life without water?

The Anthropic Principle takes another whack. [sub. req.]

As a cradle for life's basic biochemistry, water is without a doubt an amazing molecule, but not everyone believes it is unique. "Whether water is necessary for life is, I think, very dubious," says Christopher McKay, a planetary scientist at NASA Ames Research Center in Moffett Field, California. Astrobiologist Dirk Schulze-Makuch at Washington State University in Pullman agrees. He thinks that Earth life's reliance on water is coincidental. "Life on Earth learned to work with water because it's the only liquid that is really abundant. I don't think there's anything magic about it," he says. On a warmer planet, oceans of sulphuric acid might do the job, or on a cooler planet, oceans of methanol, ammonia, or even methane.

While the European and US space agencies methodically hunt for Earth-like, water-based life on Mars, evidence is emerging elsewhere to suggest that life doesn't need water. Engineers who harness enzymes in the manufacture of industrial chemicals are seeing at first-hand that these essential biological catalysts can also function in hydrocarbon fluids such as hexane, indicating water might not be as essential as we thought.

At the same time, many point out that not all of water's properties are unique. A handful of other liquids, such as hydrogen fluoride, sulphuric acid, ammonia and even hydrogen peroxide share water's ability to carry around hydrogen ions which catalyse chemical reactions that are crucial for cells to digest nutrients - and all have been proposed as liquids for life. For example, people have suggested that hydrogen peroxide-based microbes inhabit the Martian soil (see "Life on Mars after all?"), and that the clouds of Venus may harbour sulphuric acid aliens (see "Acid drinkers"). Life in non-water solvents might just be possible, at least from the standpoint of getting basic cellular machinery such as enzymes to function....

So where would be the most likely place in our solar system to look for genuinely alien biochemistries? Both McKay and Schulze-Makuch have proposed that microbes on the surface of Saturn's moon Titan might consume a gas called ethene that is produced high in the atmosphere by sunlight, and release methane as a waste product. These microbes' cells would be filled with liquid methane or ethane. Before we send a probe to Titan to test directly for life, most people think another mission will be needed to improve our understanding of the non-biological chemical reactions that happen there - so we can distinguish them from genuine signs of life. "If there's life on Titan, it's probably quite exotic, quite different from life on Earth," says Schulze-Makuch. "We would have to refine our understanding, so we know what to look for."

Suffice it to say that the non-water hypothesis is tentative, and we'll likely not see experimental results in this decade, or observational results within our lifetime. Theoretically, though, the size of the universe and the fresh list of potential requisite traits means that our current estimates of habitability need serious revision.