨作为生命的基础存在一些问题。 氨分子之间的氢键比水中的弱，使氨的汽化热仅为水的一半，表面张力为水的三分之一，并通过疏水作用降低了浓缩非极性分子的能力。 杰拉尔德·芬伯格和罗伯特·夏皮罗对氨能否将益生元分子很好地结合在一起以允许出现自我繁殖系统提出质疑。 氨在氧气中也是易燃的，并且在适合有氧代谢的环境中无法持续存在。
Titan’s theorized internal structure, subsurface ocean shown in blue
A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual in relation to life on Earth. Life on Earth 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). Chemical reactions generally proceed more slowly at a lower temperature. Therefore, ammonia-based life, if it exists, might metabolize more slowly and evolve more slowly than life on Earth. On the other hand, lower temperatures could also enable living systems to use chemical species that would be too unstable at Earth temperatures to be useful.
基于氨的生物圈可能存在于相对于地球生命而言极为不同寻常的温度或气压下。 在正常压力下，地球上的生命通常存在于水的熔点和沸点内，介于0°C（273 K）和100°C（373 K）之间； 在常压下，氨的熔点和沸点介于-78°C（195 K）和-33°C（240 K）之间。 化学反应通常在较低温度下进行得更慢。 因此，以氨为基础的生命（如果存在的话）可能比地球上的生命更慢地代谢和进化。 另一方面，较低的温度也可能使生物系统使用在地球温度下太不稳定而无法使用的化学物质。
氨在类似地球的温度下可能是液体，但压力要高得多。 例如，在60个大气压下，氨在-77°C（196 K）时熔化，在98°C（371 K）时沸腾。
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.
甲烷和其他碳氢化合物Methane and other hydrocarbons
Methane (CH4) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos: hydrogen and carbon. It has a cosmic abundance comparable with ammonia. Hydrocarbons could act as a solvent over a wide range of temperatures, but would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested in 1981 that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane. Lakes composed of a mixture of hydrocarbons, including methane and ethane, have been detected on the surface of Titan by the Cassini spacecraft.
甲烷（CH4）是一种简单的碳氢化合物：即，宇宙中两种最常见元素的化合物：氢和碳。 它具有与氨相当的宇宙丰度。 碳氢化合物可以在很宽的温度范围内充当溶剂，但缺乏极性。 生物化学家和科幻小说家艾萨克·阿西莫夫（Isaac Asimov）于1981年建议，多脂类化合物可以替代非极性溶剂（例如甲烷）中的蛋白质。 卡西尼号航天器在泰坦表面上发现了由碳氢化合物（包括甲烷和乙烷）组成的湖泊。
There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia. Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell. However, water is also more chemically reactive and can break down large organic molecules through hydrolysis. A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way. Also, the water molecule’s tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules. Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules. Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry.
与水或氨相比，甲烷和其他碳氢化合物作为生命溶剂的有效性存在争议。   水是比碳氢化合物强的溶剂，使物质更易于在细胞中运输。 但是，水的化学反应性也更高，并且可以通过水解分解大的有机分子。 一种溶剂是碳氢化合物的生命形式，不会面临以这种方式破坏其生物分子的威胁。 同样，水分子形成强氢键的趋势会干扰复杂有机分子中的内部氢键。 使用碳氢化合物溶剂可以在其生物分子中更多地利用氢键。 此外，生物分子中氢键的强度将适合于低温生物化学。
Astrobiologist Chris McKay has argued, on thermodynamic grounds, that if life does exist on Titan’s surface, using hydrocarbons as a solvent, it is likely also to use the more complex hydrocarbons as an energy source by reacting them with hydrogen, reducing ethane and acetylene to methane. Possible evidence for this form of life on Titan was identified in 2010 by Darrell Strobel of Johns Hopkins University; a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward diffusion at a rate of roughly 1025 molecules per second and disappearance of hydrogen near Titan’s surface.
天体生物学家克里斯·麦凯（Chris McKay）以热力学为依据认为，如果使用碳氢化合物作为溶剂，在泰坦表面上确实存在生命，还可能通过将更复杂的碳氢化合物与氢反应，将乙烷和乙炔还原为氢来使用能源。 甲烷。 约翰·霍普金斯大学的达瑞尔·斯特罗贝尔（Darrell Strobel）于2010年发现了泰坦上这种生活方式的可能证据； 与较低的层相比，Titan的较高大气层中的分子氢丰度更高，认为向下扩散的速度约为每秒1025个分子，并且靠近Titan的表面氢消失了。
As Strobel noted, his findings were in line with the effects Chris McKay had predicted if methanogenic life-forms were present. The same year, another study showed low levels of acetylene on Titan’s surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms reducing acetylene to methane. While restating the biological hypothesis, McKay cautioned that other explanations for the hydrogen and acetylene findings are to be considered more likely: the possibilities of yet unidentified physical or chemical processes (e.g. a non-living surface catalyst enabling acetylene to react with hydrogen), or flaws in the current models of material flow. He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery.
正如Strobel所指出的那样，他的发现与克里斯·麦凯（Chris McKay）所预测的是否存在产甲烷生命形式有关。   同年，另一项研究表明泰坦表面的乙炔含量较低，这是克里斯·麦凯（Chris McKay）解释的，与有机物将乙炔还原为甲烷的假设相符。 在重申生物学假设时，McKay告诫说，应该更多地考虑其他有关氢和乙炔发现的解释：尚未确定的物理或化学过程的可能性（例如，使乙炔与氢反应的非活性表面催化剂），或 当前物质流模型中的缺陷。 他指出，即使是在95 K下有效的非生物催化剂，其本身也将是一个惊人的发现。
A hypothetical cell membrane termed an azotosome capable of functioning in liquid methane in Titan conditions was computer-modeled in an article published in February 2015. Composed of acrylonitrile, a small molecule containing carbon, hydrogen, and nitrogen, it is predicted to have stability and flexibility in liquid methane comparable to that of a phospholipid bilayer (the type of cell membrane possessed by all life on Earth) in liquid water. An analysis of data obtained using the Atacama Large Millimeter / submillimeter Array (ALMA), completed in 2017, confirmed substantial amounts of acrylonitrile in Titan’s atmosphere.
在2015年2月发表的一篇文章中，计算机模拟了一种假设的细胞膜，该膜被称为能够在泰坦条件下在液态甲烷中起作用的偶氮体。 液态甲烷在液态水中的柔韧性可与磷脂双分子层（地球上所有生命拥有的细胞膜类型）相媲美。  对使用阿塔卡马大型毫米/亚毫米阵列（ALMA）于2017年完成的数据进行的分析证实，泰坦大气中存在大量丙烯腈。 
Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Its melting point is −84 °C, and its boiling point is 19.54 °C (at atmospheric pressure); 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 and Carl Sagan.
像水一样，氟化氢（HF）是极性分子，由于其极性，它可以溶解许多离子化合物。 熔点为-84℃，沸点为19.54℃（常压下）。 两者之间的差异略大于100K。HF还会与它的邻居分子形成氢键，水和氨也会如此。 Peter Sneath 和Carl Sagan 等科学家认为它可能是生命的溶剂。
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. 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.
HF对于构成地球生命的分子系统是危险的，但是某些其他有机化合物，例如石蜡，也很稳定。 像水和氨一样，液态氟化氢支持酸碱化学反应。 根据溶剂系统对酸度和碱度的定义，硝酸在添加到液体HF中时起碱的作用。
However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.
Hydrogen sulfide is the closest chemical analog to water, but is less polar and a weaker inorganic solvent. 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. On a planet with hydrogen-sulfide oceans the source of the hydrogen sulfide could come from volcanos, 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.
硫化氢是最接近水的化学类似物，但极性较小，无机溶剂较弱。 硫化氢在木星的月亮Io上非常丰富，并且可能以液态形式存在于地表以下一小段距离。 天体生物学家Dirk Schulze-Makuch建议将其作为可能的生命来源。 在具有硫化氢海洋的行星上，硫化氢的来源可能来自火山，在这种情况下，硫化氢可以与少量氟化氢混合，从而有助于溶解矿物。 硫化氢的寿命可能使用一氧化碳和二氧化碳的混合物作为碳源。 它们可能产生并依靠一氧化硫生活，后者类似于氧气（O2）。 硫化氢像氰化氢和氨一样，处于液态的较小温度范围，尽管它像氰化氢和氨一样随着压力的升高而增加。
二氧化硅和硅酸盐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 could have lower boiling points than silica. 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.
二氧化硅，也称为二氧化硅和石英，在宇宙中非常丰富，在液态时温度范围很大。 但是，其熔点为1,600至1,725°C（2,912至3,137°F），因此不可能在该温度下制造有机化合物，因为它们都会分解。 硅酸盐类似于二氧化硅，并且某些沸点可能低于二氧化硅。 杰拉尔德·芬伯格和罗伯特·夏皮罗提出，熔融硅酸盐岩可以用作具有基于硅，氧和铝等其他元素的化学物质的生物的液体介质。
其他溶剂或助溶剂Other solvents or cosolvents
Sulfuric acid (H2SO4)
Other solvents sometimes proposed:
- Supercritical fluids: supercritical carbon dioxide and supercritical hydrogen.
- Simple hydrogen compounds: hydrogen chloride.
- More complex compounds: sulfuric acid, formamide, methanol.
- Very-low-temperature fluids: liquid nitrogen and hydrogen.
- High-temperature liquids: sodium chloride.
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.
液态硫酸是强极性的。 它在比水高的温度下仍保持液态，在1个大气压下，其液体温度范围为10°C至337°C，但在300°C以上时，它会缓慢分解。 已知硫酸以气雾滴的形式富含在金星云中。 在使用硫酸作为溶剂的生物化学中，两个碳原子通过双键连接的烯基（C = C）在水基生物化学中的功能类似于羰基（C = O）。[35 ]
A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent. A 61.2% (by mass) mix of water and hydrogen peroxide has a freezing point of −56.5 °C and tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.
有人提出可能存在火星上的生命，并使用水和过氧化氢的混合物作为其溶剂。 水和过氧化氢的61.2％（质量）混合物的凝固点为-56.5°C，并且倾向于过冷而不是结晶。 它也具有吸湿性，在缺水环境中具有优势。 
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.
有人提出可能存在火星上的生命，并使用水和过氧化氢的混合物作为其溶剂。 水和过氧化氢的61.2％（质量）混合物的凝固点为-56.5°C，并且倾向于过冷而不是结晶。 它也具有吸湿性，在缺水环境中具有优势。 
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. These studies indicate that blue plants would be unlikely, however yellow or red plants may be relatively common.
物理学家指出，尽管地球上的光合作用通常涉及绿色植物，但各种其他颜色的植物也可以支持光合作用，这是地球上大多数生命所必需的，并且在接受不同恒星辐射混合的地方，其他颜色可能更可取 比地球。  这些研究表明，蓝色的植物不太可能，但是黄色或红色的植物可能相对常见。
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. 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, 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. 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.
例如，在寒冷气候下，青蛙的大部分身体水处于冻结状态，它们可以存活更长的时间，而澳大利亚的沙漠青蛙在干燥时期会变得不活跃和脱水，最多损失其体液的75％ 但通过在潮湿的时期迅速补水来恢复生活。 对于任何缺乏检测低水平新陈代谢的敏感手段的人，在休眠期间，每种类型的青蛙都将表现出生化惰性（即不活着）。
基于粉尘和等离子体Dust and 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. 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”.
Vadim N. Tsytovich及其同事在2007年提出，在太空中可能存在的条件下，悬浮在等离子体中的尘埃颗粒可以表现出逼真的行为。  计算机模型表明，当尘埃带电时，颗粒会自组织成微观的螺旋结构，作者提供了“ …螺旋晶粒结构复制的可能模型的概图”。