Mono Lake in California - not really a site of alien biochemistry, but nicely photogenic all the same.

Astronomy Without A Telescope – Why Water?

Article Updated: 24 Dec , 2015

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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.

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.

Water's polar nature - and acting as a solvent. Credit: Addison-Wesley.

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?

Further Reading:
Meadows et al The Search for Habitable Environments and Life in the Universe.
Wikipedia Hypothetical Types of Biochemistry.



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Aqua4U
Member
January 8, 2011 8:57 PM

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?

Aqua4U
Member
January 8, 2011 9:02 PM

Ahem… no doubt innumerable in our universe?

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 5:43 AM

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.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 8, 2011 6:38 PM
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,… Read more »
Foote
Member
Foote
January 8, 2011 10:06 PM
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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 5:53 AM

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! grin

Uncle Fred
Member
Uncle Fred
January 10, 2011 11:22 AM
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… Read more »
Excalibur
Member
Excalibur
January 10, 2011 5:13 AM

Yes, we can ignore those russian and german researches since they simply failed at showing what they claimed.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 10, 2011 8:49 AM

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?

Excalibur
Member
Excalibur
January 10, 2011 9:50 AM

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…

damian
Member
January 8, 2011 10:09 PM

Get your dose of pseudo-science here. smile
http://topdocumentaryfilms.com/water-great-mystery/

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. smile Wish there was more experimentation with this medium on the ISS.

Lawrence B. Crowell
Member
Lawrence B. Crowell
January 9, 2011 5:14 AM
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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 6:26 AM
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,… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
January 10, 2011 5:07 AM
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… Read more »
The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
January 9, 2011 6:26 AM
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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 7:02 AM
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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 7:06 AM

That is “pKw”, or pKw, obviously.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 7:07 AM

HTML fail for “sub” tag.

The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
January 9, 2011 7:44 AM

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!

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 8:28 AM

Ah, now the pot is (not) cooking! grin

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.

HeadAroundU
Member
January 9, 2011 7:26 AM

What atmospheric pressures other planets have? Super earths, for example.

Is alternative life more likely around red dwarfs?

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 9:09 AM
Good questions. 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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 9:19 AM

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).

Tramman
Member
Tramman
January 9, 2011 11:27 AM

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?

The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
January 9, 2011 11:44 AM

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!

The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
January 9, 2011 11:47 AM

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.”

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 9, 2011 8:46 PM
Yes, it’s a great question! [He says, having raised it himself on occasion… ] 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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 10, 2011 8:47 AM
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… Read more »
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