Terrestrial life, the only one currently known to us, is based on a huge variety of carbon compounds. Meanwhile, this is not the only chemical element that may underlie life.
The existence of other forms of life, 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 literature.
However, science fiction writers are not limited to this - they come up with both exotic forms of traditional (carbon) life and its no less exotic foundations - say, living crystals, disembodied energy field creatures or organosilicon creatures.
In addition to science fiction writers, scientists are also engaged in the discussion of such issues, although they are much more cautious 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 Carl Sagan said that it is completely wrong to generalize statements about earthly life in relation to life in the entire Universe. Sagan called such generalizations "carbon chauvinism", while he himself considered silicon as the most likely alternative basis for life.
The main question of life
Organosilicon life form from the science fiction series "Star Trek"
Promotional video:
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, several important conditions must be met.
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 enormous.
Secondly, these compounds must be thermodynamically stable or at least metastable, that is, their lifetime must 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 to accumulate and release it.
Fourth, for the self-reproduction of life, a mechanism of heredity is required, in which a large aperiodic molecule acts as a carrier of information.
Erwin Schrödinger suggested that an 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 reactions (metabolism) due to diffusion.
Traditional alternatives
In the case of carbon, all these conditions are fulfilled, but even with the nearest alternative - silicon - the situation is far from being so rosy. Organosilicon molecules can be long enough to carry hereditary information, but their diversity is too poor in comparison with carbon organics - because of the larger size of atoms, silicon hardly forms double bonds, which greatly limits the possibilities of attaching various functional groups.
In addition, saturated hydrogen silicones - silanes - are completely unstable. Of course, there are also stable compounds such as silicates, but most of them are solid substances under normal conditions.
With other elements, such as boron or sulfur, the situation is even more sad: organoboron and high-molecular sulfur compounds are extremely unstable, and their diversity is too poor to provide life with all the necessary conditions.
Under pressure
“Nitrogen has never been seriously considered as the basis for life, since under normal conditions the only stable nitrogen-hydrogen compound is ammonia NH3,” says Artem Oganov, head of the MIPT's computer-aided materials design laboratory, professor at Stony Brook University in New York and the Skolkovo Institute of Science and Technology (Skoltech).
“However, recently, while simulating various nitrogen systems at high pressures (up to 800 GPa) using our USPEX (Universal Structure Predictor: Evolutionary Xtallography) algorithm, our group discovered an amazing thing.
It turned out that at pressures above 36 GPa (360,000 atm), a number of stable hydrogen nitrogen appears, such as long one-dimensional polymer chains of N4H, N3H, N2H, and NH units, exotic N9H4, forming two-dimensional sheets of nitrogen atoms with attached NH4 + cations, and molecular compounds N8H, NH2, N3H7, NH4, NH5.
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 diversity than the traditional organic one under normal conditions."
Step to life
This hypothesis of Artem Oganov's group opens up completely unexpected possibilities in terms of a non-carbon basis of life.
“Hydrogen nitrogen 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 the consideration in our models, and this, possibly, will open the way to nitrogen analogs of carbon proteins, albeit the simplest ones for a start, without active centers and complex structure.
The question of energy sources for nitrogen-based life is still open, although it may well be some kind of redox reactions still unknown to us, taking place under high pressure conditions. In reality, such conditions can exist in the bowels of giant planets such as Uranus or Neptune, although the temperatures there are too high. But so far we do not know exactly what reactions can occur there and which of them are important for life, therefore we cannot accurately estimate the required temperature range."
Living conditions based on nitrogen compounds may seem extremely exotic to readers. But suffice it to recall the fact that the abundance of giant planets in stellar systems is at least no less than that of rocky earth-like planets. And this means that it is ours, carbon life in the Universe that can turn out to be much more exotic.
“Nitrogen is the seventh most abundant element in the universe. There are quite a few of it in the composition of giant planets such as Uranus and Neptune. It is believed that nitrogen is present there mainly in the form of ammonia, but our modeling shows that at pressures above 460 GPa, ammonia ceases to be a stable compound (as it is under normal conditions). So, perhaps, in the bowels of the giant planets, instead of ammonia, there are completely different molecules, and this is the chemistry we are now investigating."
Nitrogen exotic
At high pressures, nitrogen and hydrogen form many stable, complex and unusual compounds. The chemistry of these hydrogen-nitrogen compounds is much more diverse than hydrocarbon chemistry under normal conditions, so it is hoped that nitrogen-hydrogen-oxygen-sulphide compounds can surpass organic chemistry in richness.
The figure shows the structures N4H, N3H, N2H, NH, N9H4 (pink - hydrogen atoms, blue - nitrogen). Monomer units are framed in pink.
Living space
It is possible that in search of exotic life, we will not have to fly to the other end of the universe. In our own solar system, there are two planets with suitable conditions. Both Uranus and Neptune are shrouded in an atmosphere of hydrogen, helium, and methane, and appear to have a silica-iron-nickel core.
And between the core and the atmosphere is a mantle, consisting of a hot liquid - a mixture of water, ammonia and methane. It is in this liquid at the right pressures at the appropriate depths that the ammonia decomposition predicted by Artem Oganov's group and the formation of exotic hydrogen nitrogen, as well as more complex compounds, including oxygen, carbon and sulfur, can occur.
Neptune also has an internal source of heat, the nature of which is still not clearly understood (it is assumed that it is radiogenic, chemical or gravitational heating). This allows us to significantly expand the "habitable zone" around our (or another) star, far beyond the limits available for our fragile carbon life.
Dmitry Mamontov