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Can Life on Titan Thrive Without Water?New discoveries have a way of messing with old definitions. Take, for example, the concept of a habitable world.
The standard definition of a "habitable world" is a world with liquid water at its surface; the "habitable zone" around a star is defined as that Goldilocks region — not too hot, not too cold — where a watery planet or moon can exist.
And then there's Titan. Saturn's giant moon Titan lies about as far from the standard definition of habitable as one can get. The temperature at its surface hovers around 94 degrees Kelvin (minus 179 C, or minus 290 F). At that temperature, water is a rock as hard as granite.
And yet many scientists now believe life may have found a way to take hold on Titan. Water may all be frozen solid, but methane and ethane are liquids. In the past few years, instruments on NASA's Cassini spacecraft and images captured by ESA's Huygens probe have revealed an astonishing world with a complete liquid cycle, much like the hydrologic cycle on Earth, but based on methane and ethane rather than on water.
"What Cassini actually found on Titan, from 2004 onwards, was a methane-ethane cycle that very much echoes the kind of hydrologic cycle we see on the Earth," says Jonathan Lunine, currently at the University of Rome Tor Vergata while on leave from the University of Arizona. Cassini has revealed rivers and lakes of methane-ethane, the lakes evaporating to form clouds, the clouds raining hydrocarbons back down onto the surface, flowing through rivers and collecting in lakes. It is the only world in our solar system other than Earth where a liquid cycle like this takes place. There's just no water.
But there are plenty of hydrocarbons. Methane and ethane are the simplest hydrocarbon molecules. By themselves, they are of limited biological interest. But hydrocarbons are versatile: they can assemble themselves into fantastically complex structures. Indeed, complex hydrocarbons form the basis of what we call life. So one has to wonder: has hydrocarbon chemistry on Titan crossed the threshold from inanimate matter to some form of life?
One thing is for certain: if there is life on Titan, it is not life as we know it. There is no way that terrestrial life could have originated or could survive on Titan.
"DNA and RNA," says Lunine, "form out of compounds that require oxygen and phosphorus, and there's very little oxygen in the Titan system." And the very structure of DNA depends on liquid water. "DNA forms a helix because of its water-loving and water-repellant ends." So life on Titan "would have to find other molecules that carry information."
