How likely are non-carbon life forms?

How likely are non-carbon life forms General Knowledge

The origin of life is a big mystery. But we can say that life arises as a result of the self-organization of matter when special complex chemicals spontaneously arise that self-organize and acquire the ability to self-reproduce, that is, certain chemicals will be able to process what surrounds them, producing their own copies.

And we know that this happens in the life forms we know. All forms of life known to us are based on carbon chemistry, which we call organic. And by analyzing what happens in the forms of life known to us, several generalizations can be made that should be applicable to other possible chemical forms of life, namely: for the existence of life, a huge chemical diversity is necessary, generated by a small number of chemical elements. In the case of organic chemistry, these are carbon, hydrogen, nitrogen, oxygen, sulfur. A huge number of organic compounds are known. This number can be increased virtually to infinity.

Second condition: although these compounds do not have to be thermodynamically stable, they must be at least metastable and exist for a long time.

Third, for life to be possible, there must be a reaction to store energy. In particular, in all known forms of life, it is adenosine triphosphate, which is a form of energy storage and the breakdown of which allows living cells to do work.

There must also be a reaction that would extract energy from the environment, from those substances that come from the environment. In particular, in aerobic life forms, this is the oxidation of glucose, which is accompanied by the release of energy. In anaerobic organisms, these are other reactions: the processing of sulfur compounds, and so on.

There must be some mechanism of heredity. Its material carrier is some kind of large aperiodic molecule (Schrödinger, anticipating the discovery of the structure of DNA, pointed to an aperiodic crystal). This aperiodic crystal, as we would say now – a polymer, or more precisely – a copolymer, is a DNA molecule. In principle, this role can be played not only by linear copolymers of the DNA type, but also by two-dimensional polymeric structures. Three-dimensional – it’s already hard to believe in it. One-dimensional, two-dimensional – quite.

And one more condition is that these substances, on the basis of which we want to imagine new forms of life, must be in a liquid or close to liquid state, so that the diffusion of molecules is fast enough, so that the products of vital activity are removed, and substances from the environment. environments could easily enter this living organism.

We recently found something that surprised us: we found extremely rich chemistry in nitrogen and hydrogen compounds. The fact is that if you look at the chemistry of hydrocarbons, the very chemistry, the diversity of which is the foundation for the diversity of all organic chemistry, you will get all organic chemistry by adding or replacing atoms of oxygen, sulfur, nitrogen.

So, for compressed hydrogen nitrogens, we found a much more diverse chemistry than is known for hydrocarbons.

There are a lot of nitrogenous hydrogens in the Universe. In fact, the planets Uranus and Neptune are approximately 8% by mass composed of ammonia, that is, the simplest hydrogen nitrogen. This is much more than the carbon on earth. Nitrogens also have a low melting point, which increases with pressure (as does the temperature in the planetary interior). Covalent nitrogen compounds with very strong directional bonds will also be metastable – in other words, not only is there an unusually large number of stable compounds under pressure, there will also be an almost unlimited number of metastable compounds. And if you start adding other atoms there: oxygen, sulfur, then the chemical diversity will exceed the diversity of organic chemistry. This is the area of ​​chemistry that we still practically do not know and which came out of our calculations.

Studying the variety of hydrogen nitrogens, we were amazed at how many of them there are. Moreover, there are many of them in a thermodynamically stable state. For hydrocarbons, under normal conditions, there is only one thermodynamically stable compound – methane. For hydrogen nitrogens, there are more than a dozen such stable compounds. And they have extremely diverse chemistry, even more diverse than hydrocarbons. If we add oxygen and sulfur, maybe carbon in small quantities, then we will most likely have a chemistry that is many times or even orders of magnitude more diverse and rich than organic, which in principle can serve as the basis for new others. life forms.

Whether life is possible on planets such as Uranus and Neptune – where 8% by mass is ammonia – we do not know. A potential problem is that the lifetime of metastable compounds under planetary conditions (high temperatures and pressures) may not be long enough.

Nitrogenous compounds are by far the best, but probably not the only candidate for non-carbon life. Their advantage over, for example, silicon compounds is a more diverse chemistry, greater proximity to the liquid state, and usually a longer lifetime of metastable states due to stronger and more directed covalent bonds.


Alexander Stephenson

Candidate of Chemical Sciences, editor-in-chief of Lecturer at several international online schools, member of the jury of chemistry competitions and author of scientific articles.

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