Sure, I think it’s quite possible. As a premise, let’s grant that an evolved life form would need a fluid–water or gas–to be biologically viable. A rock monster or what have you may also be possible, but let’s consider “juicy creatures” first, the only life we have on Earth.
All life forms (plants, animals, and protazoa) on Earth use water. (Not all require oxygen for respiration.) But here are some other possibilities:
Hydrogen fluoride (HF). It’s possible that there are planets with HF oceans. Cold environment required.
Ammonia (NH[sub]3[/sub]) on a cold planet or moon.
Methane (CH[sub]4[/sub]) on a cold planet or moon.
Ethane (C[sub]2[/sub]H[sub]6[/sub]). There may be liquid ethane bodies on Titan, the largest moon of Saturn. It’s cold there.
Any of these liquids has some nice goodies in it (carbon, nitrogen, and hydrogen) that could help life along.
Not allowing any water is only marginally more restrictive than not allowing liquid water. The biochemical purpose of water is as a solvent – the medium in which all structural and catalytic components of an organism are dissolved and in which all reactions take place. Since water vapor and ice can’t act as solvents, allowing them (but not liquid water) doesn’t make a difference. Life (as we understand it) is just as impossible in a world of ice as it is in a world with no water at all.
Even more so than water, I think life is impossible without carbon. Science fiction authors may play around with silicon-based and even more exotic organisms, but there are many reasons why I think life must be carbon-based. First, the chemistry of carbon-containing molecules (i.e. organic chemistry) is vastly more complex and varied than the chemistry of any other element. The very unique ability of carbon to form long chains with itself (concatenation) is one of the main reasons for this. Other elements can concatenate, but in a much more limited way than carbon. Even silicon, because of its larger atomic radius, has a hard time forming long chains with itself. Further, carbon reacts readily with a variety of other elements (H, O, N, F, Cl, etc.) that permit functionalization of carbon-based molecules. This allows them to have a much more diverse chemistry that is conducive to life. This ability is determined by the energies of the C-C bond and the bonds of carbon with other elements as well as other properties. These properties are set by physical laws, which are (to our knowledge) constant throughout the universe.
If you’ll permit me to assume that all life must be carbon-based, then we can work with the question of whether water is the only possible solvent. Certainly, laboratory organic chemistry can easily be done in other solvents. The polarity of water and its ability to support acid-base reactions better than other potential solvents make it a very good choice, but I can’t be sure it’s the only choice. I think life might be possible in other, less polar solvents. The most likely possibilities would probably be polar aprotic solvents such as dimethyl sulfoxide. Perhaps even non-polar solvents such as methane or ethane could support a very exotic biological chemistry. As for heavier solvents such as hexane or benzene, or even something like acetic acid, I can’t be certain that such a liquid could exist in large enough quantities to act as a solvent on a planet-wide scale.
Anyway, I don’t think life without water is absolutely unimaginable. Life without a solvent certainly is.
Aeschines: I think HF and NH[sub]3[/sub] would be poor choices (erm, unlikely to support the development of life). HF is strongly acidic and reactive with many classes of carbon compounds, which would severely limit the range of chemistry possible. That is, you could have acid-catalyzed reactions, but not base-catalyzed ones. Also, a lot of types of organic molecules just wouldn’t develop because of the extensive halogenation that would occur. Conversely, ammonia would permit base-catalyzed reactions but disallow acid-catalyzed ones.
Methane and ethane are probably viable. Life on Earth has been very good at taking advantage of aqueous organic chemistry. Life on a methane world would probably be able to adapt to wholly non-polar chemistry.
I think DMSO is a really intriguing possibility. The polarity would allow a wide range of solutes, and the solvent could even participate in some reactions (though not as well as water can). Reduction of DMSO to dimethyl sulfide might even be used as a biological energy source provided the environment was right for it. I’m not sure how likely a world with DMSO oceans would be, though.
In the collection The Left Hand of the Electron, Isaac Asimov included an essay title “The Thalassogens”, a term he coined for life-supporting liquids. In it, he discusses the subject at length, including postulating oceans of methane and ammonia and whatnot. Among other problems, the liquid range of temperature of most of these is too small. For example, at a pressure of one atmosphere, ammonia boils at -33 degrees C, and melts at -77 degress C, giving it a liquid “range” of only 44 degrees. Merthane’s liquid range is even smaller (at one atmosphere) at about 21 degrees. Speculating on life existing in either of these requires a much steadier temperature than can be found on Earth. Water is more tolerable to temperature changes, staying liquid across a 100-degree range.
In all, I expect life-forms that evolved in an ammonia or methane environment would be far too vulnerable to temperature variation. One bad “season” where the temperature dipped 20 or so degrees would force mass extinctions. On Earth, we just put on coats.
dropzone: The argument goes something like ‘the number of possible organic compounds is unlimited because you can always add something’. If you can make X, you can make X-CH[sub]3[/sub], and if you can make that, you can make X-CH[sub]2[/sub]CH[sub]3[/sub], and if you can make that, you can make X-CH[sub]2[/sub]CH[sub]2[/sub]CH[sub]3[/sub], and so on. Or X-CH[sub]2[/sub]CH[sub]2[/sub]OH, or anything else you might imagine. So there is no ‘most complex possible organic compound’, and the size and diversity of organic chemicals is limitless.
Aha! the chap who posed this question did not appear to specify that the life concerned evolved spontaneously; I can imagine a water based carbon intelligent lifeform designing silicon and metal explorer robots which have the capacity to self replicate and adapt to their environment; send enough of those out and ater a few million years they could be present on every solid object in the Galaxy.
Now if that could happen why could the same thing not conceivably happen spontaneously?
Spontaneously evolving microprocessors?
Given enough montmorillionite and enough time you never know…
For those interested in this topic: Harvard Neurologist Athena Andreadis wrote a popular science book titled To Seek Out New Life which discusses at length the ramifications of waterless environments and non-carbon-based molecules on the creation of life.
Central to Andreadis’ theme are the same points that Roches and Bryan Eckers made above.
Would freezing the liquid methane or ammonia for a while kill the creature or just slow/stop its processes until conditions improved? Neither substance expands when it freezes so you needn’t worry about cell walls (if it has them) breaking.
People, people, I’m disappointed that some of you are not getting into the spirit of this. You are taking the writings of a neurologist at face value? And a biochemist, even one whose sideline was imagining alternatives, is going to have blind spots and preconceptions imposed by his training about what is possible. You’re as smart or smarter than they–let your imaginations soar! (Within the bounds of the exercise, of course.)
I only mentioned the book as a suggestion for those interested in the subject. I wasn’t using mention of the book as an argument. I’m not really debating here.
Don’t knock the book until you’ve tried it. While Dr. Andreadis is fan of the various Star Trek shows as entertainment, she repeatedly and tenaciously eviscerated the shows’ writers when they employ poor science (which is often).
“You’re as smart or smarter than they”? I don’t make that claim for myself. I can discuss this matter only at a very broad layman’s level. I, for one, can’t come up with the boiling and freezing points for various chemicals off the top of my head.
I think a number of relatively inert polar solvents might fit the bill, as Roches brilliantly outlined above. The thing is, chemical reactions depend on basically two things: molecular motion, and proper configuration. I can’t see much going on in the solid phase. In addition, I have a hard time picturing anything like a cell membrane developing in the gaseous phase. What would cause it to self-assemble, and what would keep it cohesive and support it against collapse? Maybe I lack imagination, but it’s hard for me to conceive of “life” without some kind of compartmentalization, where something separated the substance of the living thing from its environment sufficiently to make it an identifiably discreet body of some sort. I’ve seen some whimsical pictures of imaginary aliens that floated in the atomosphere of a gas giant, either on wings or suspended from gas bladders of some sort. I could see something like these evolving from an ocean-based life form, but I just can’t see how you get cells outside of a liquid environment. Sure, you can blow soap bubbles into the air, but only from a little tub of it in solution. Liposomes (or some analog) purely derived from and suspended in a gas? Seems pretty much impossible to me.
I agree with the bias towards looking for life where there’s water. We’ve got pretty good evidence here on earth that life can arise spontaneously in a watery environment, and, even more exciting, there are or were other watery environments in the solar system. I’d put my money on the watery spots first.
An interesting question might be: Would we be able to recognize something as “alive” if it didn’t fit some sort of cellular paradigm? I mean, energy beings or smart dust-devils or whatever? They might be too alien.
I can’t personally imagine a first generation lifeform (e.g. ex nihilo evolved lifeform) that doesn’t require water, however I can certainly imagine a second generation (or beyond) lifeform, artificially created by the first generation lifeform that does not require water (and maybe not even matter).
I did give some consideration to the ‘spontaneous evolution’ rule. Allowing an existing intelligent lifeform to design some organism that doesn’t require water significantly increases the likelihood of it happening, because you eliminate the variable of the probability of abiogenesis occurring on this waterless planet. I think water tends to favor abiogenesis – for example, oceans of water can support geothermal vents which give rise to a diverse chemistry at relatively high temperature.
Bordelond, speaking of boiling points, I suppose the boiling point of a potential solvent is a major factor. A high-boiling solvent will lead to reactions happening much more quickly, which makes abiogenesis seem more likely.
One other thing I just thought of is this – does this planet have no water, or are small amounts present? (A hypothetical planet will probably have some liquid water if its average temperature is within 0-100°C.) Without water, a lot of possible organic molecules (alcohols, carboxylic acids, ketones, etc.) probably won’t form, which makes life far harder to imagine.
dropzone: I admit my training (in biological/organic chemistry) is going to lead to some preconceptions. I really can’t imagine organic molecules that are entirely different than the ones we’ve already prepared. So I think what I’m considering here is more like “Would something similar to life as we know it be possible on a planet without water?” rather than “Is life possible without water?” I’m also working with the basic assumption that ‘life’ must consist of something analogous to life on Earth, with these very basic components:
chemical components (mostly catalytic proteins/enzymes)
data storage/genetic components (nucleic acids)
I suppose it’s possible that something could be considered ‘alive’ that used an entirely different paradigm of life. I imagine extraterrestrial life as being very similar to life on Earth mostly because I don’t think extraterrestrial chemistry is different than chemistry on Earth. But terrestrial chemistry has shown us some possibilities which could potentially, somehow, lead to ‘living’ things. For example, in a world where silicon or organic or some other semiconductors formed spontaneously, you might eventually get ‘energy beings’ that were essentially living computers. But I agree that planets with water are almost certainly the best place to look.
Yeah, it’s a given that planets with liquid water are the best bets and I don’t want my hard-earned tax dollars chasing the remote possibility of oddball lifeforms when we haven’t found any more “normal” ones off off Earth, but this thread is is as much a Pitting of that guy I heard on Nova as it is a chance for youse guys to brainstorm. “We can’t imagine life without liquid water,” eh? Well, I know people who CAN! And some who can’t.
Along the same lines as the writings by Andreadis and Asimov, chapter 17 Worlds Without End by planetary scientist John S. Lewis speculates about alternative biochemistries. As above, ammonia, liquid carbon monoxide, and various sulfur and hydrogen compounds are considered, and all are found to be inferior to liquid water for fulfilling the need for a solvent.
So we have to have a solvent, and it pretty much has to be a liquid. Water is far and away the best alternative. If we reject it, we’re left with the second-best alternative, sulfur dioxide. (Actually, the second best is ammonia, but it will almost invariably be found with water.) Lewis proposes that liquid sulfur dioxide may plausibly be found beneath the surface of Io, for example.
He follows the same reasoning to conclude that carbon is the most preferable molecular building material, superior to sulfur, iodine, chlorine, and so on. Silicon, of course, is widely considered second best after carbon, and Lewis’s analysis corroborates this. The OP doesn’t demand that we exclude carbon, though.
So if we keep carbon, but reject water, we’re talking about a limited-biosphere life form in an extremely alien environment, such as a hypothetical low-temperature sea under the surface of a volcanic world like Io. The substance of the sea would normally be frozen, but is kept in liquid form by high gravity and tidal action.