If we want to find life elsewhere in the solar system that has arisen independently of life on Earth, where should we look? According to Jonathan Lunine, at the University of Arizona in Tucson, the somewhat surprising answer is Saturn’s giant moon, Titan.

That’s not quite as crazy as it sounds. Lunine’s argument is based on the idea that the important question for humanity is not whether there is life elsewhere in the solar system but whether there is life elsewhere in the universe. And that is an important difference.

We know that life has arisen once in the solar system, but unfortunately this tells us nothing about its ubiquity elsewhere. Life on Earth may be a one-off.

But were we to find evidence that life had independently arisen twice in the solar system, it would provide strong evidence that the universe is teeming with ET creepy crawlies. The key word here is “independently.”

For many astrobiologist, the most obvious place to look is wherever there is liquid water. Top of their list is Mars, which once had a warmer and wetter climate and may still be damp in some places beneath its crust.

The trouble with Mars, says Lunine, is cross-contamination. Any life on Mars may well have been sent there from Earth by large meteor impacts (and vice versa).

Then there is Europa, Jupiter’s moon, which hides a huge salty ocean some 100 kilometers beneath its icy crust. Europa’s oceans hold about twice as much water as Earth’s. Cross-contamination is less of a concern with Europa (although it cannot be ruled out entirely) but the real problem is penetrating its thick icy surface. If Europa’s oceans contain life, it’s going to be mighty difficult to reach it, although NASA has recently grasped the nettle by announcing a major new mission to the moon.

Which leaves Titan, an object with a nitrogen-based atmosphere about four times as dense as Earth’s. Titan’s allure is based on its extraordinary climate. At -176 degrees centigrade, the water on Titan is safely locked up as ice. But on the surface, ethane and methane play the role that liquid water plays on Earth. There are ethane oceans and methane rain and cloud.

Could life have arisen in these conditions? If it has, Lunine suggests, it would look very different to our own, perhaps based on hydrogen bonds rather than the covalent bonds that hold life on Earth together. And if so, then there can be no question of contamination.

What’s more, Lunine argues that Titan-like climates may be much more common in the Milky Way than Earth-like ones, because the most common stars are smaller and cooler than the Sun.

So Titan-like life would be much more widespread than Earth-like life.

With its dense atmosphere and low gravity, Titan ought to be much easier to explore than Europa.

That’s an interesting argument but it raises some questions, not the least of which is how to make an unambiguous detection of such a life-form. That would be hard, given our ignorance of every aspect of its existence.

One idea that Lunine ignores is panspermia–the idea that life on Earth was seeded from space. We know that some interstellar clouds are riddled with ice and organic molecules. We also know from lab experiments that, when zapped with UV light, these organic molecules can form cell-like vesicles, even at the freezing temperatures of interstellar space.

Could the solar system have passed through one of these clouds soon after it formed? Possibly, and if so, the problem of contamination may be much bigger than Lunine suspects.

Ref: arxiv.org/abs/0908.0762: Saturn’s Titan: A Strict Test for Life’s Cosmic Ubiquity