“CHON?”, you might ask; “What’s that?”. It’s an affectionate acronym for the four most common vital chemical elements in the biochemistry of life as we know it. To wit, carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). The human body is 95..6% CHON by mass, 99.1% by number of atoms. Famously, our form of life is carbon-based, and the solvent we use, water, is H2O. More CHON.
It’s often been supposed that carbon-water biochemistry is not the only kind out there, and that alien life-forms might exist that use a base other than carbon or drink a solvent other than water. A distinct possibility, in the enthusiastic amateur mind of yours truly…but rarely is it asked: just how truly alien are these alternative biochemistries?
Let’s go down one list which Wikipedia has helpfully assembled. Ammonia as a solvent instead of water? Ammonia is NH3; like water, entirely made up of “CHON” elements, just assembled differently. Methane or hydrocarbons as a solvent? CH4 or various C+H combinations; again, entirely CHON. Liquid nitrogen? That’s N, which is a CHON element. Carbon dioxide as a solvent? Sorry, once again, entirely made up of CHON. Hydrogen fluoride? Hydrogen sulfide? Slightly more imaginative, but hydrogen is H. More CHON. Even silicon dioxide still contains oxygen, which is the O in CHON.
Okay…this is hard. So what plausible solvents wouldn’t contain any of the CHON elements? Not much; even Robert Freitas’s grab-bag list is very CHON-heavy. Elemental sulfur is perhaps the most commonly proposed biological solvent that doesn’t make use of any of these elements; helpfully sulfur is an abundant enough element in desiccated environments like Venus or Io, but oceans of the stuff in elemental form have yet to be observed. It’s possible, though. But again, how alien would such a life-form be?
Sulfur, at Earth atmospheric pressure (1 bar), melts at 239 degrees Fahrenheit and boils at 832 degrees Fahrenheit; a surprisingly decent liquid range. Beyond where we could take the stability of carbon-based biochemical backbones for granted. So what else is out there beyond carbon?
Robert Freitas has perhaps still the best write-up on the topic in his “Xenology”, and the list extends well beyond silicon, the usual go-to analogue to carbon. Inconveniently, silicon in polymerized form, which is the sort that would be useful for life-forms, is usually expected to occur either in the form of silicones (alternating atoms of Si and…O; more CHON…), or silanes (polymers of Si and…H; more CHON!). Boron-based biochemistry? A fascinating possibility, but boron would tend to form complex structures analogous to organic chemistry in the form of the boron nitrides, which as the name suggests, contains nitrogen, i.e. N. More CHON! Even a biochemistry as exotic as metal oxides, metallic compounds known to form complex structures that can withstand high temperatures, incorporate oxygen, i.e. the O in CHON, and have been proposed to potentially be highly compatible with silicon dioxide, i.e. molten lava, as a solvent. Which contains lots of oxygen, i.e. the O in CHON.
Clearly, it’s tough to escape CHON’s reach. Inconveniently, almost any plausible liquid ocean that could chemically react with much of anything would tend to make use of one or more of the CHON elements. From molten lava to liquid water to liquid methane to liquid nitrogen, to everything in between, it’s all CHON all the time, it seems. Why is this? One reason is that there’s just so damn much of the stuff.
Let’s look at the abundance of the chemical elements in the universe. Hydrogen (the H in CHON) is by far the most abundant element, comprising 74% of the total. Helium is second at 24% and is not part of CHON but is famously unreactive; life based on chemistry can’t use it. Third is oxygen (the O in CHON) at 1%. Next up at fourth place is…carbon, the C in CHON, making up 0.46%. See a pattern yet? Fifth place is neon, another noble gas that chemical processes basically can’t use. Iron is in sixth place, the most abundant reactive element that’s non-CHON, making up 0.1% of the total. In seventh place, making up another 0.1%, is…nitrogen, the N in CHON, rounding out the total.
So out of the top five elements that ever react with much of anything, the CHON elements represent four. Iron is the only exception. In all fairness silicon, magnesium, and sulfur aren’t very far behind, but there are reasons we see the same components over and over, just reassembled in different configurations. So is there hope for entirely non-CHON-based life?
Maybe. You can look at the solvents and working fluids we use for industrial processes as a guide to what’s out there that does anything “interesting” chemically, and out of them, one jumps out at me: molten salts. Most prominently used in nuclear reactor design, these “ionic liquids” can be eerie doppelgängers to water, visually, only they’re liquid at hotter temperatures. It depends on the precise chemical composition, but typically these ionic compounds are liquid in the hundreds of degrees Fahrenheit. Exactly the sort of environment where it would be too hot for water…or most of the other “volatile” compounds, which tend to be CHON-heavy. Look at the outer solar system and the composition of the fluids found in the outer gas giants, their moons, and the comets: it might as well be CHON central. A hot, anhydrous inner-solar-system environment is precisely where we would expect to find an exotic but still chemistry-based life-form. Such as one that uses molten salts.
The classic case of salt is, of course, sodium chloride, but many other combinations are possible: calcium chloride, lithium fluoride, and more. Notice the total lack of any of the CHON elements from this thalassogenic chemical soup; it’s an entirely new regime compared to what we’re used to. It gets even better: the elements that could make use of the chemistry offered by an ionic solvent would tend to be various metals…and without oxygen to provide an oxide scaffolding. In such fluids complex branching structures can be formed purely from electrically conductive metals. No input from CHON elements needed at all.
If you ask me that’s by far the most promising possibility, before you leave the realm of chemistry entirely. Various other non-CHON-based systems could be envisaged, but they tend to be made out of much rarer elements that realistically aren’t going to form oceans and concentrated deposits, from a geochemical point of view. With molten salts, all you need is for a salty ocean to boil away, leaving pillars of salts behind; picture the lake that left the salt flats of Death Valley behind, but on a global scale, with the salts themselves starting to melt once it got hot enough. Liberate free metals from surrounding rock, which are readily at hand on any terrestrial planet, and there you go.
It would be a rather peculiar (and hot!) sort of world, but it would be one full of chemically exotic life-forms…in an environment that superficially might seem familiar. A global ocean of molten salts would be transparent, and with light viscosity; it would form waves and surf, lapping up on the beaches. It would be lethally hot, yes, but most molten salts would be nowhere near hot enough to glow red from its own heat, so to the human eye, it would look oceanic. It might even look drinkable. On a molten-salt world you might even see life swimming in the fluid as the sun dapples through the surface. Only chemically it’s alien from top to bottom: the basic elements have nothing in common with our own.
I confess, I have no idea to turn it into a story, a novel, or a book, but what a premise for a science-fiction world, and a rather original one. Feel free to use it…or even delve into the edges of chemistry and take up the challenge: forget repudiating carbon chauvinism or water chauvinism, that’s just too easy. Now, CHON chauvinism? Breaking that is hard. 😉