Why it is necessary to have water on a planet to have life

[kybeards]

Fully explain why it is necessary to have water on a planet to have life. For example why wouldn't it be possible to have life on a planet where there is liquid H2S and no H2O assuming that the lower temps to allow for liquid H2S would not be a factor???

[Paul Robbins]

We need to make presumptions about life in order to begin answering the question.

We presume that life will be in solid form with internal liquids carrying metabolic chemicals in solution. Earth based life's internal liquids are dissolved in water, which is liquid at 0 to 100 ^oC.

Hydrogen disulfide is liquid at -82 to -60 ^oC. Metabolic processes would be incredibly slow at these temperatures. Probably too slow for life to evolve. Ammonia, which some people have proposed as a viable solvent, has a better liquid range than H₂S, but it's still uncomfortably low at -77 to -33 ^oC.

Water is one of the very few substances that expands when it freezes. If it didn't, ice would sink to the bottom of the oceans and gradually the oceans will fill up with ice almost entirely. This would reduce the prospects of life evolving. Having a solvent that expands when it freezes seems to be highly desirable for life and water's out there on its own as a solvent that expands when freezing.

Water also has a very high heat capacity, so a planet with a lot of it has a much more stable temperature than it would otherwise - very helpful for the evolution of life.

That's a start....