THE 6TH BEST ATOMIC FLAVOR - CARBON
Ah... Carbon, the Periodic Table's main subject for most high-school students together with Oxygen and Hydrogen.
Carbon is an interesting sort of element, and I think the vast majority of you are used to its amazing properties. Carbon is an extremely stable element, it is able to form different lattices, which vary from graphites to diamonds, when in the presence of other elements it can make up to 4 electronic bonds with them. For the industry apart from these characteristics, carbon is extremely important in base reactions to refine metals, build polymers, and drastically increase the heat resistance of certain components, among other uses.
In general, carbon plays an important role in our society, and as it is of common knowledge at this point, our ecosystem and life as we know it.
There is one question that utterly bothers me about carbon, and has kind of always been in sci-fi writer's heads for quite a while, "is carbon-based life is the only way to go?".
That often if not ALWAYS come with the extra of questioning the place of hydronium oxide as well, a stable and highly corrosive substance - also known as Water - as a solvent for life.
A SUBSTITUTE FOR CARBON?
Wikipedia cites at least 17 hypothetical alternatives to the Water-Carbon based life (among other affairs) we have on Earth. Talking about building blocks, ie, substitutes for Carbon, we often stumble upon Boron and Silicon, some outliers go as far as to consider Arsenic and Sulphur too.
BORON
Boron and Borane are interesting interactions, but their abundances nonetheless rule them out as a likely possibility at all, Boron is even rarer than Fluorine, akin to Vanadium.
Take for example diborane (B2H6), it is only liquid between -164,8°C and -92,5°C, a 72,3 K window in the energy spectrum and a very low energy window. Needless to say, diborane cannot exist in an environment with oxygen or water too, as it reacts violently to form boron trioxide (a powder) and boric acid (also a powder).
SILICON
Silicon is notable for making 4 bonds like Carbon does - but unlike carbon, organosilicon reactions have to take place at low temperatures because the optimal medium for these reactions are alkenes, alkynes and ketones. If the medium is acetylene, the temperature of the world has to be stable at -80°C and at 1,27 atm. For ethylene, the temperature must be between -170°C and -103°C. And for Acetone, it is between -95°C and 56°C.
The major problem here is that the most energetic molecules that can be derived here need a lot of energy to be broken and release their energy in exchange, see Ketoses for example, way above the boiling point of many of the proposed solvents.
Si-C bonds are weaker than C-C and C-O bonds, but Si-O bonds are stronger than the others, hence why we find silicon mostly in rocks bound to a given amount of oxygen - and even if somehow there is any free silicon around the planet, we can discard its viability as building block as it is substantially less electronegative than hydrogen. Which helps maintaining molecules together but not so close, in which case Silicon becomes the positive end of the molecule. It is so "useless" for base life chemistry because it is very damn stable, and silicon chemistry requires lots of energy and sometimes even Platinum catalysts.
ARSENIC AND SULFUR
These two elements oxidize each other, and react with water and oxygen as well, both are unstable chemicals and their basic compounds are like time-bombs that run faster the hotter it gets (really fast at +100°C), from which Arsenic is the most interesting. Arsine is stable at narrow range of temperatures (-111°C to -62°C). Arsenic compounds tend to also form metallic complexes and crystals which are by default absurdly stable and nonreactive given the planetary conditions. Arsenic is also a billion times rarer than Carbon.
A SUBSTITUTE FOR WATER?
Oxidane, or chemically H2O, is a very peculiar molecule in several ways, in general that is linked to its electronegativity differential.
Oxygen is one of the most electronegative elements in the periodic table, that is, it has a strong tendency to attract electrons to itself when bonding, that's what gives oxygen its oxidative properties. In turn, hydrogen cannot hold its single electron near the oxygen, so the electron pair is mostly present around the oxygen while the nude protons cluster on one side of the molecule, giving the molecule an overall positive and negative ends. This is an important quirk of water, because it allow water to stick to itself through hydrogen bonds. So in water, every water molecule is connected to at least other four molecules, and so forth.
From this, water is particularly resistant to atomic motion - that is - resistance to heating up. Water has an absurdly high heat capacity (4,2 J/g°C), and it helps it to remain liquid in wide array of pressures and temperatures.
But let's look at other elements near oxygen, if those characteristics are from polar substances, sure of water's neighbors might be just as good.
If you add Hydrogens to Nitrogen, Oxygen and Fluorine - you get Ammonia (NH3), Oxidane (H2O), and Fluorane (HF).
Based on sheer electronegativity and bonding strength, fluorane should be stronger than other substances. Except it is not, because molecule geometry also matters, for the maximum strength of bonds, fluorane would have bond linearly with other molecules, which it doesn't because fluorine is so much electronegative that it keeps from bonding with more than 2 molecules at a time - instead bonding in zig-zag patterns, like so:
Likewise, when we look at Ammonia, its molecule can only bond with 3 others...
So actually, the boiling point temperature goes like: H2O > NH3> HF because of the number of bonds they are able to make.
The polar nature of water also makes it an optimal solvent for the majority of molecules, because the water molecules can nudge themselves into the weaker bonds of substances and separate their components through hydrolysis, take for example how paper dissolves in a water because the water molecules break the cellulose bonds separating the glucose monomers.
Oxygen is also
way more abundant in the universe than any of the other two elements, oxygen is tens of times more abundant than nitrogen, while nitrogen is hundreds of times more abundant than fluorine.
WHAT ARE POSSIBLE EXOTIC WORLDS?
Given what we've seen here so far, there is reason to believe that Water-Carbon based life is, if not dominant by a vast margin (say like, "90%"), the only type of biochemistry possible in the Universe as we know it.
Besides the small-scale chemical properties of oxidane, it also serves a huge temperature buffer substance, narrowing down the temperature variation in the atmosphere of the planet because of its high specific heat capacity, the other fluid that is just as good in this is Ethanol (and it is just half as good). Any planets sporting other volatiles would have wild temperature variations between day and night or throughout the year unless they were extremely distant from their stars with circular orbits.
The only other water-analog molecule I cannot find strong evidence against for possibility biochemistry is hydrogen sulfide (H
2S) - all other hydrides can be easily destroyed by the primary atmosphere of a young planet, such as how
selane can be created by aluminum compounds and water in the early crust, but dissociated into fine selenium ash and water when reacting sulfur dioxide from volcanic activity.
So we have team of carbon as a building block, and either oxidane, ethanol, and sulfane as solvents (heat capacity 4,2 > 2,4 > 1,0 J/g°C), each with decreasing boiling points and decreasing commonality.
OTHER TYPES OF EXOTIC BIOCHEMISTRY...
Within our established limitations here, we can have:
ALTERNATE CHIRALITY
Chirality is an asymmetric property of certain molecules, that is, if your superimpose the reflections of this certain molecule on top of another, they won't match.
In the example above, molecules 1 and 2 have the same structure and chemical formula - but they are mirror images of the other, molecule 1 has the red radical on the Right side while molecule 2 has the red radical on the Left side.
It might not be that big of a deal at first, after all, it is chemically identical, right? Well, it happens that molecular machinery in life will not be able to fit the lock&key mechanism of their chemical reactions half of the time, and so it adapts to only accept and use one chirality of a molecule at a time, and sometimes molecules have drastically different effects if they are flipped on their chirality to one organism - and so the ecosystem of Earth has adapted to use only one chirality of glucose, only one chirality of ATP, and only one chirality of vitamin B, and so on so forth.
Because of this, had luck or the conditions of our planet been any different, it is possible that life worked its way around other sets of chiralities, and so the same may be valid for other planets. Imagine Earth food being poisonous to aliens because it reacts different within their organism, and vice-versa - there is N possibilities within this field.
ALTERNATE DNA STRUCTURE
Under general circumstances, a DNA or RNA world is a given to form with 4 base pairs, ATGC - but these are not the only possible base pairs for a DNA-like structure, under very specific conditions (a lab as far as we know nowadays), the base pairs PZBS or d5SICS-dNaM may be included, partially mixed, or be the only base pairs akin to ATGC is in our DNA, and so depending on the nature of life in a certain planet, DNA may be partially compatible or not compatible at all.
If this really holds true, assuming 4 bases is the minimum optimal number of bases you need, there are at least between 4 and 15 other possible DNA alphabets than our own, opening a world of exotic amino-acid synthesis and gene expression.
"CARBON CHAUVINISM"
Carbon chauvinism is a relatively recent term to designate the quality of those who assume that extraterrestrial life must be similar to life on Earth. In particular, the term applies to those who assume that the molecules responsible for the chemical processes of life must be based on carbon as the main structural element.
This attitude suggests that humans, as carbon-based life forms that have never found life beyond their planet, may have difficulty conceiving the existence of alternative biochemicals. The term was first used in 1973, when Carl Sagan described this and other human chauvinisms that limited his imagination about possible distinct forms of extraterrestrial life in his book The Cosmic Connection.
However there is sufficient mathematical and experimental proof of carbon's superior thermodynamic and chemical properties. Such ideas are not akin to Eugenics or the belief in Miasma, but more like the theory of relativity, which holds its predictions, and has taken several decades for the nearly full understanding and observation of described phenomenon, such as space curvature and black holes. It is such a fundamental property of chemistry one would need to completely re-write physics in order to make non-carbon based life possible at all.
It is not a matter of preference, but a matter of understanding that the surrounding chemistry of life, such as energetic molecules like ATP and structural cell membranes can only exist within the boundaries of water-carbon chemistry. One cannot completely rule-out the possibility until we have explored a substantial number of planets, but recognize other hypothetical biochemistries as significant possibilities is outright foolish and a pseudo-skeptical posture - alike people that "doubt" the existence of outer space like planets and other suns because the sky is so "fundamentally" different from the earth that our physics should just break. Or doubting the existence of atoms, despite chemical and physical phenomena being accurately described by atomic theory.
Now, I'm not willing to touch on the subject of aplanetary life, like microscopic string-particle things inside stars or in deep space, because it is not scientifically falsifiable, and thus one cannot put themselves through the problem of discussing it.
But I can see how one sees faces carbon problem like how we see today the ancient Greeks discussing the possibility of atoms in the first place. The problem with the idea that we may only be on the tip of the iceberg is that nowadays, we have methods and a vast disparity of technological superiority. We do understand the fundamentals of physics, and arguing against those fundamentals is like arguing against trigonometry - one just can't...
- M.O. Valent, 14/11/2021
