RAFI BRISTER
In the search for both extraterrestrial life and possible habitable extraterrestrial bodies, there are many contributing factors but one of the most important ones is the chemical composition of the body. All life on Earth needs water as a solvent for biological reactions to take place and about 95% of all living matter is built on carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur with all other elements being found in trace amounts and a total of 29 playing an active role in biological reactions. However, while modern life has evolved to live with the atmospheric concentrations of gases (primarily oxygen and nitrogen) as they are, this means that they could not or would struggle to survive on a planet where this balance was not present (including for example the Earth during the Cretaceous Period). This means that in the search for extraterrestrial life, planets which have the required concentrations of these elements detectable are higher priority candidates. However, even on Earth there are some life forms which do not conform to this template.
The diatoms are a partially silicon-based family of single-celled algae with a divergent evolution. While it is only their outer wall that is composed of silicon, their existence shows that it is possible to have organisms which are significantly based on an element other than carbon. However, despite silicon being immediately below carbon in the periodic table and so very similar chemically, there are some significant problems for silicon-based life forms surviving on Earth, the main one being that silicon can bond with far fewer other elements than carbon. While this does mean it is less affected by molecules and elements which are toxic to carbon-based life, it also means that it is harder for there to be a large enough variety of silicon based molecules for life: there is no silicon equivalent of organic chemistry. On top of this silanes (which are molecules made of silicon and hydrogen and analogous to alkanes) are highly reactive with water and spontaneously decompose. However, there is a polymer type called silicones which is made up of alternating silicon and oxygen molecules which is much more stable and so it has been suggested that chemicals based on them could be much more stable in the sulphuric acid rich environment found in some extraterrestrial locations. Despite this, in interstellar space only 8 compounds based on silicon have been found compared with 84 based on carbon, which could suggest that it is harder for diverse silicon-based molecules to form and hence unlikely that diverse life forms could exist somewhere, occupying every ecological niche.
More generally, there are two proposed ways in which the chemistry behind extraterrestrial life could differ from that on Earth. The first way, an example of which is the diatoms, is non-carbon based biochemistries. In addition to silicon, other proposed non-carbon based organic molecules include arsenic, boranes, several transition metals and sulphur. However, boranes are dangerously explosive in Earth’s atmosphere but could be much more stable in a reducing environment. However due to Boron’s low abundance in the universe this would be less likely than carbon. On the other hand, arsenic, while chemically similar to phosphorus, is toxic to most life (due to this similarity it is able to replace phosphorus in some key biochemical molecules and so inhibit their functions). However, there are some life forms, such as a species of bacteria, which needs arsenic-rich conditions to survive. Some metals such as zinc have the ability to form nanotube and diamond-like structures (cubic zirconia) and others, such as heteropoly acids can form molecules with the complexity and thermal stability to rival carbon based organic molecules. In addition, some, like titanium, aluminium and magnesium are more abundant on Earth than carbon. Metal oxide based life has also been theorised to be possible in conditions where carbon-based life would be unlikely such as in high temperatures. However, there may be problems with this in terms of the variety of molecules which could bond with these molecules, which is much more limited than carbon. Lastly, sulphur is unable to form branched molecules (unlike carbon and silicon), which is a very important part of organic molecules.
The other proposed alternative way in which life could exist in different environments to those found on Earth, is if the reactions needed to sustain them could occur in a solvent other than water. The most commonly cited ones include liquid ammonia, and hydrogen sulphide. While hydrogen sulphide is the most structurally similar to water, it is less polar and a weaker inorganic solvent. This could create problems with both reaction speeds and could limit reaction selectivity (the ratio of desired to undesired product). On top of this it is toxic to humans. However, water is very important to life on Earth because of factors including: it being a liquid over a large range of temperatures, high solubility of oxygen and carbon (as well as the ability to dissolve a large range of compounds), a large heat of evaporation (meaning the possibility of stable lakes and oceans) and the solid form being less dense than the liquid form. Despite this, there are several possible alternative solvents for life. One which is similar to water is ammonia. It is very common and most organic compounds which dissolve in water also dissolve in ammonia in addition to many elemental metals and it is also able to accept or donate a hydrogen ion. However, the bonds in ammonia are weaker than those in water which means that its heat of evaporation is half that of water and its surface tension is a third. Also iit has a reduced ability to concentrate non-polar molecules. The possibility of its use as an organic solvent has also been questioned because at atmospheric pressures on Earth it has a melting point of -78 degrees centigrade and a boiling point of -33 degrees, with these only approaching those of water at 60 atmospheres of pressure.
In summary, while it has been theorised as possible for life to exist in very different conditions from those on Earth, the fact is that there are important trade offs when proposing elements or compounds to replace carbon and water. Therefore, although arguably the search should not be entirely restricted to extraterrestrial life based on what we know for certain is possible (carbon and water-based), this remains the main focus and the evolution of divergent biochemistries is considered to be far less likely.