Why does it have to be liquid?
In a gas, molecules fly apart and barely interact; in a solid, they do not move at all. A liquid offers a middle ground: particles can move, yet are packed closely enough to meet and form complex compounds.
That is why the search for life focuses on liquids. The question is not "Is there water there?" but "Is there anything liquid for long enough?" Only then do scientists look at what that substance is.
- Water: a broad range, dissolves many things
- Ammonia: liquid at lower temperatures
- Methane: liquid only in extreme cold
The band shows water's main advantage - the broad temperature range in which it remains liquid. Other candidates have a noticeably narrower range, so the conditions have to line up more precisely.
What's wrong with the alternatives
Dissolves a great many substances, stays liquid across a broad temperature range, and forms ice that is lighter than water, so it floats instead of freezing a body of water solid
Dissolve a different set of substances, stay liquid across a narrow temperature range, and support reactions that proceed extremely slowly at such low temperatures
Ice deserves a separate explanation. Solid water is lighter than liquid water, so ice floats and forms a lid over a body of water, preserving life beneath it. For almost all other substances, the solid phase is heavier and sinks, freezing everything down to the bottom.
The main objection to cold solvents is speed. At minus 180 degrees, chemistry proceeds so slowly that the age of the universe may not be long enough for anything complex to arise.
Even so, scientists study such places seriously. One of Saturn's moons has lakes of liquid methane, making it the only known body besides Earth where liquid can persist on the surface.
Scientists add one careful caveat. We look for life resembling the life we know because we do not know how to search for the unknown: it is unclear what signs would identify it. That is a limitation of the method, not a conclusion about nature.
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Sources
- Liquid ammonia and hydrocarbons are considered alternative solvents for life
- limitations include a narrow temperature range for the liquid state and extremely low rates of chemical reactions at the corresponding temperatures