ET - has to use water?

[jst]

Ok, this may be a little out on the loony fringe for this crowd (forgive me, I'm new), but this is something that has always bugged me.

Why do we assume that life on other planets will rely on water? Europa is often cited as a top candidate for life in the solar system (beyond Earth, obviously) because it has water. Why is this?

What is it about water that we think is so critical? Is it because of the theories about how life started (at a super-simplified level, that molecules in water were able to bump into each other to create replicating molecules)? Do we think the same could not happen in, say, a gas environment?

Is it something fundamental about biology? If so, why do we assume that other biologies have to follow the same structures as ours? In a different environment, might not another system have evolved?

Forgive me if this has been covered before. I am happy to read another thread on this if there is one.

[AstroSmurf]

Water does actually have some interesting properties.

1) It's a dipole, which has significant consequence for the chemistry.
2) It is lighter in solid form than in liquid form. This isn't true for many other substances.

There have been speculations about alternate biochemistries, but no firm data is available. Mostly, this is because we have only one type of ecosystem to observe; others may be possible, but without data, this is all speculative.

I'm not really up to speed on this subject; perhaps someone else can fill in.

[mike alexander]

Water is really a most remarkable substance.

It is a dipole.
The unshared pair of electrons on the oxygen let it participate as an intermediate in many chemical reactions.
It is an excellent dielectric. It dissolves many polar substances.
It can donate/receive protons in many chemical reactions.
Due to self-association it has an extended liquid range at relatively low pressures.

I personally have a hard time seeing how life could originate as anything but carbon-based in a water medium. Where it goes from there may be another story.

[Andromeda321]

I used to know the answer to this one, I'll have to go find my old bio notes and look it up...
Just thinking about it tho, I'm guessing that part of it is just because we know water is used on Earth and nature repeats itself. So instead of looking everywhere and being overwhelmed by it all if you use a factor like water in your search you can narrow things down a bit. It's pretty big out there so it's better to focus on the probable cases first based on what we know before looking elsewhere.
But, as I said, I don't remember the actual answer. Where IS that bio notebook?!? :-?

[Betenoire]

The water/carbon format is at the very least highly convenient to the formation of life. Large carbon molecules tend to be "hydrophobic". They don't like water so they cluster together very tightly (This can be seen in the globules formed by oil if you mix it with water). This behavior is how cell membranes formed (Technically formed by hydrophobic molecules with one hydrophilic end (lipids) which can form a lipid sphere between the water outside and water at the center). Non-carbon based life is a near impossibility, and nothing has the same effect on carbon molecules as water.
ATP, a molecule used to transfer energy from what a cell 'eats' to its functioning proteins, is hydrolized to release its energy. There are likely other ways of providing energy, but at least in the case of ATP water is necessary to store and release its power.

[TinFoilHat]

It may be because every life form we have ever seen needs liquid qater. While this doesn't mean that life that doesn't involve water is possible, we have no examples of viable waterless live and no idea what environmental conditions it would need or where to look for it. So we look for places where life that we know about could concievable exist, because otherwise we'd have to look everywhere, for things which we have no idea what they would look like.

[ToSeek]

Everywhere we've looked so far (which is limited to Earth, of course), water means life. If there's water, there's life. If there's no water, there's no life. It's a perfect correlation. Now, there may be life somewhere that finds a way to get by without water, but since we haven't found any yet, that doesn't seem like the way to bet.

[informant]

I'm guessing that part of it is just because we know water is used on Earth and nature repeats itself.

Do we? We've only got one "repetition" from which to infer.

[Ilya]

The broadest definition of "life" is simply a self-organizing, self-reproducing structure. By that definition a computer virus is alive. While not carbon-based, it (so far) requires a silicon template built by carbon-based life forms to exist.

Personally I think all chemistry-based life must use carbon, and almost certainly water. But chemistry (interactions between molecules) is not the only way to make complex structures capable of converting energy into more of themselves. Magnetic structures possessing same trait are at least theoretically possible, and would be most likely found inside stars and perhaps even inside Earth mantle. Trouble is, how would we go about looking for them? Or even recognize them? The most sensitive magnetometer we could devise or even imagine would see nothing unusual about a stable, self-reproducing magnetic whorl in Sun's photosphere - it would register as merely another magnetic whorl.

In the same vein, if a hypothetical intelligent magnetic being (to which conditions of Earth's oceans are as deadly as photosphere is to us) built an "Earth-probe", it would probably not recognize a fish or a whale as alive. After all, it is just water with a few impurities.

[pi is exactly 3]

Can someone describe polar and dipolar to me? I know I failed that part of my last chemistry lab. What is so special about polar and dipolar?

Perhaps someone could make a dot structre since the visual might be helpful

Thanks

[mike alexander]

A polar molecule is one where there is some inhomogeneity in the electron distribution. A chemical bond between atoms of differing electronegativity (C-O, for example) is polar, negative toward the oxygen, positive toward the carbon. Polarity can also be induced by external factors. Van der Waals attraction is the result of instantaneous inhomgeneities in the electron distribution of a molecule, which can then induce similar inhomogeneities in an adjacent molecule.

A dipole can be either permanent or induced. In the case of water, the electronegative oxygen atom is essentially attracting the electrons toward itself, making it more negative. This partially unshields the positive charge on the protons, maing them slightly more positive. Negative (ish) on one end, positive (ish) on the other: a net dipole.

The oxygen also makes the O-H bond weaker. Pure water is dissociated to the extent of 10^-7 (hence pH 7 of pure water).

[Lunnalkann]

Interesting thread, mind if I derail it?

It is related to this somewhat. How low are the possibilities of non-carbon lifeforms? Silicon in particular.

[mike alexander]

How low are the possibilities of non-carbon lifeforms? Silicon in particular.

Grad school professor had a mantra: silicon is not just heavy carbon!

Silanes (-SiH2-SiH2-SiH2-) are not analogus to hydrocarbons (-CH2-CH2-CH2-) in terms of stability or polymer formation. They are unstable with respect to oxygen and water. There is no stable silicon analog to the amide bond in proteins, say.

Not to mention that the analog of CO2 is SiO2. Quartz.

In short. Very low. Sorry.

[Betenoire]

Mike,

I've been wondering: Is sulphur versatile enough to have an outside chance of giving rise to sulphur-based life?

[Swift]

Short answer - no. Longer answer - sulfur chemically is closest to oxygen. Could one have life based on a carbon-sulfur-hydrogen chemistry? It seems unlikely, there is just not enough possible compounds there. Humans would also find it very smelly . Sulfur as a substitute for carbon - its not even close.

Carbon chemistry is really amazing, even beyond the fact that life on Earth is based on it. Because of the variability of carbon-oxygen, carbon-carbon, and carbon-nitrogen bonds, the number of compounds one can make, with a huge range of sizes, chemistries (polar vs. non-polar, for example) and structures, its just hard to beat.

What about carbon based life in a non-water environment? Water is a great solvent and seems very common in the universe, but I can't think of a quick answer to completely rule it out. It doesn't happen in earth-life, but people do a lot of carbon chemistry in non-aqueous systems.

[jokergirl]

<nitpick>

2) It is lighter in solid form than in liquid form. This isn't true for many other substances.

It isn't "lighter" but it has its densest point at +4°C, whereas it freezes at 0°C. The mass of the same volume is different.

</nitpick>

[eburacum45]

If you have a self perpetuating system which can store data and replicate itself using that data, and respond to stimuli, you have something very similar to life;
in this fiction I took slow oscillating reations and magmatic rheolith crystals , mixed them up and confected a semimolten crystalline self-organising system, which doesn't rely on silicones or anything so degradable;
http://www.orionsarm.com/xenos/Rheolithoids.html

other people have imagined selforganising systems of plasma in stars and so forth.

My colleagues at OA seem to think that ammonia might have enough versatility to support life and living systems;
http://www.orionsarm.com/xenos/Muuh.html
this seems remotely possible,

but I would expect that life aswe know it might also hold surprises, for example the heat, cold and radiation tolerant extremophiles might find themselves selected for on a planet with severe environment stress.

[Iain Lambert]

the analog of CO2 is SiO2. Quartz.

In short. Very low. Sorry.

Thats the killer part for me, definitely. Its really hard to figure out how you'd do an energy cycle for a silicon-based lifeform that doesn't have it trying to either breathe or excrete quartz at some point.

[Betenoire]

Short answer - no. Longer answer - sulfur chemically is closest to oxygen. Could one have life based on a carbon-sulfur-hydrogen chemistry? It seems unlikely, there is just not enough possible compounds there. Humans would also find it very smelly . Sulfur as a substitute for carbon - its not even close.

Carbon chemistry is really amazing, even beyond the fact that life on Earth is based on it. Because of the variability of carbon-oxygen, carbon-carbon, and carbon-nitrogen bonds, the number of compounds one can make, with a huge range of sizes, chemistries (polar vs. non-polar, for example) and structures, its just hard to beat.

What about carbon based life in a non-water environment? Water is a great solvent and seems very common in the universe, but I can't think of a quick answer to completely rule it out. It doesn't happen in earth-life, but people do a lot of carbon chemistry in non-aqueous systems.

Darn :-( I was hoping for non-carbon analogies formed by sulfur, what with its six bonds and all.

[mike alexander]

Ammonia as a liquid substrate for life has marginal possibilities but can't be completely ruled out. It has a narrower liquid range (in terms of temperature) than water, about -80 to -30 degC at 1 atm, and roughly half the heat of evaporation. Obviously, the liquid range can be extended by increasing pressure. It's polar, has good hydrogen bonding. It has a dielectric constant about 1/4 that of water, so it can dissolve ionizing compounds fairly well. Its autoprotolysis constant is 10^-30 compared to water at 10^-14.

One problem with ammonia is that the solid form is denser than the liquid form. This means that an environment that exists near the freezing point would probably be metastable. On earth, where water is near the freezing point, extended liquid areas freeze from the top down. This means that solar input, for example, will tend to melt the top layer of ice first (the ice also insulates the lower liquid layer from further freezing). In an ocean of ammonia, any freezing would mean the solid would fall to the bottom, continually exposing fresh liquid to more freezing. So in the winter, the pond would freeze from the bottom up.

People always come back to carbon and the second row of the periodic table not just because we are made of it, but also because the bonding properties are such that complex molecules can arise in the broadest range of environmental conditions. Once you get down to the third row and that extra layer of electrons, stable complexity becomes MUCH harder to sustain (i.e., the range of environments conducive to stability becomes much narrower and more particular).

[Ian Goddard]

The definition of habitable zone -- “The region surrounding a star within which the star’s heat can maintain one or more solvents in the liquid state” (Goldsmith & Owen, 556) -- is not restricted to water. As the term suggests, the habitable zone defines a region wherein life might be able to occur.

Another drawback of ammonia (in addition to that cited by Mike) versus water is that unlike ammonia, water can potentially shield against ultraviolet (UV) radiation. Incoming UV photons from the local star disassociate some H2O molecules releasing oxygen atoms, some of which link into ozone (O3) molecules that then block UV radiation that destroys organic molecules. On the other hand, ammonia disassociation yields nitrogen which does not block UV radiation. So H2O has this built-in shielding capacity in addition to its many other advantages for life. (Goldsmith & Owen, 245-6).

Another solvent that has been suggested as a possible basis for life is methyl alcohol, which has an even broader liquid temperature range (159C) than water (100C). (Goldsmith & Owen, 243) But I don’t know about its other advantages or disadvantages.

Source: Goldsmith, Donald and Tobias Owen. The Search for Life in the Universe. Third Edition. Sausalito, California: University Science Books, 2001.

[Betenoire]

Does methanol hydrogen bond? (I bet I've been told this before, but...) The O-C bond would definitely have a dipole moment, but I expect it doesn't float when frozen.

[Swift]

Methanol (methyl alcohol) does hydrogen bond (believe or not, I've actually measured it with NMR as part of my graduate work) but it is a much weaker bond than water. But how likely are we to get planets with seas of methanol?

What we need is a planet with seas of ethanol

[Betenoire]

I'll call up Magarathea.

Where'd you go for grad work?

[TriangleMan]

But how likely are we to get planets with seas of methanol?
What we need is a planet with seas of ethanol

Would be great for a party, just remember to tell the guests they can't smoke!

So to get it back to jst's topic, does anyone have any speculations for any other non-water medium for life? I initially figured no, there have been previous threads from months ago discussing non-carbon life, but I didn't realize that ammonia could form a very-slim-but-possible replacement for water for a carbon-based life form. Anything else?

[Betenoire]

I'd say that hydrogen bonding is necessary, and a relatively small size, and abiotically occurring (that is, it has to have a source before life goes about generating it). That really narrows down the field.

[mike alexander]

Betenoire and IanG add, I think, really good points. There are inherent chemical limitations and environmental limitations.

You keep coming back to water because it has so many unique properties. Acid-leveling, proton donor-acceptor in so many reactions. Great ability to hold things in solution. Cosmically fairly abundant.

It occurs to me that the liquid range may not necessarily be as bad a limiting factor as I used to think. If the atmosphere is thick enough the liquid range will be extended by the pressure. Might even go over into the supercritical fluid range, which might actually have advantages. I wouldn't have thought it as reasonable until I saw those hydrothermal vent bacteria happily multiplying at 100+C. There is an obvious upper temperature limit imposed by the energy of the bonds, but it looks like life can take more heat than most of us ever thought.

[Betenoire]

Mike, you seem to be a chemist. Any idea how much salt concentration can lower the bottom end of the liquid range?

[Swift]

I'm not Mike, but I can give you an answer.

Freezing point depresson, the amount the freezing point is lowered by adding a solute (salt) is defined by

delta T = i Kf m.

i depends upon the number of ions in the salt, so, for example, sugar has i = 1, NaCl (table salt) i = 2, MgCl2 i=3, etc.

Kf depends on the solvent/liquid. For water its 1.86.

m is the concentration of the solute.

Generally we are talking a couple of degrees, up to maybe 10 or 20C tops.

[mike alexander]

Swift is fast and correct. It's known as Raoult's law.