Why do we search for extra-terrestrial water?

Question

When searching for signs of extra-terrestrial life, the key primary challenge as of now seems to be searching for signs of water on other planets. Presumably, this is because on Earth, water is necessary for all life.

However, what if we discovered organisms on another planet which exhibited respiration, reproduction, or even social interaction, in a similar way life on Earth does, but which has evolved without the need for water? We would still consider this a form of "life", and certainly it would be a worthy find.

It seems perfectly possible that other substances besides water could come together to create something that we would perceive as "life". This could either be through other substances which contain Hydrogen and Oxyen, or because there are other elements (including elements not yet discovered) which could combine in a way that appears to create "life", just from a different direction.

"Life", in a broad sense -- and certainly in the sense that extra-terrestrial scientists would be interested in -- is just a series of chemical reactions which happen to have allowed for reproduction / respiration etc.; but this has no relation to the existence of water. In fact, given the vastness of the universe, the vastness of the number of potential new chemicals and new chemical reactions out there, and the relative insignificance of Earth and its beloved water amongst all of this, it seems unlikely that if there is extra-terrestrial life, it would be water-based.

So why, then, do we focus so much of our efforts on searching for extra-terrestrial water? Is it because it's a good starting point and we don't know what else to look for? Or is it because I am wrong in thinking that other forms of "life" could have evolved without water?

(This question relates only to the search for extra-terrestrial life, not for other uses of extra-terrestrial water, such as colonisation.)

Answer 1

Virtually all chemical reactions take place in a liquid or gaseous phase. Uncontained gasses aren't a likely candidate for life, so we're looking for solvents for life-related reactions to take place in.

A good solvent is (a) common, (b) dissolves a wide variety of substances, and (c) is liquid at a wide range of temperatures. Very few liquids meet even two of these criteria, much less all three. The likely solvents for life are water and ammonia, and maybe methane.

Answer 2

Life depended on water is what we know. Until we find life not dependent on water we don't know what to look for. We have to act within our experience.

Cosmic exploration is expensive and the risks are high. To minimize the chances of failure in looking for life it's less risky to look for "life as we know it".

Answer 3

The other two answers give one basic, primary reason: If we're looking for signs of life, water is assumed to be an essential building block and a good hint the place has any chance to host life.

Then there are two secondary concerns:

• if we ever plan manned exploration, water will be one of the scarce, expensive resources needed to sustain human life. Lifting it off Earth is very costly. Any bases/stations/etc would be much easier to sustain with local water source.

• water is hydrogen and oxygen, and that means rocket fuel. In more distant perspective, water-rich destinations are possible locations where the craft could refuel. We're still pretty far from that point, but having a good list of the places already won't hurt.