Why does life need to have water?

[bondinthesand]

Just because our earth has water, why does that mean that life anywhere in the universe needs to have it? I no its most likely it will need it but how do we know? we are one plant of so many life could be out of carbon for all we know.

[russ_watters]

It doesn't necessarily require water - perhaps it could be based on liquid methane. But water has some unique properties: it is an extremely good solvent, exists on earth in all 3 states, and is lighter when solid than when liquid.

[MATLABdude]

Ammonia might also be an alternative, for similar reasons (good solvent, low molecular weight, yet high-ish boiling point owing to hydrogen bonding) except that it'd need to be for low-temperature systems.

Wikipedia's article on speculative biochemistries (the first crystal Stegosaurus image comes out of a book whose title I can't recall, but is probably written / illustrated by the author of the originating website. Really neat coffee-table style book on what alien life might look like):
http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry

[joelupchurch]

I not so sure that the mechanisms for spontaneous creation of silicon based lifeforms are plausible, but we are already creating artificial silicon based lifeforms called computers.

[Bob S]

We actually have two basic life forms:
1) plants that take in water and CO2, and with sunlight create saccharides (sugars etc) and O2;
2) Animals that consume O2 and saccharides, and dissipate energy.

These two life forms exist (mostly) in equilibrium, and work on the principle of converting sunlight into stored chemical energy, and then using that chemical energy.

Sunlight has to have enough photon energy to electrolize (break the bond(s) of) water, and at the same time have so little photon energy (meaning UV radiation) that it does not damage the biological systems it helps create. (Atmosphere helps filter it).

So in all cases, would life elsewhere need to have two symbiotiic forms; one for absorbing "sunlight" and storing concentrated energy, and another form for consuming that energy? Or would life elsewhere be like Mars rovers, have photocell-like panels that convert "sunlight" to electrical energy, store, and use it?

[Phrak]

So in all cases, would life elsewhere need to have two symbiotiic forms; one for absorbing "sunlight" and storing concentrated energy, and another form for consuming that energy? Or would life elsewhere be like Mars rovers, have photocell-like panels that convert "sunlight" to electrical energy, store, and use it?

With rules come exceptions like cave dwelling bacteria that eat rocks.