Non-carbon life forms: silicon or nitrogen?
The existence of other life forms that are fundamentally different from our earthly presence, location and number of paws, eyes, teeth, claws, tentacles and other parts of the body is one of the favorite topics in science fiction. However, science fiction writers are not limited to this only - they come up with both exotic forms of traditional (carbon) life and no less exotic foundations of it - say, living crystals, incorporeal energy field creatures or organosilicon creatures.
In addition to science fiction writers, scientists are also discussing such issues, although they are much more careful in their assessments. After all, so far the only basis of life that is precisely known to science is carbon. Nevertheless, at one time, the famous astronomer and popularizer of science, Karl Sagan, stated that it was completely wrong to generalize the statements about earthly life in relation to life in the entire Universe. Sagan called such generalizations "carbon chauvinism", while he himself, as the most likely alternative basis for life, considered primarily silicon.
“Nitrogen is the seventh most abundant element in the universe. It is quite a lot in the composition of giant planets such as Uranus and Neptune. It is believed that there nitrogen is mainly in the form of ammonia, but our simulation shows that at pressures above 460 GPa, ammonia ceases to be a stable compound (which it is under normal conditions). So, perhaps, completely different molecules exist in the bowels of the giant planets instead of ammonia, and it is this chemistry that we are now investigating. ”
Life's main issue
What is life? It would seem that the answer to this question is obvious, but oddly enough, there are still discussions about formal criteria in the scientific community. Nevertheless, a number of characteristic features can be distinguished: life must reproduce itself and evolve, and for this it is necessary to observe several important conditions. First, the existence of life requires a large number of chemical compounds, consisting mainly of a limited number of chemical elements. In the case of organic chemistry, these are carbon, hydrogen, nitrogen, oxygen, sulfur, and the number of such compounds is huge. Secondly, these compounds must be thermodynamically stable or at least metastable, that is, their lifetime should be long enough to carry out various biochemical reactions. The third condition is that there must be reactions to extract energy from the environment, as well as its accumulation and release. Fourth, for the self-reproducibility of life, a mechanism of heredity is required, the information carrier in which is a large aperiodic molecule. Erwin Schrödinger suggested that the aperiodic crystal could be the carrier of hereditary information, and later the structure of the DNA molecule — a linear copolymer — was discovered. Finally, all these substances must be in a liquid state in order to ensure a sufficient rate of metabolic (metabolic) reactions due to diffusion.
In the case of carbon, all these conditions are met, but even with the closest alternative - silicon - the situation is far from so rosy. Organosilicon molecules can be long enough to carry hereditary information, but their variety is too poor compared to carbon organics - due to the larger size of the atoms, silicon can hardly form double bonds, which greatly limits the possibility of attachment of various functional groups. In addition, the limiting silicon hydrogen - silanes - are completely unstable. Of course, there are stable compounds, such as silicates, but most of them are solid under normal conditions. With other elements, such as boron or sulfur, the situation is even sadder: the organoboron and high molecular weight sulfur compounds are extremely unstable, and their diversity is too poor to provide life with all the necessary conditions.
“Nitrogen has never been seriously considered as the basis for life, since under normal conditions the only stable nitrogen-hydrogen compound is ammonia NH 3, ” says Artyom Oganov, head of the MIPT computer design laboratory at Professor at Stony Brook University in New York and the Skolkovo Institute of Science and Technology (Skoltech). - However, recently, when modeling various nitrogen-hydrogen systems at high pressures (up to 800 GPa) using our USPEX algorithm (Universal Structure Predictor: Evolutionary Xtallography, Universal Structure Predictor: Evolutionary Crystallography, see "PM" No. 10'2010), our group discovered an amazing thing. It turned out that at pressures above 36 GPa (360, 000 atm) a whole series of stable nitrogen hydrogen atoms appear, such as long one-dimensional polymer chains of N 4 H, N 3 H, N 2 H and NH, exotic N 9 H 4, forming two-dimensional sheets nitrogen atoms with attached NH 4 + cations, as well as molecular compounds N 8 H, NH 2, N 3 H 7, NH 4, NH 5 . In fact, we found that at pressures of the order of 40–60 GPa, nitrogen-hydrogen chemistry in its diversity significantly exceeds the chemistry of hydrocarbon compounds under normal conditions. This allows us to hope that the chemistry of systems involving nitrogen, hydrogen, oxygen and sulfur is also richer in its diversity than traditional organic under normal conditions. ”
Step to life
This hypothesis of the group of Artyom Oganov opens up completely unexpected possibilities in terms of the non-carbon basis of life. “Nitrogen hydrogen can form long polymer chains and even two-dimensional sheets, ” explains Artem. - Now we are studying the properties of such systems with the participation of oxygen, then we will add carbon and sulfur to our models, and this, perhaps, will open the way to nitrogen analogues of carbon proteins, albeit for starters the simplest ones, without active centers and complex structures. The question of sources of energy for life based on nitrogen remains open, although it may well be some redox reactions that are still unknown to us and occurring at high pressures. In reality, such conditions may exist in the bowels of giant planets such as Uranus or Neptune, although the temperatures there are too high. But so far we don’t know exactly what reactions can occur there and which of them are important for life, therefore we cannot accurately assess the required temperature range. ”
The “living” conditions of living creatures based on nitrogen compounds may seem extremely exotic to readers. But it’s enough to recall the fact that the prevalence of giant planets in stellar systems is at least no less than stony earth-like planets. And this means that in the Universe it is our carbon life that can turn out to be much more exotic.The article “Carbon Chauvinism” was published in the journal Popular Mechanics (No. 5, May 2015).