While bike riding on one of my usual routes, I stopped at one of the many ponds that dot the Lake County Forest Preserve system. In the water there was an abundance of small fish, and on the surface were several ducks and an occasional turtle. Dragonflies raced back and forth above the surface and now and then a frog's croaking could be heard in the rushes lining the pond. So much life. That got me thinking about the importance of water to life on Earth. As we know, life sprang from the sea and it so happens that the seawater of long ago had the same level of salinity as human blood. This could be what led oceanographer John Delaney to proclaim: "An ocean is the womb of a planet".
Scientists are now scanning the universe looking for the chemical signatures of life - water vapor, carbon dioxide, ozone, and methane. We find it a mental stretch to imagine life forms on other planets that are not tied to liquid water. But the water some planetologists think might exist elsewhere is either in the form of ice or vapor, or lies well below other surface material.
Philip Ball, author of the book, Life's Matrix: A Biography of Water, observed that liquid water is essential for the kind of delicate chemistry that makes life possible. Certainly, there are liquids other than water, and some molecular processes required for life can be duplicated in non-aqueous solvents. But many of the molecular reactions required by even simple organisms are facilitated by water in a very unique way.
Thinking out of the box, is any liquid really needed? Mobility seems to dictate life forms based on some sort of liquid - for lubrication or as a solvent. Mobility is an implicit requirement for technological achievement, which serves as a mark of intelligence. I suppose in theory there could be another intellect out there that is as mobile as a rock! However, it also implies that a being who lacks mobility isn't likely to have a recognizable technology, so there would be no signals for SETI to pick up. That wouldn't fit the definition of "life", as we understand it...organic life, that is.
Laws of probability say there's life elsewhere. But everything we now know suggests that liquid water is critical for any life. Small wonder that naturalist Loren Eiseley wrote: "If there is magic on this planet, it is contained in water." And small wonder that a mantra for many space missions is "FIND WATER!"
Funny how ponds make you think.