As far as is known, water is required by all living things on Earth and life cannot survive without it. NASA used to have the adage "to find life, follow the water" and until we have another example of life which operates independently of water then water will continue to be an intrinsic requirement for life. This is not to say it is the only requirement and there may be life in the universe which does not require water, we just haven't found it yet.

We know that water is a very effective chemical solvent, fundamental in dissolving, mixing, and transporting several chemical elements and molecules which build up terrestrial life forms, on several length scales (from cellular to oceanic). Depending on the length scale you are considering, your question can split in a biological question or in a geophysical one.

From the geophysical point of view, surface liquid water can gather a lot of chemical elements from surface minerals and atmospheric gases, and guarantee the molecular mobility needed to form the more complex compounds. In principle, another liquid could have the same role, as far as the different solvent effectiveness in different thermodynamical environment is concerned. However, there could be in any case enough compounds to be dissolved, transported and gathered to build up complex molecules. The only extraterrestrial body known to host surface liquids is Titan, the largest satellite of Saturn: several extended lakes have been unveiled by the Cassini spacecraft few years ago. But their are not made of water (surface temperature is of the order of 90K with a surface pressure of about 1.4bar), but of a mixture of methane/ethane. I don't know if liquid methane could be, in that environment, a chemical solvent as effective as water on Earth, but in any case the solid surface's bulk composition is very different from the Earth's one (dominated by water ice instead of rocks). This poorer compositional variability could reduce the need of a very effective solvent on the surface. In this case, atmospheric (heterogeneous) chemistry may be invoked to form larger, organics molecules, which are actually known to form at high altitude and then settle on the surface overall the satellite. The surface liquid in this case could only be essential in gathering the organic compounds (settled directly on it, or transported by rivers) and let them interact each others for enough time. Therefore we may already know a place where the (bio)geophysical role of water could be replaced by another molecule.

The biological aspect of the question is still very hard to assess with our terrestrial-biased knowledge. Is a cellular biochemistry (even based on carbon) but (almost) without water conceivable? Very open question...

If you are searching for life anywhere search the water in a very preliminary step.

Life as we know it yes, as water is quite unique (see article below from NOVA), but we can of course think outside that box. Alcohols are a fairly good class of solvents, as is liquid methane or ethane, as are acetic acid and acetone(all miscible in water) or even liquid ammonia which seem to be readily available in the cosmos. However, most of these also have a very small liquid range (without additives), and thus a creature with say ammonia in its system, could walk outside in 10-degree warmer weather and its "blood" would literally boil, or even just by over exerting itself! Water in contrast stays liquid over almost 100 degrees (C), depending on pressure. [Where I live water boils at 94 C. :-) ] But this also presupposes life based on organic chemistry. What about Boron or Silicon, both of these elements can support very complex chemistry with hydrogen or halogens, but my guess is Earth would be very hostile to these beings. (Borane for example is very flammable!) But under the right conditions who knows what is truly possible for life, and given the fact that we have discovered life like Archea growing deep in the Earth and in other "unlivable conditions" we have know idea what extremes Earth life can adapt too let alone alien life!

http://oceanservice.noaa.gov/facts/extremophile.html

http://science.howstuffworks.com/life/cellular-microscopic/extremophile.htm

Nice article related to water and life...

http://www.pbs.org/wgbh/nova/evolution/liquid-of-life.html

The answers so far make good points- thank you. I suppose you could extend the liquid range of other solvents under high pressure so we need to keep an open mind. I am not sure about Silicon life given the few different complexes it makes compared to carbon complexes and the difficulty of making silicon double bounds that seem so key to carbon-based life. Boron is an interested idea as it can make lots of different complexes and bonds, but I am not sure if it is as plentiful as is carbon and this may be an issue.

In the literature suggestions have included methane and other hydrocarbons along with formamide, ammonia, sulfuric acid plus for low temperatures liquid nitrogen or liquid hydrogen. Ammonia is a good solvent and would be liquid at a range of temperatures at high pressures but it does not have strong hydrogen bonds compared to water. Hydrogen sulfide would seem to be a reasonable water chemical analogue, but I’m not sure its solvent properties are comparable. I would value chemical and biochemical views on how versatile these or other possibilities might be compared to water

Water is abundant and ubiquitous. Even Mercury

and our moon possess water deposits. And there

are more places where it could exist in liquid form

than the traditionally habitable zone: it isn't necessary

that a star provides the heat. Geothermal energy,

radioactive decay, gravitational kneading, even

dark matter could keep water liquid.

Of course there might be aliens based on

silicon, like robots. If I had to guess how alien

life will be discovered: probably by seasonal

changes of an exoplanet's atmosphere

caused by vegetation.

Regards,

Joachim

The fact that water is very abundant does not imply anything about its necessity for life. It could only be relevant in a statistical view, which is not so significant since we know only the case of terrestrial life. Maybe, if you imagine an hypothetic case in which we discover life on many planets, then most of them could need water simply because aqueous environments are more abundant and water availability greater. But we can say "most" of them, not "all".

My answer is no!

We only know life with some nucleic acids (DNA or RNA), proteins, water to be necessary to store and divide reproductive information, i.e. genes with information and cells or similar as vehicles or slaves for the information. That may need some water for all life we currently know. But there could be other systems - just very different from what we know or currently can imagine, so that water could be totally negligeable or even toxic to the reproductive information system and its carrier.

There are so many different forms of life on earth, which are hard to understand, which developed outside oxygen containing water, without light as energy source, but chemicals, so in a very abstract sense many other things are thinkable.

And with the prions, who could imagine 40 years ago, that there could be some "reproductive" information stored outside nucleic acids, in infectious proteins. I know, such proteins are far away from being considered as forms of "life" - without metabolism.

Do we know of any organism on earth that can live without water?

I admit the question is leading, because water existed prior to life on earth so life most likely adapted to involve the abundance of water.

Earth-like life needs water. Whether you can have life using other solvents requires far more information than we have right now. And until we actually find non-water based life there is no way you can give a definite 'yes', even if we can formulate something in the lab, so astrobiological missions to places such as the hydrocarbon lakes on Titan are essential.

Personally I'm trying to work out what compounds hydrocarbon-based life could use as cell membranes, which could be useful in finding any life in the hydrocarbon lakes on Titan. Hopefully I'll publish some results early next year, but if anyone is interested I have a review article which presents my idea for a type of reverse liposome that may be used by hydrocarbon-based biota, as well as an overview of the possibilities of water-based life on Titan.

http://onlinelibrary.wiley.com/doi/10.1111/j.1468-4004.2011.52139.x/pdf

Dear John,

Water is not necessary either in the atmosphere or at the celestial object surface. As for the underground water, the direct necessity in liquid water is absent for DNA formation under the condition of the occurrence of methane hydrate, niter, and phosphate. However, by many reasons, liquid water can't be absent in the volume where living matter exists. To understand my answer in detail, please, read one of our review papers that are devoted to the problems of living matter origination and development. A number of such our review papers are available at the ResearchGate site at my and Elena Kadyshevich pages and in a number of international scientific journals.

Sincerely.

Another thing that is common to all life that we know of is compartments. Something to separate the organisms from the external environment. It could be something akin to a cell membrane or even a viral protein coat. So in thinking about what liquids could support life, it might be helpful to think of liquids that could support the formation of membranes or other means of molecular compartmentalization. Especially membranes that can form spontaneously. No doubt life in water has advantages because simple lipids spontaneously form membranes and vesicles. These are helpful for fighting entropy. Cheers.

Dear Paul,

Sorry, your consideration is out of the content of the question that is put by John. Living matter can exist under ground and it exists there. I address you to any our review which relates to the problems of living matter origination and development. They are available at ResearchGate and contain consideration of the problem in its entire its volume.

Regards.

Dear Victor,

Sorry, but I fail to see how my previous comment was "out of the content" of the question at hand. The question is whether water is necessary for life or whether other liquids might be suitable. I merely suggested that one property any life-supporting liquid would probably have is that it should support the formation, preferably the spontaneous self-assembly, of some kind of membrane or other nanosheet that can act as a semipermeable boundary. If a liquid other than water is found that supports complex mixtures and the spontaneous self-assembly of membrane-like structures, there seems to be the possibility of it supporting some form of life. All life that we know of is encapsulated in some way, in order to maintain disequilibrium with the surrounding environment. If you have a theory of life that does not require some form of semipermeable boundary I would enjoy learning of it. With kind regards.

Dear Paul,

Sorry, you are young and there are no frames in scientific discussions for you. It is a rule for me, who is an old grumbler, and for the persons similar to me, to be too verbose, but this doesn't relate to you.

Come to the point! You are not the first to think about this. In Russia, in the Karpov Institute of Physical Chemistry, Prof. A. Shatenshtein studied different reactions in liquid phase in the NH3 presence. One of his explanations of such a direction of his works was just the statement that NH3 could be the medium for the life development in other worlds. I think that he was not unique in this idea.

As for me, I think that fantasies are necessary and productive in technique and in engineering (if they do not contradict the thermodynamics) but are not productive and even dangerous in the studies of the origins. When studying at school, I laughed up my sleeve when listening to the teachers speaking about Oparin's ideas on life origination and Eddington's ideas on the causes of the solar activity. And, only half a century later, after studying different sciences and performing scientific works in different fields, I ventured to propose new hypotheses for these Origins (I advice you, at least, to look through our (together with E. Kadyshevich) review works on these subjects at ResearchGate).

Best wishes.

Remember that 60% of your body is made of water. Only dead species can be dry.