Titan: A World of Its Own
Februrary 3, 2005
By Seth Shostak - Senior Astronomer, SETI Institute
OK, everyone anticipated that Titan was going to be interesting, but few expected it to be weirder than Michael Jackson.
Two weeks ago, as the Huygens probe parachuted through this distant moon’s oily, pumpkin skies, a less-than-consumer-grade 0.04 megapixel camera was trained on the landscape ten miles below. It saw a hostile shoreline, riven with tributaries, and what appears to be a (possibly dry) lake.
Imagine the luck: a shoreline and a lake. Try dropping a penny on a big map of your home state, and see how often it lands on a bit of shoreline topography. Not often, unless you live in Minnesota. The implication is that Titan is pockmarked with ponds; it’s Minnesota trapped in the mother of all winters. Daytime temperatures are an unpleasant -180 C (-290 F).
But the cold hasn’t immobilized this moon. Titan’s crater-free surface, the tributaries, the broken topography… indeed the whole scene tells you that something is going on here, despite the big chill. There’s geology on the move. This is not Europa’s frozen rictus, or even Mars’ gelid, dust-blown desert.
“This is a place with a combination of stability and dynamic flow,” says David Grinspoon, of the Southwest Research Institute. “It’s the kind of location where we might find life.”
Life? Now there’s a thought. For years, textbooks and talking heads have explained why a mission to Titan was worth the euros because it might reveal something about the start of life on our planet. The claim was that Titan’s atmosphere is rather similar to Earth’s early air. So maybe this hydrocarbon-coated moon could help us unravel the genesis of terrestrial biology, at least if atmospheric chemistry was an important factor.
Well, you can forget that, says Bruce Jakosky, of the University of Colorado. “The reasons we originally looked to Titan are long gone. The putative reducing atmosphere of the early Earth – one that was oxygen poor and hydrogen rich – is no longer in fashion.” Titan is its own world, and not a time capsule of our own.
That’s one reason why the existence of life on Titan would be big news. It would be life-as-we-don’t-know-it.
A Tough Environment
Fair enough. But is it there? Is there really any chance that microbial Titans are tucked away in this moon’s sticky, cruddy landscape? Most pundits are doubtful. They wave their arms at two major difficulties:
To begin with, there’s no liquid water on the surface. It’s frozen harder than a graduate math exam. That seems to be a real problem for biologists. Rocco Mancinelli, of the SETI Institute, notes that “the solvent system for earthly life is water. Sure, there might be lakes of liquid ethane and methane on Titan, but the biological compounds we know about just don’t dissolve in those.”
The other problem is the temperature, lower than a lounge lizard’s moral character. This is not a matter of comfort, but of chemistry. If you recall what was taught you in high school, you’ll know that chemical reaction rates slow down as the thermometer drops. A quick calculation using the Arrhenius equation suggests that, at –180 C, chemistry is 16 orders of magnitude more sluggish than at room temperature. In other words, even if life cooked up on Earth in five minutes, it will still be another 100 billion years before it might do so on Titan. (Mind you, this calculation is included here merely to draw the ire of chemists, who will undoubtedly e-mail me with a come-uppance. But there’s no doubt that cold will be a hindrance to metabolism.)
Of course, you might solve both problems by going underground. There’s a decent chance that ammonia-water aquifers lurk under Titan’s surface, and that could be good news for life. “An ammonia-water mixture is, to me, just as exciting as liquid water reservoirs,” says Grinspoon. But Jakosky points out that any sudsy, subterranean retreats would be far from the surface, where the food supply might be.
Food and Warmth
Which raises the point: what could tiny Titans possibly eat? The surface is choked with hydrocarbon compounds, and while you might prefer to use them for heating your house or fueling buses, they could presumably be food for the right kind of microbes. Grinspoon notes that ultraviolet light from the sun will convert some of Titan’s methane and ethane-rich upper atmosphere into acetylene. “These energy-rich, big acetylene molecules would fall to the surface and accumulate,” he suggests.
It would be kind of like having wheat fields in the sky. Acetylene manna from heaven. “And if microscopic Titans feed on this stuff, maybe they’d give off some body heat, making their own little liquid water holes.”
That’s not something you see every day: acetylene-powered life, staving off the cold with its own body heat. But if metabolism doesn’t provide enough warmth, Jakosky offers some other ideas for beating the sub-zero conditions of the surface. “After all, you have impacts from incoming space rocks that careen into Titan. The impacts will melt the landscape, keeping it locally warm for thousands of years.” That might make for episodic surface life, or maybe life that toughs it out in spore form until the next warm spell.
And then there’s the possibility that earlier in its history, Titan’s atmosphere was thicker (there’s some evidence for that, notes Jakosky). If so, maybe there was once more of a greenhouse effect – and possibly temperatures salubrious enough to sustain liquid water – and spawn life.
The bottom line is that there’s still no bottom line. The up-close-and-personal photos of Titan’s landscape show rocks and chunks of ice, and nothing that seems to be alive. But quite obviously, that doesn’t rule out Titans, either extant or extinct. If someday we find that this anti-hellish world has spawned even the simplest biology, we could claim more than just some diminutive, solar system neighbors. We would have hard proof that life is no delicate flower, no finicky accident resulting from extraordinary circumstances, but a common fact of the universe.