Why is H2O the sine qua non of life on other planetary bodies?

It’s not only Mars. I just heard on the radio a JPL researcher say that his best bet would be to explore a satellite of Saturn, Enceladus, where he said that it could be discovered “non-DNA” life form.

But always this requires water. Why?

Water is an extremely good solvent for complex chemistry. It is very low energy, so it doesn’t participate much except for catalyticaly. It dissolves most ions and any polar organic. Non-polar organic molecules don’t usually have many functional groups to react with anyway. It is also a relatively well ordered solvent, so where energy isn’t in your favor, entropy can make things happen. When I left synthesis, aqueous chemistry was having a bit of a renaissance, not because it’s green, but because complex structures would spontaneously form.

alternate solvents have problems. Methanol will easily oxidize to formaldehyde, formic acid and carbon dioxide. Ammonia will oxidize, act as a nucleophile and turn to nitrogen irreversibly. Hydrogen sulfide will reduce things, act as a nucleophile and become a very strong acid. Methane just doesn’t dissolve anything.

There’s a few “objective” properties water has from a chemical standpoint. It’s considered the largest universal solvent. Also oxygen and hydrogen are incredibly abundant in the universe over other elements. Oxygen, as an element, is requires for a host of chemical reactions not only in life, but as a major catalyst all around. Furthermore, it’s a compound that has a relatively larger temperature range for its liquid state… and, It’s Refreshing!™

There might be more “objective” qualities, but anything further would most likely come from our singularly biased view of carbon-based life as we know it.

What about the life cycle that Robert Silverberg postulated in “Point of Focus”, where a humanoid life form was discovered that used a carbon-tet/chlorine cycle: carbon-tet in place of water, chlorine in place of oxygen? Would that be workable, or did Silverberg just come up with a cycle that fit his plot?

The simplest answer is that Earthly life all requires water, and we know so little about non-Earthly life that pretty much the best we can do is assume that it’s mostly similar to the Earthly kind.

That said, though, water is a decent solvent for a variety of interesting substances, and it’s a lot more common in the Universe than other similar solvents. So as wild-assed guesses go, it’s probably not a terribly bad one.

WarmNPrickly said it way better.

In case no one was aware, organic molecules/chemistry are those substances and compounds that involve the element carbon (which is almost without exception, the basis of all life on earth, of course). Despite organic compounds being used in plastics, hydrocarbons and other, toxic even, applications.

Addendum: It’s not really a point against methanol, ammonia, etc. to say that they oxidize easily, since that’s presupposing an environment rich in elemental oxygen, which is not necessary even for Life As We Know It.

And carbon tetrachloride is nonpolar, and hence won’t dissolve ionic compounds. You might be able to work around that somehow, but it’d be tough.

Also, I don’t know if this factors in much, but water-ice is less dense than liquid water, so it floats. This isn’t usually the case for other basic compounds or elements. It could, perhaps, be a natual advantage for abiogenesis, or even survival of water dwelling lifeforms as most lakes/oceans would freeze over during cold seasons (or climates), while able to huddle around thermal vents or activity at the sea floor.

The point is that in terms of ox/redox chemistry, methanol is not anywhere near its minimum energy. Once methanol gets to CO2, it’s at it’s minimum and not going back. Methane is also in a thermodynamic sink despite being relatively high enthalpically. The oxidation states for a single carbon molecule are:

Methane (0) < methanol (1) < formaldehyde (2) < formic acid (3) < carbon dioxide (4)

The middle three may exist throughout the universe, but there aren’t, so far as I know, lakes of them anywhere. In terms of geological time scales, they are quite reactive.

It also means that ice on an ocean/lake of water will be exposed to the sun and therefore more likely to melt than non-water “ice” in an ocean of something else.

Water also has the advantage of a large liquid phase; slight temperature variations don’t cause everything to freeze or boil away. I’ve heard sulfuric acid given as a possible alternative to water for that reason.

(bolding mine)
What are the exceptions?

My mistake. All life is carbon-based as far as we know. :smack:

(I think that report from NASA about finding bacteria with arsenic instead of phosphorous in their DNA was in the back of my mind, but that’s not the same, and I think even that claim has been rescinded, IIRC)

In addition to what everyone else has said, water’s high specific heat is extremely valuable to the viability of life. Water can absorb or release a lot of energy without changing temperature, so lifeforms made of water can freeze at night or stay out in the sun without their cells frying or having ice crystals rupture their cell walls. This temperature regulation is the only thing that allowed Earthlings to crawl out of the oceans in the first place.

Do substances dissolve in liquid helium or liquid hydrogen anywhere in the range of pressures observed on Saturn? Do they dissolve when hydrogen is in a supercritical fluid state, as on Jupiter?

For that matter, what are the prospects for life on Neptune, whose pressures heat up its ammonia/superionic water?

We expect places with water to be more likely to have life than places without water. Maybe even “much” more likely, as has already been explained in this thread. But no scientist worth his salt water would say that life could not exist without it. It is not the “sine qua non” of life.

It’s true, but at that point we will need to redefine life unless it reaches out and communicates with us. An edy in the suns corona could be life, but how the hell would we know?

I am dubious about this statement. Life as we know it is based, first of all on Carbon (the real sine qua non for life), and on large complex carbon-based molecules. Take a look at the drawings of lots of these, and a great many consist of carbon chains capped on one end by an H and on the other end by an OH.

Oxidizing these (burning or metabolizing) basically chops out the C’s in the middle, but creates a new HOH. Synthesis (photosynthesis or whatever other process) requires water, and not just as a catalyst. Water gets used up! New long carbon-chain molecules are built, consisting of an HOH with a bunch of C’s in between.

So I think water does get chemically involved in biochemical reactions.

It does, which is why I left myself wiggle room with “much”. Lets face it, there are few statements that can be made absolutely about life chemistry. My point was that water is an extreme energetic minimum. Any place where it does participate is probably taking energy to do so.

This on-line book has some interesting possible alternatives to water-based biochemistry;


including ammonia, sulphuric acid, formamide, supercritical hydrogen and nitrogen…

…of course the fact that we can imagine them does not mean that they exist anywhere in the real universe. Some potentially useful solvents, such as fluorine and chlorine compounds, are probably less likely because those elements are much less common in the universe than hydrogen, oxygen, carbon or nitrogen.

Water is not simply a solvent, but an important reaction partner in most important chemical reactions in life as we know it: amino acids are connected to form protein strands by eliminating water - the reverse reaction, hydrolysis or cleavage of a peptide bond by the addition of a water molecule, is exothermic. Sugars form complex carbohydrates such as starch and cellulose through elimination of water, the backbone of RNA and DNA is formed by elimination of water. All these macromolecules are metastable: Cell expend a lot of energy to correctly form these bonds, and spontaneous cleavage of these bonds is only prevented by the high activation energy required. In the presence of an appropriate catalyst (enzyme), all these macromolecules can easily be broken down to their constituent building blocks by addition of water molecules without expenditure of energy.