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jst
2003-Oct-14, 03:05 PM
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
2003-Oct-14, 03:14 PM
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
2003-Oct-14, 04:37 PM
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
2003-Oct-14, 04:41 PM
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
2003-Oct-14, 06:07 PM
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
2003-Oct-14, 07:34 PM
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
2003-Oct-14, 08:20 PM
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
2003-Oct-14, 09:01 PM
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
2003-Oct-14, 09:14 PM
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
2003-Oct-14, 09:42 PM
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
2003-Oct-14, 10:39 PM
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
2003-Oct-15, 12:04 AM
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
2003-Oct-15, 12:17 AM
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
2003-Oct-15, 01:21 PM
Mike,

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

Swift
2003-Oct-15, 01:45 PM
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
2003-Oct-15, 03:01 PM
<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
2003-Oct-15, 03:14 PM
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
2003-Oct-15, 03:18 PM
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
2003-Oct-15, 03:36 PM
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
2003-Oct-15, 04:27 PM
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
2003-Oct-16, 12:56 PM
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
2003-Oct-16, 01:56 PM
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
2003-Oct-16, 02:05 PM
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 :lol:

Betenoire
2003-Oct-16, 02:37 PM
I'll call up Magarathea.

Where'd you go for grad work?

TriangleMan
2003-Oct-16, 02:58 PM
But how likely are we to get planets with seas of methanol?
What we need is a planet with seas of ethanol :lol:

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

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
2003-Oct-16, 03:11 PM
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
2003-Oct-16, 06:00 PM
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
2003-Oct-17, 06:38 PM
Mike, you seem to be a chemist. Any idea how much salt concentration can lower the bottom end of the liquid range?

Swift
2003-Oct-17, 07:14 PM
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
2003-Oct-17, 08:25 PM
Swift is fast and correct. It's known as Raoult's law.

ToSeek
2004-May-26, 07:16 PM
The San Diego Union-Tribune has a good overview article about the relationship between water and life:

Water: Medium of Life (http://www.signonsandiego.com/news/science/20040526-9999-lz1c26water.html)


All life on Earth, the only kind we know about, requires three things: molecules that readily link up with other molecules, a medium in which they can hook up, and energy.

On this planet the molecules that fit the bill are made from long chains of carbon atoms called organic carbon. Carbon atoms are the fundamental building blocks of life because of their ability to form long, complex chains and link up easily with atoms of other elements such as oxygen, hydrogen and nitrogen.

Liquid water, meanwhile, provides an ideal medium in which everything comes together.

Energy, which organisms convert into tissue, comes to the Earth in the form of sunlight. For some creatures at the bottom of the ocean, energy comes from thermal vents that release heat from the Earth's interior.

Mars
2004-May-27, 02:24 AM
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

Water is bent, sort of like a boomerang or ^ symbol. The Oxygen is in the center. Oxygen is one of the most electronegative atoms, because electro negativity increases from left to right and from bottom to top, making Fluorine the most electronegative compound.

Because of this there is a partial negative charge on the O and partial positives on the H's, making 2 partial positives. Like a magnet with one S pole and two weaker N poles.

Polar is when you have a partial negative and a partial positive, usually referring to single poles such as a magnet.

Mars
2004-May-27, 02:26 AM
A semester and a half of Organic Chem mechanisms are coming in handy!

Mars
2004-May-27, 02:27 AM
http://webbook.nist.gov/cgi/cbook.cgi?Name=Water&Units=SI

TravisM
2004-May-27, 11:59 AM
All you need is a solvent and some kind of molecular chain, right? I would like to believe that there are stranger things out there than we can imagine, and I think we've found this to be true in other arena's time and again. Life in the universe is one of the neatest chemical reactions that happens naturally. I like to think of it as rocks with mold hanging on them. :)
All this talk of pressure/temperature got me thinking. How may solvents have we tried with what compounds and at what temperatures have we 'tested'? I can't think we've done too many permutations of this, but I've been wrong a bunch more than I've been right.
BTW, I like the whole magnetic self organization in stars, that's way left field! But, then, I'm WAY left field also...

Betenoire
2004-May-27, 05:10 PM
All you need is a solvent and some kind of molecular chain, right? I would like to believe that there are stranger things out there than we can imagine, and I think we've found this to be true in other arena's time and again. Life in the universe is one of the neatest chemical reactions that happens naturally. I like to think of it as rocks with mold hanging on them. :)
All this talk of pressure/temperature got me thinking. How may solvents have we tried with what compounds and at what temperatures have we 'tested'? I can't think we've done too many permutations of this, but I've been wrong a bunch more than I've been right.
BTW, I like the whole magnetic self organization in stars, that's way left field! But, then, I'm WAY left field also...

The thing is water is an amazing solvent. Solvents are generally in two groups, organic and inorganic. Organic solvents (kerosene, gasoline, more or less any liquid organic compound) are really good at disolving organic compounds. But when it comes to anything inorganic, well...
Water, on the other hand, can dissolve the minerals and salts that provide a lot of chemical energy for life (Generally energy for life comes from chemical gradients or light, and it's really hard to evolve bluegreen algae from scratch).
Other inorganic solvents could do the same thing, except they lack the hydrogen bonding that makes water relatively unique and gives it certain properties that make life more likely.

Kaptain K
2004-May-27, 09:57 PM
The basic dichotomy betwen solvents is polar vs nonpolar. Nonpolar solvents are best at dissolving nonpolar substances and vice versa. Water is "quasi-polar". It is polar enough to dissolve (most) polar substances but nonpolar enough to dissolves (most) nonpolar substances. It is the closest thing to a "universal solvent".

eburacum45
2004-May-28, 02:54 PM
It is just possible that most life in the universe does not use water as a solvent;

consider this; one possible way that has been proposed for humanity to explore distant stars is to build self replicating, autonomous machines; this may have already been tried, in some distant galaxy long ago and far away.

The universe might be slowly filling up with these Von Neumann machines, which (it seems to me) display all the characteristcs of primitive life forms.

Depending on how well they are programmed they might not be so primitive; they might even have been given the potential to evolve, and so may mutate into a myriad forms throughout the universe.

A lot of maybes, but it is possible, either now or in the future.

If these Von Neumanns are silicon based then the predominant form of life in the universe may well use little water or carbon in their 'metabolism'.

Just a thought.

Betenoire
2004-May-28, 04:19 PM
Sure. But what built them?

Erm... okay, chemical life:
It has to be dynamic (er... highly mobile. Even if the organism sits still, stuff inside of it has to move, to transport nutrients and the like). So there has to be some sort of solvent to allow things to move and diffuse through it.
It has to have lots of chemical complexity (I mean, mind-numbingly complex), and this is where the big "Well isn't this possible..." option of silicon comes into it. Carbon, and only carbon, provides the chemical complexity needed by life. Not even silicon comes close.
So, yeah, wet, carbon-based life is sorta essential, at least for spontaneously-arising life.