These are some good questions.
I've heard it asserted that life could potentially arise without the presence of water.
This is occasionally suggested. I have seen no particularly likely examples, but you don't know what you don't know.
Our life is carbon-based. Carbon is good for life because it is in the middle of the periodic table and so can form 4 covalent bonds, which allows for complex chemistry based on carbon chains. In fact, let's look at the relevant bit of the table:
Across the top you can see:
Carbon (4 bonds)
Nitrogen (3 bonds)
Oxygen (2 bonds)
Flourine (1 bond)
CNO are, along with hydrogen, the dominant compounds that make up life. The next most common elements are Phosphorus and Sulfer in the next row.
Chemical properties are similar vertically, so it is sometimes suggested that you could have silicon-based life instead of carbon-based life (some would say that computers could be silicon-based life, but this is a different category entirely as they are not using complex silicon chemistry in computers).
Whether silicon-based life could occur in water or some other solvent I have no idea.
But I've also heard it asserted that, if it wasn't for the fact that water expanded when it froze, the oceans would freeze solid.
Lakes would clearly freeze solid in cold places (whereas with real water just the top freezes. Whether this would happen to Earth's oceans seems to me somewhat debatable, issues like global circulation, hot tropics, geothermal heat, the effects of pressure from accumulating ice, etc. make this a very complex hypothetical question. I think only 1% of the Earth's free water is frozen right now.
So, how could life be sustained for long on a world where the basic solvent was ammonia or something other than water?
*If* you had, say, carbon based life that worked in an ammonia solvent, it would probably last as long as the ammonia, which could be billions of years in a cold environment.
I think the key difficulty with ammonia is that it has to be very cold to have liquid ammonia, and when it is that cold, none of the activation energies for the various carbon covalent bond reactions procede at any reasonable rate. This might be overcome with some kind of energy input to drive reactions, e.g. cosmic rays or something, but it is a big hurdle to overcome.
Perhaps you could get liquid ammonia at a reasonable temperature under high pressure, e.g. in a small gas giant with an Earth-sized core like Neptune, I have no idea. It would be interesting to have a chart showing the temperature/pressure ranges where various small polar molecules are liquid.
More likely solvents that occur to me (I have not investigated any of their actual properties, I just have general reasons/recollections to think they might be liquids in vaguely the right temperature range:
H2S (actually boils at -67 C, perhaps under higher pressure though)
You could imagine similar compounds including sulfur or nitrogen also. I expect there are known reasons why some of these wouldn't work, and most of them are more complex than water (though most are probably found prebiotically, e.g. titan). Different mixtures and pressure might make some of them possibilities.
Another area to investigate would be high-temperature chemistry, where things like silicon and silicon dioxide are liquid. Perhaps you could get complex chemistry in one of those situations. On the other hand, perhaps only small molecules would be stable at that temperature.
And, if life can't arise without water, shouldn't the existance of a universal solvent that is the only substance in existance which expands when frozen be considered an element of fine-tuning by an intelligent designer?
Who knows? Maybe. However, keep in mind that this kind of "cosmic fine-tuning" contradicts the intelligent design movement's "special creation" arguments.
If you combine the two, you get this scenario: the Intelligent Designer is going to oh-so-carefully fine-tune the physical laws and chemistry of the universe so that after the big bang, after you wait billions and billions of years, you have a universe huge enough that all of the right conditions combine to get a planet in the "just right" zone where there is lots of liquid water...and then the IDer snaps his fingers and "poofs" bacteria into existence? And then waits several billion more years to "poof" eukaryotes, then another billion to "poof" multicellular creatures (or to "poof" the modifications or whatever you like), then the major vertebrate classes, and finally at long last, a few dozen poofs later, humans?
It just doesn't make much sense. Anthropic coincidence "fine-tuning" arguments were born and raised based on a cosmological picture where evolution produced life, they become internally incoherant if you introduce divine intervention. The modern ID movement has latched on to them, I think, because their own arguments about biology have just not gone anywhere, and they hope they'll get taken a bit more seriously on the anthropic bandwagon.
Or is the nature of water dependant on the physical laws of our universe? In another universe, with different physical laws, could H20 be a less effective solvent? And maybe in another universe, it's ammonia which expands when frozen? I don't know, which is why I'm asking...
No one knows. As I understand it we can't even calculate the properties of atoms and molecules in our own universe (except hydrogen) from first principles; so why should we be able to do it in another universe?
....but if water is the only concievable solvent, regardless of varying physical laws, which could expand when frozen, then I'd have to say that the existance of water is one of the stronger arguments for fine-tuning.
As mentioned, water is not really unique here. I posted this on t.o. awhile back:
5 elements have solid forms less dense than their liquids, according
to this page:
Element Solid Density (g/cc) Liquid Density (g/cc)
Arsenic 4.70 5.22
Bismuth 9.80 10.07
Gallium 5.90 6.09
Germanium 5.32 5.60
Silicon 2.33 2.51
The page also says "a number of compound materials (like salts and
alloys) have this property."
Another page says, "Water and, whatever some books may say, many other liquids expand when they freeze."
A few other examples mentioned in newsgroups, metal alloys mostly. I
would particularly like to see the solid vs. liquid densities for
things like ammonia, low MW hydrocarbons, and other simple compounds,
particularly those that might exhibit hydrogen bonding.
And, the Earth's crust "floats" on the mantle -- although this is a somewhat different deal it shows that floating hard stuff on squishy stuff is actually quite common. See much more informed opinion in the rest of the thread.
BTW, Admin, I don't know where this thread should go.......it contains questions on both chemistry (the composition of water) and cosmology (varying physical laws). Wherever you want to place it is fine, I suppose.
The kinds of things to look out for that might actually advance your questions beyond speculation:
1. Small molecules that are liquid at reasonable temperatures, especially under heavier pressures that on Earth
2. Energy sources that might exist in cold, Titan-like environments (i.e., what if a Titan-like was orbiting a neutron star or something? Or had lots of radioactive materials in the crust? Or, speaking of Titan, are any there any good geothermal energy sources from radioactivity or tidal heating?
3. Solutions that include water but with a large percentage of something else, e.g. salts, sulfuric acid, ammonia, etc. These kinds of things might exist under "odd" regimes but support life.
Edited to format link, restore page to normal width
This message has been edited by AdminHambre, 11-04-2004 06:04 AM