Most of the Universe is really dark and cold, so life which might exist in such conditions is worth thinking about. For every world with oceans of liquid water there are dozens of planets and moons with only ice.
There's an upper temperature limit for As-We-Know-It life: above about 333 Kelvin (60 C, or 140 Fahrenheit) proteins denature, and I suspect bad stuff happens to nucleotides, as well.
But I don't know any lower temperature limit for DNA-based life. DNA has to exist in solution, so as long as whatever it's dissolved in doesn't freeze, I think you can have DNA replication, protein synthesis, and so forth. (Any biochemists reading this PLEASE let me know if I'm off base.) So let's go down the temperature scale and see what's there.
Ammonia: Just colder than liquid water is liquid ammonia (NH4). Ammonia's liquid range is a bit narrower than water's, but not excessively so: 196 to 239 Kelvins (about -77 to -34 Centigrade, or -106 to -29 Fahrenheit). It's very common in the universe, so it's entirely possible to have celestial bodies with ammonia seas or oceans. The DNA and proteins which would form in such an environment would be very different from what we see on Earth, but it seems likely that evolution could come up with a system that would work about the same way, at least on the macroscopic level.
Ammonia and oxygen react fairly readily, so I don't see an ammonia-ocean world developing an oxygen atmosphere, even if something analogous to photosynthesis evolves there.
In fact, photosynthesis on an ammonia world would be a planet-killer: plants turning ammonia and carbon dioxide into sugars would release oxygen and nitrogen into the atmosphere. The oxygen would combine with ammonia, turning that into water and more free nitrogen. The free nitrogen just goes into the atmosphere, no problem, but in a liquid-ammonia environment water is a rock. It would accumulate in a planetary ice cap, permanently taking hydrogen and oxygen out of the environment. Gradually this dries up the ammonia ocean, until eventually all the available hydrogen and oxygen is locked up in glaciers and everything dies.
All of which means that life on an ammonia world would probably use lower-energy chemosynthesis for power — either from the start, or after photosynthesis becomes impossible.
It seems plausible that ammonia-based life would make greater use of nitrogen compounds than Earth life does. This is more than a cosmetic detail: notable complex nitrogen compounds include things like TNT and ammonium nitrate. They go boom, even without an oxygen atmosphere. In a cold ammonia world environment they would likely be stable ways to store energy in living tissues, but an ammonia-based being getting exposed to a hot oxygen atmosphere could go off like a bomb!
Chlorine: Chlorine (Cl2) is liquid at a temperature range similar to ammonia, 173 to 239 Kelvin. However, as we've already discussed, it's a lot less abundant in the Universe. I can't easily think of a mechanism which would create large amounts of free chlorine, especially since in most cold environments one could expect a decent amount of hydrogen.
However, hydrogen plus chlorine gives us hydrogen chloride (HCl), which is liquid at a temperature range of 160 to 188 Kelvin, somewhat colder (about -172 to -121 Fahrenheit). In the presence of water, hydrogen chloride forms hydrochloric acid. Where most water is frozen, I'm going to guess you would have oceans of liquid hydrogen chloride with a lot of acid mixed in.
Proteins don't like strong acid any more than they like high temperatures, so life in a hydrogen chloride sea would have to use some other kind of molecules. Non-polar organic compounds like oils or lipids could possibly withstand that environment, so perhaps more complex molecules might form inside globules of oil — or maybe evolution could produce an oily, non-polar version of DNA and proteins.
Sulfur Again: We associate sulfur with hot worlds, but there are sulfur compounds which are liquid at very cold temperatures indeed. Hydrogen sulfide (H2S) is a liquid between 188 and 213 Kelvin (-85 to -60 Centigrade).
Carbon disulfide (CS2) is liquid across a large range of temperatures, from 161 to 319 Kelvin, or -170 to 114 Fahrenheit. It is another non-polar molecule, which means it is more compatible with oils and similar compounds rather than water and salts.
Both compounds can play a part in sulfur-reduction biochemistry. If one could come up with a light-powered mechanism for making complex sulfur compounds, that could serve as the basis for a cold environment sulfur biosphere.
Note that it's nasty stuff on Earth, so any humans visiting a cold sulfur world, or cold sulfur beings visiting human outposts, will need very good protective suits.
Liquid Methane: Methane is extremely common in the Universe, and is abundant on the colder bodies in our own Solar System. It has a fairly narrow liquid range: 91 to 111 Kelvin, or -296 to -260 Fahrenheit. Though on massive worlds like the gas and ice giants the high atmospheric pressure can likely keep methane liquid even at higher temperatures. It's another non-polar liquid, so again, part of the "oily" family rather than "watery" polar solvents.
As already noted, this could support a biosphere based on lipid-type chemicals. Since methane can't co-exist with oxygen (that's literally how the SpaceX Raptor rocket engine is powered), biological processes on a methane world would have to depend on hydrogen reduction reactions. But hydrogen is exceedingly abundant, so that's not a problem.
However, as we get down into the extremely cold conditions within shouting distance of absolute zero, a new problem arises. Most of the processes within living cells are driven by random molecular motion. RNA molecules or the equivalent don't zoom around the cell nucleus like little forklifts carrying pallets labeled "Amino Acids." They just kind of bounce around via Brownian motion until they bump into some other molecules and interact.
Brownian motion is driven by heat, which thus means the colder the environment is, the slower biological processes go. This isn't a huge problem — in fact, in a low-energy environment it's almost an advantage, since energy sources we would disdain as too weak could be fine when integrated over time — but it does suggest that low-temperature biospheres will be slow. Slow-growing, slow-moving, slow-spreading . . . and slow-evolving. If each generation takes ten times longer, then natural selection is ten times slower.
This should come as a huge relief to us Earthlings. The fraction of the Universe where low-temperature life might arise and thrive is vastly bigger than the tiny portion where life like ours can exist. If low-temperature life could grow and evolve as fast as we do, we probably wouldn't be here. Earth would be a mining outpost supplying heavy elements for some ammonia-drinking entrepreneurs. Presumably if we ever do discover low-temperature organisms out in the chilly wastes beyond Jupiter's orbit, they will be considerably less advanced than we are, not just in technology but in actual biological complexity.
Liquid Nitrogen and Liquid Hydrogen: These two are primordial gases, and are very abundant. Hydrogen in particular basically makes up the whole Universe with a few trace elements. Both are liquid at very cold temperatures: nitrogen (N2) at 64 to 77 Kelvin, hydrogen (H2) at 15 to 20 Kelvin.
But. Both elements are certain to exist in fairly large amounts in giant planets, where the massive atmospheric pressures can keep them liquid at much warmer temperatures. Much much warmer: part of the interior of Jupiter is thought to be liquid hydrogen at thousands of degrees Kelvin!
What this means is that any gas giant world — and exoplanet searches have turned up hundreds of them already — could have vast oceans of liquid hydrogen at fairly high temperatures. So even if liquid-hydrogen life is vanishingly rare, the number of places it could arise are immensely greater than the possible origin points for water-based life, and have likely been around longer. If hydrogen-based life is possible, our Universe is optimized for them, not us.
The trouble with liquid hydrogen is that if conditions are cold enough for hydrogren to condense, then just about any other substance other than helium is frozen. And if you raise the pressure, the temperatures increase enough to make most complex molecules impossible. But it may be possible for other structures to exist in a liquid hydorgen or liquid nitrogen environment — physical structures like chains of vortexes in the liquid, or complexes of magnetic fields persisting at superconducting cold temperatures. Again, improbable, but the sheer amount of hydrogen in the Universe means lots of chances.
That wraps up the discussion of possible biochemistries. I would very much like to expand these entries, so please contact me if you have more information. Next time I'll tackle what an alien ecosystem might look like.
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