Tiny microbial life forms that may exist elsewhere in the solar system don't exactly scream out to be noticed by passing spacecraft. The traces they might leave behind as evidence to their existence are difficult to detect, even in the most sophisticated laboratories on Earth.
So in trying to answer questions about whether life ever existed elsewhere in the solar system, scientists have agree to search only in the areas where it is most likely to occur: where liquid water exists, or once existed.
Astrobiologists generally agree that life requires liquid water to form and to survive.
"Ice won't do it. Water vapor in the air won't do it. Somehow there has to condense at least a microlayer of water," for life to evolve and survive, said Gene McDonald a scientist at the Astrobiology Research Element at NASA's Jet Propulsion Laboratory.
The insistence on water is not so much a peculiar egocentricity or some sort of narrow-mindedness that keeps scientists from imagining fundamental alternatives to life as we know it. It's simply a limitation imposed by the laws of chemistry, McDonald said.
"If you're going to do biochemistry, you have to move things around. And it's a lot easier to move molecules around in liquid than it is in solid."
It is not likely that the complex chemistry required to form living organisms can occur when molecules are locked in a solid matrix.
Vapor poses another barrier to developing life. "If you're in the air, if you're just floating around, then it's hard to keep all of your parts together," McDonald said. "If you're a cell, you've got to keep your cell machinery together. So just on a physical basis, it makes the most sense to have life in a liquid medium."
Scientists have suggested a few other solvents that might work in biochemistry for forming living organisms. Ammonia is the most promising of these, but liquid ammonia exists well below the freezing point of water, at temperatures where molecules and chemical interactions move only very slowly.
Ammonia melts at minus 107 degrees Fahrenheit (minus 77 Celsius) and evaporates at minus 28.3 degrees Fahrenheit (minus 33.5 Celsius). If life did exist in a liquid-ammonia environment, it would face major challenges doing the simple chemistry it needs to fuel its own metabolism, McDonald said. "But it's not absolutely impossible," he added.
Organic molecules -- those in which carbon is a key component -- are the building blocks of life, but life based on other types of molecules have been proposed. Chief among them has been silicon-based molecules, but silica compounds are not as reactive as organic compounds because they are not very soluble, certainly not in water.
It is unclear what kind of solvent silica-based molecules would need. Water or ammonia wouldn't do much.
The general consensus among scientists is that the kind of life likely to be found on other planets is carbon-based life "that looks at least something like life on Earth," McDonald said.
One reason carbon is given the best chance is its versatility. "Carbon can make a lot of different compounds with a lot of different chemical properties, and it can have a lot of different functions," he said. "The same thing with water. Water has a very unique set of properties that make it a good solvent to do the kind of chemistry that you need to start life and then have life living in it."
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