http://www.slate.com/id/2137875/
The Cassini spacecraft has found evidence of what may be geysers of liquid water on a moon of Saturn, project scientists said on Thursday. "If we are right," said one of the Cassini researchers, the moon "might possibly have conditions suitable for living organisms." Liquid water is generally considered one of the likely preconditions for extraterrestrial life, along with sources of heat and organic materials. But why are we so sure water is crucial for the development of life?
Every living thing on Earth needs water to survive. That doesn't mean life on other planets would necessarily be based on liquid water, but it gives us one of our best clues as to what to look for. Since water works so well for us, we may as well focus our attention on planets or moons that have it, too.
What makes water so useful? First of all, it serves as a substrate for all the chemical reactions you need to make a living thing. To get something as complicated as biology, you've got to have a system that allows a wide variety of molecules to interact in a wide variety of ways. Water, which is a polar molecule—i.e., it has both positively and negatively-charged ends—acts as a "universal solvent." That means it can dissolve many chemicals—including the organic compounds that are the building blocks of life on Earth—and allow them to recombine or attach to one another in various arrangements.
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