The assumption that alien biochemistries probably require liquid water may seem a little Earth-centric. But given the chemical possibilities available from the most abundant elements in the universe, even an alien scientist with a different biochemistry would probably agree that a water-solvent-based biochemistry is more than likely to occur elsewhere in the universe – and would be the most likely foundation for intelligent life to develop.
Based on what we know of life and biochemistry, it seems likely that an alien biochemistry will need a solvent (like water) and one or more elemental units for its structure and function (like carbon). Solvents are important to enable chemical reactions, as well as physically transporting materials – and in both contexts, having that solvent in its liquid phase seems vital.
Remove All Ads on Universe Today
Join our Patreon for as little as $3!
Get the ad-free experience for life
We might expect that common biochemically useful solvents are most likely to form from the most common elements in the universe – being hydrogen, helium, oxygen, neon, nitrogen, carbon, silicon, magnesium, iron and sulfur, in that order.
You can probably forget about helium and neon – both noble gases, they are largely chemically inert and only rarely form chemical compounds, none of which obviously have the properties of a solvent. Looking at what’s left, the polar solvents that might be most readily available to support a biochemistry are firstly water (H2O), then ammonia (NH3) and hydrogen sulfide (H2S). Various non-polar solvents can also be formed, notably methane (CH4). Broadly speaking, polar solvents have a weak electric charge and can dissolve most things that are water-soluble, while non-polar solvents have no charge and act more like the industrial solvents we are familiar with on Earth, such as turpentine.
Isaac Asimov, who when not writing science fiction was a biochemist, proposed a hypothetical biochemistry where poly-lipids (essentially chains of fat molecules) could substitute for proteins in a methane (or other non-polar) solvent. Such a biochemistry might work on Saturn’s moon, Titan.
Nonetheless, from the list of potentially abundant solvents in the universe, water looks to be the best candidate to support a complex ecosystem. After all, it is likely to be the most universally abundant solvent anyway – and its liquid phase occurs at a higher temperature range than any of the others.
It seems reasonable to assume that a biochemistry will be more dynamic in a warmer environment with more energy available to drive biochemical reactions. Such a dynamic environment should mean that organisms can grow and reproduce (and hence evolve) that much faster.
Water also has the advantages of:
• having strong hydrogen bonds that gives it a strong surface tension (three times that of liquid ammonia) – which would encourage the aggregation of prebiotic compounds and the development of membranes;
• being able to form weak non-covalent bonds with other compounds – which, for example, supports the 3d structure of proteins in Earth biochemistry; and
• being able to engage in electron transport reactions (the key method of energy production in Earth biochemistry), by donating a hydrogen ion and its corresponding electron.
Hydrogen fluoride (HF) has been suggested as an alternative stable solvent that could also engage in electron transport reactions – with a liquid phase between -80 oC and 20 oC at 1 atmosphere pressure (Earth, sea-level). This is a warmer temperature range than the other solvents that are likely to be universally abundant, apart from water. However fluorine itself is not a very abundant element and HF, in the presence of water, will turn into hydrofluoric acid.
H2S can also be used for electron transport reactions – and is so used by some Earth-based chemosynthetic bacteria – but as a fluid it only exists in the relatively narrow and cold temperature range of -90 oC to -60 oC at 1 atmosphere.
These points at least make a strong case for liquid water being the most statistically likely basis for the development of complex ecosystems capable of supporting intelligent life. Although other biochemistries based on other solvents are possible – they seem likely to be limited to cold, low energy environments where the rate of development of biological diversity and evolution may be very slow.
The only exception to this rule might be high pressure environments which can sustain those other solvents in fluid phase at higher temperatures (where they would otherwise exist as a gas at a pressure of 1 atmosphere).
Next week: Why Carbon?
Meadows et al The Search for Habitable Environments and Life in the Universe.
Wikipedia Hypothetical Types of Biochemistry.
28 Replies to “Astronomy Without A Telescope – Why Water?”
Apart from ammonia, which may have had reasonable impact on early Earth environment as well, I’ve never been much interested in alternative chemistries. I would be quite happy with observing other water biospheres at first. However alternative chemistries do extend habitability, if perhaps marginally, so it is reasonable to look at these environments as well.
That said, I believe recent advances makes the case for other solvents much weaker. A series of results culminating in an understanding of how probiotic biochemistry establishes itself points out the importance of water.
Viz., earlier textbook ideas of how chemical reactions behaves with temperature change seems to be an observer effect. (Of choosing rates and temperatures that allow easy observation.) In reality, the slowest reactions increases most in rate, and reaction half-life tend to ~ 10 years at 100 degC. Even for those reactions that would take billions of years at 25 degC. (See fig 1.)
Not only makes this nearly *all* putative pro- to protobiotic scenarios viable. (If you hear the popping sounds, that is the empty heads of creationists that reacts to hearing of this research.)
But it brings in a selection effect that results in early enzymes. Namely, when the planet in question globally cools, it automatically increases rate enhancement in the same subset of enzymes cells use today, enthalpic enzymes.* (See fig 2.)
The paper points out that these enzymes depend on polar bond effects most pronounced in water (electrostatic and hydrogen bonds). This doesn’t necessarily rule out, say, ammonia. But it makes water the environment with the richest probiotic set of “metabolic” pathways.**
* It is interesting to note that the only known exception, the core PTC of the ribosome that decodes mRNA to protein, is but a slight provider of reaction enhancement by the entropic effect. This “water trap” (uses two trapped water molecules instead of metal ions for chemical and mechanical action) seems selected as an entropic “placement machine” with a specific assembly production task, being the exception that proves the ‘rule’ that cellular functions are mostly exaptations for other tasks.
Btw, this is another test for the RNA world, its core enzyme can differ from previous and later metabolic enzymes because the RNA world was emergent on existing pathways and in later low temperatures. (20-40 degC, as the putative early amino acid set seems to prefer, see “Parallel adaptations to high temperatures in the Archaean eon” by Boussau et al.)
** Also, if I understand the earlier paper “Temperature Dependence of Enzyme Rate Enhancements” by Wolfenden et al correctly, a big if, the resulting enthalpic enzyme set results instead in the textbook temperature non-sensitivity of reaction rates, “the Harcourt line”. If this was the case it would undoubtedly help stabilize protocells until more full regulatory mechanisms were in place, as well as benefit robustness of later cells. This is then another benefit from water chemistry.
Extraterrestrial chemistry… Now THAT is an interesting subject!
Unexpected extraterrestrial chemistry like the liquidy deposits on the landing legs on the Phoenix Lander come to mind. Extraterrestrial chemistry must include the unknown effects of chemical reactions in a variety of gravitational, pressure, radiation/temperature or magnetic fields densities. Combination’s of ion exchanges under the influence of any or all of these factors are a challenge to estimate but no doubt innumerable?
Ahem… no doubt innumerable in our universe?
Well, yes, in principle we can reproduce them but the cornucopia of effects would be practically impossible to elucidate. Look at life from CHNOPS compounds (recent post): who ordered that? I.e. how did that came about from “humble beginnings”?
Which is the very question we are interested in here. I would say that we can get a handle on that subset of effects by comparing water chemistry with other chemistry. And I did an attempt above.
So what about water being particularly able to produce geometric structures, via transient hydrogen bonding, in response to resonance on many frequency ranges. I feel this is an area that has strong implications for life, particularly if you believe in the chemistry of life evolving at underwater geothermal vents, which does seem to be the most likely explanations for terrestrial origins.
That said, panspermia must be given some credence, and we can also contemplate the origins of life being a direct by product of the nature of the universe and the fundamental ways in which higher energies operate with the physical dimension.
That said also, what about the spontaneous generation experiments done in Russia with white hot sand in a vacuum sealed tube?… Oh thats right, we can just ignore the Russian and German research that doesn’t fit in with the western social control dynamic. (said a bit simplistically, and i really shouldn’t go toting non-cosmological stuff on this site)
As I commented on above, modern research seems to point out the strong hydrogen bonds of water especially as vital for probiotic chemistry.
I don’t know what you mean by water and geometric structures, resonances and frequency. What would be the resonating system, and why would volume water geometry be important for life as we know it? Similarly for “higher energies” and “physical dimension”; similarly for “spontaneous generation” in “a vacuum sealed tube”. I don’t see how they connote something real, and the absence of references tells me they don’t.
Transpermia doesn’t solve OOL, it modifies it at best. And spontaneous generation was soundly rejected by Pasteur 1859, after being in doubt since 1668. Do try to keep up! 😀
I don’t understand how people can get so off the scientific track. It’s as if they live in their own little world. It’s frankly stunning that every article on here has commentators who actually believe vibrations, “higher energy” plasmas and eathers and other junk is real.
I’m not anywhere near as knowledgeable as many of our heavyweight commentators such as LC or Torbjorn, but even I know this stuff is pure fantasyland.
I just don’t get it. With all the information available today, there is really no excuse. Perhaps UT can do an article on this? Maybe there is an evolutionary purpose that drives a percentage of individuals to think outside the box, even if “outside” means nowhere near the box at all!
Yes, we can ignore those russian and german researches since they simply failed at showing what they claimed.
We can ignore russian and german researches that doesn’t exist – no references were given.
I already said that, btw. Iz my writing skilz too much for you?
Torbjörn, actually you didnt say anything of the like, iz your writing skilz failed ?
btw, you may not have noted so ill point it out – sarcasm intended in original post, learnz to readz…
Get your dose of pseudo-science here. 🙂
Read about Masaru Emoto’s awesomely beautifully experiments that have been unanimously debunked regarding water memory. But fun to consider nonetheless.
Then Visit Wikipedia for some context.
For me, seeing Water experiments on the ISS have been some of the most intriguing. Particularity how water behaves and responds in zero G. 🙂 Wish there was more experimentation with this medium on the ISS.
The polar properties of H_2O make for pretty compelling reasons to consider it the solvent for life. Another reason is that it has a pH of 7, in the middle of the acid-base range, which makes it the perfect solvent for the transport of protons in acid-base reactions.
One of the early developments in pre-biotic chemistry was the formation of a lipid bubble that contains the cytosol of a cell. At the core this appears to require the interaction of polar and nonpolar liquids. Life also requires a molecular coding structure. DNA and RNA are double stranded by the hydrogen bond. The properties of nucleic acids and DNA are tied then to their polar structure and solubility in water with a similar hydrogen bonding.
It is possible that other liquids could promote complex chemistry. It is difficult to say whether any of that would constitute what we call life.
Indeed, thanks for bringing that up!
The properties of water you mention is the other side of the coin, its possibilities in protobiotic chemistry. As you say, water seems perfectly suited for self-assembly of membrane vacuoles and setting up redox systems to explore anabolic and catabolic metabolism.
A self-assembling protocell complements the core RNA world enzyme (ribosome) in that it isn’t all enthalpic driven (nor entropic, AFAIU), but a low temperature (self) selected mechanism. I realized that yesterday but I had kludged my previous comment enough.
*If* probiotic chemistry provided enough local concentration of molecules, something which the Stockbridge et al paper seem promising to show, these structures should form. And if the probiotic chemistry is metabolic like, again alluring hypothesis following from Stockbridge et al, there would be selective forces for ribozymes/ribosomes + protein enzymes.
In fact, all we need per Shostak et al is for the protocells to cycle in a natural “PCR” environment like hydrothermal vents. Semipermeable uptake, copy isolation and phosphorylation activation of of nucleosides would then take the protocells from a chemical selection process to a natural selection process by self-copy. Here is an old but endearing presentation of a (AFAIU) still viable variant. (If you don’t like the initial creationist take down, you may want to enter at around 2:45 and turn the annoying music off.)
Since that presentation, Shostak et al has gone on to show that spontaneous cleaving is what you get from spontaneous protocell growth. At least two out of four nucleosides have known natural synthetic pathways. So “all” the pieces of the grand puzzle is there, or nearly so.
The pre-biotic world was probably a natural PCR world with RNA. RNA has gained a lot of attention of late, for it has complex interactions with polypeptides. Some mRNAs are coded up not to produce some protein, but just to bind onto other polypeptides to facilitate structures. Ribosomes are nice complexes of this sort. At some point the random assemblies of RNA corresponded to replicase, which was then selected for on a molecular level.
Of course this is rather speculative at this time. It is my hope that these remnants of water environments on Mars might contain some preserved record of this pre-biotic chemistry. Of course it will require some very complex probe capable to performing a wide range of chemical assays to ferret this out. We might have to wait until the 2020 or 2030 time frame before there is any prospect for that.
Actually it is more simple than this article implies.
In a nutshell, it all depends on the dissociation constants (ionisation constant) of the chemical reactions. Without this very useful ionic reversible reaction, there cannot be any transfer of energy to form active molecules in solutions.
For any life to actually exist, the critical value of the dissociation constant of water (or other solvent) must be consistent across a narrow range of temperatures and pressures , which for water in aqueous solutions is across 0° C to about 40°. The general process is also called autoprotolysis.
As long as molecules can dissociated between these two states (called an ampholyte), it is possible for life system to evolve with different or more complex molecules.
So if it ain’t liquid, it is unlikely to change dissociating molecules fast enough to exhibit any form of respiration.
Interesting point on catabolism (or even anabolism assembly), but I don’t get it.
Okay, I knew that the isoelectric point of water is pH 7, which is what Lawrence describes above. Water is naturally ampholytic and so buffering at that pH. [Oy vey, chemistry was a long time ago!]
I didn’t know that water was an ampholyte only under a certain temperature range.
I’m plotting pKW against T and it’s a smooth declining function. In fact its derivative is lower for higher temperatures. (And looking at its self-ionization wiki entry, there is a minimum at higher temperatures at these pressures.)
What am I missing?
– Is it “the critical value”? Which means what, btw?
– Is it “in aqueous solutions”?
– Or do you mean that outside of the convenient temperature range, the buffer pH differs from the usual? By its very nature, buffering acts against such a change. And I don’t believe I am making that up, I believe the entry I mentioned above on self-ionization makes that very point!)
[I’ll also add in this context that I goofed on the temperature range “the putative early amino acid set seems to prefer” in an earlier comment. It was actually 0-40 degC, the very range we discuss here!]
That is “pKw”, or pKw, obviously.
HTML fail for “sub” tag.
Oops, i meant 100°C NOT 40°C.
“I didn’t know that water was an ampholyte only under a certain temperature range.” Obviously, it cannot act as an ampholyte if it is an gas or as ice!
Ah, now the pot is (not) cooking! 😀
As for the phase changes, I dunno. Solids can make the durnedest things, even act as electrolytes and proton transport media. The problem is that they don’t move and self-replicate much (if all solid).
Gas chemistry can still be interesting under sufficiently high pressures, but yes, it wouldn’t be the same now, would it.
What atmospheric pressures other planets have? Super earths, for example.
Is alternative life more likely around red dwarfs?
1) “atmospheric pressures”
There is a whole slew of scale laws that pops up when you take Earth and scale up (or down).
Crustal and atmosphere pressure scales, for the near surface volumes we are most interested in, as planetary gravity or mass or radius^3. But there are other scaling considerations, such as initial atmosphere collection and later leaking that scales with area or radius^2.
Long analysis short, “Earth analogs” seems to be between 0.5 – 2 Earth radius.
Note that this doesn’t tell us how well cells withstand pressure.
In fact the most sturdy cells look like fullerene cages (floor balls) in their cell wall skeletons of proteins with large openings, surrounded by cell membranes on both sides. They take differential pressures of 40 bars easily (so could have developed while osmotic et cetera regulation was still haphazard).
But pressure adapted bacteria (as well as multicellulars) are found all the way down the 10 000 meters of the Mariner deep, so can take 100s to 1000s of atmospheres non-differentially. (“One atmosphere for 10 meters of water.”)
2) “red dwarfs”
I would think so, for biospheres in general.
– We know that the number of such M stars outnumbers the rest.
– We know that the habitability zone of M stars gives tidal interlock after ~ 1 Gy.
Before that super Earths with high initial rotation and denser atmosphere could shield from these small stars sometimes unruly beginnings. (At some 100 times larger Coronal Mass Ejections than our type of larger star.)
Afterwards the locked rotation rate is ~ 1/30 of ours, resulting in ~ 1/1000 weaker fields. (According to the dynamo theory, the field scales as 1/w^2, w = angular frequency.)
Now it pays to have a denser atmosphere! But also, mature M stars may have weaker CMEs than ours, and it is mainly CMEs that erode atmosphere. M stars mature in ~1 Gy (I think, what says the experts?), and many or most are very active and not good for life.
There is no show stopper to biospheres around M stars that I know of.
– We know that archaeabacteria can extract energy from far IR, and that cyanobacteria can do bona fide photosynthesis in near IR of M stars (by last years find of chlorophyll f).
There is no show stopper to oxygenic atmospheres around M stars, and so multicellularity, that I know of. (Multicellularity requires endosymbiosis of the mitochondrial type to feed a large enough genome, which in turn requires oxygen to feed the voracious appetites of protein producing genes.
Oops. I’m beginning to tire I think. Some errors in there:
– “atmosphere collection” is a bad example of scaling, depends on the actual process which is somewhat up in the air (sic!).
– “proteins” – carbohydrates (mostly).
At higher pressures, water will still be a liquid at temperatures >100°C, so uncatalysed reactions will be even faster.
Could complex organisms evolve at temperatures which stayed high?
Yes, but the problem with pressurised solutions is the inhibit reactions and formation with other biological important molecules. For life on earth, this would be for the amino acids, where temperatures above 300°C (570F). However, for proteins to form, the temperature would have to be much lower. (At rough guess probably 150°C (300F)) I.e. The highest known is for the hyperthermophilic bacteria that will still grow at up to 122°C (250F)!
For alien chemistry using compounds other than water, this would be equally as complex.
Really great question, though!
Oops! (Must be tired….)
I meant to say; “For life on earth, this would be for the amino acids, where temperatures above 300°C (570F), life would not not survive.”
Yes, it’s a great question! [He says, having raised it himself on occasion… :-D]
I’ve been wondering about that. Some astrobiology texts mentions the water triple point, as if going critical would be a problem in itself. The absence of a phase (gas+liquid -> “gas-liquid”) would mean less phenomena, but shouldn’t be taken as a no go I would think.
HSBC notes some more relevant problems. I didn’t realize hyperthermophiles were so close to protein fold formation stable temperatures. I would have thought there was a leeway to find sets of high temp stable proteins. Is there a reference for this?
As for generally, full development under such conditions at the very extremes, I think there are some problems. The route I gave in my first comment depends on precisely lowered temperatures. Absent that, there would be one more function (enzymes) that would have to evolve later. And we don’t know how feasible that would be.
Now you can argue that you don’t need enzymes at high temperatures, and that the set of realistic (high enough rate) reactions are greater. And yes, including inhibiting and/or decomposing reactions.
But then I have a harder time see how cells would institute reaction control in the first place! Without enzymes you would have to go the other route and inhibit all reactions instead. (And start of from scratch to boot, not naturally given a fully working subset of such compounds, outside some inhibits.) That seems like a much taller order!?
And you guys prompt me to note that it didn’t happen on Earth, hydrothermal vents and other high temperature and/or high pressure habitats are all populated by ordinary cells. If other cells naturally evolved as hyperthermophilic they would have had a tremendous advantage when our own mesophilic lineages started to colonize other environments. But we see exactly zero such alternative life.
Speaking of hyperthermophilic cells, one should be slightly wary of using them as evidence of possibilities of high temperature life. A cell is an enormously crowded chemical compartment, which is diffusion limited and even employ active transport for functionality.
The evolved hyperthermophile solutions works there, in fact it is likely that they depend on such crowding in the first place. (For example, as a means to stabilize protein folding.)
But it is another question if there are pathways that goes from high temperature environments to crowded cells in the first place. Now I happen to think that the set of possible pathways is so large that it doesn’t matter much. But I don’t know how to test that (without finding actual such life).
One other point to add is that we can see that high temperatures was a tremendous problem in the first place. Archaeabacteria, the dominant and highest temperature tolerant set of species, seems to be the youngest major clade bar eukaryotes, which they are sisters with.
Eukaryotes likely goes back < ~ 1 Ga to a time just before their incessant attempts at multicellularity, with success at least 3 independent times. If they are older, and there are no fossils that are unequivocally eukaryote that old, we can't explain why there are no older multicellulars. (And again, see the links in my first comment how the enabler of multicellularity likely defined eukaryotes in the first place.) Oxygen was plentiful long before that.
Looking at archaea themselves they are a) AFAIK fairly reliably phylogenetically resolvable by using a few genes instead of whole genome approaches b) they have a deep split into two clades. Being able to make those observations in my mind point to the youngness of the clade. [But I'm no biologist.]
Comments are closed.