WHEN the spacecraft Cassini approaches Titan next week it will be entering unexplored territory, preparing to fill in the greatest remaining blank on our map of the solar system. We know so little about Saturn’s giant moon that, just like medieval cartographers doodling beyond the fringes of the known world, we are free to endow our space-age terra incognita with imaginary continents, seas and mountains. And even outlandish creatures. For icy, smog-bound Titan might really hold life. “It is a hypothesis so alluring that one can hardly exclude it,” says astronomer Toby Owen of the Institute for Astronomy at the University of Hawaii in Honolulu, one of the Cassini team.
Titan has two special qualities that life might like. It is seething with organic chemicals, perfect material for building the molecules of life. And it has a thick, sheltering atmosphere. It is the only moon in the solar system with an atmosphere worth the name, and one of only four rocky worlds to have one, along with Earth, Venus and Mars. In some ways, Titan’s atmosphere is the most Earth-like. It is composed mostly of nitrogen and has only slightly higher pressure. It even has clouds, although they are made of methane and other hydrocarbons rather than water.
Titan’s atmosphere is thought to be similar to that of Earth shortly after our planet’s birth, some 4 billion years ago. Many astrobiologists are eager to study it as a prototype terrestrial atmosphere, hoping to find out how complex organic molecules were synthesised before life emerged here on Earth. Interesting as that is, it’s hardly as fascinating as the possibility of life growing, reproducing and squirming around on this distant moon.
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Pessimists point out that despite the atmosphere and the organic chemistry, Titan has drawbacks as a habitat. Most obvious is a surface temperature of around 95 kelvin, or about 180 °C below freezing, which sounds ferociously inhospitable. Certainly no life we know of could survive there.
But life has two possible ways out of this fix. The cowardly way is to lurk deep within the moon. Buried under many kilometres of rock-hard ice, a watery ocean probably girdles Titan. According to some computer simulations, the gentle heat of radioactive decay in the moon’s rocky core combined with a liberal lacing of antifreeze could be enough to make the ocean liquid. The ocean is thought to be roughly 15 per cent ammonia, which would keep it liquid at temperatures as low as -30 °C. It could be as much as 200 kilometres deep.
This ocean is perishingly cold and utterly dark. The ammonia makes it rather caustic, with a pH estimated at 11. Yet there are examples of life on Earth that can cope with all these conditions. Some bacteria thrive in a pH as high as 12, and in Antarctica others live in pools of salt water at -50 °C.
“Our concept of life is very narrow. We would be foolish to look for life like Earth’s on other planets”
In any case, there are probably vents at the base of Titan’s ocean where much warmer fluids seep or gush out. These sites could provide habitats somewhat similar to the well-populated black smokers on our own ocean floors, where the food chain begins with bugs that feed on chemicals spewed out by vents. There is no reason why volcanism on Titan shouldn’t provide a similar kind of bug food. Photosynthesis is not a culinary option for micro-organisms this far below the surface. Yet Dominic Fortes of University College London has calculated that the total energy this thin chemical gruel would provide could support more life than Jupiter’s moon, Europa, where we know there is a sub-surface ocean.
Life on Titan has a second option that is denied on airless Europa. It may not need to cower in some deep ammoniac shelter. Images from the Hubble Space Telescope and the Arecibo radio telescope in Puerto Rico hint that parts of Titan may be awash with liquid ethane. And there are astrobiologists who believe creatures could survive on the surface of Titan, swimming happily through the ethane seas.
It is a controversial view though. Even if the seas are made of ethane, Bruce Jakosky of the University of Colorado at Boulder and François Raulin of the University of Paris XII believe there is no chance of life existing on the surface of Titan. They argue that at such low temperatures chemical reactions proceed at a glacial rate. Ordinary organic reactions would take millions of years or more, so how could life exist, depending as it does on trillions of reactions every second?
The same question could be asked about life here on Earth. Many chemical reactions essential to life are intrinsically slow. They rely on enzymes, which grab other molecules and push them into the right configuration for rapid reaction. Enzymes can accelerate some reactions a trillion times. So could super-enzymes oil the wheels of life on Titan? “You could imagine a situation with enzymes that operate at extremely low temperatures, and speed up reactions to a point where life is possible,” Owen says. They would have to do an even better job than earthly enzymes, but according to Steven Benner, an astrobiologist at the University of Florida in Gainsville, we don’t know if there is any limit to the acceleration enzymes can generate.
“Some earthly bacteria thrive on the kind of complex organic compounds produced in Titan’s atmosphere”
Benner thinks the chill of Titan could actually benefit life. “Obviously, reactions are slower. But so are bad reactions. On Earth, biological systems are constrained in part by the fact that undesired reactions are also rapid.”
It is all very well speculating about super-enzymes in life that is already up and running, but how could life start? One possibility is that it could have begun when Titan was young and warm, perhaps when there was still liquid water at the surface, and later adapted to the cooling temperatures. But Benner thinks that life could have taken its time to start up, only emerging after the surface had cooled, helped by the fact that destructive reactions are slowed down as much as constructive ones. “In a living organism repair is possible, but for the origins of life, it is not. Hence, low temperatures are not bad,” Benner says.
What is less clear is how life might have overcome the other big obstacle presented by conditions at Titan’s surface: the absence of water. All life on Earth needs liquid water as its solvent, for organic molecules to move and react in. But is that true on alien worlds? Many astrobiologists feel that there is no point speculating about weird life that doesn’t use water because we simply do not have any evidence to go on.
But others feel that the lack of evidence is exactly why we should speculate about non-watery creatures (New Scientist, 29 November 2003, p 8). “We have to admit that the life we see around us is really just different manifestations of same thing – the same 20 amino acids, the same genetic code – so our concept of life is very narrow,” Owen says. “We would be foolish to look for life like Earth’s on other planets.”
So what might quench the thirst of Titanians, if not water? Benner thinks that ethane might act as a solvent for life. Nobody knows how such a biochemistry would work in detail. “It would have to be very exotic,” is as far as Owen will speculate.
He and Benner are not alone in considering life at the surface of Titan a possibility, albeit a slight one. Planetary scientist Chris McKay of NASA’s Ames Research Center in Moffett Field, California, who led the team that announced evidence for fossils in a Martian meteorite in 1997, has a suggestion for what these creatures might eat: hydrocarbons raining down from the sky. Chemicals such as acetylene and ethylene are created in Titan’s upper atmosphere when ultraviolet radiation from the sun breaks down methane. Microbes might be able to metabolise this manna from heaven, producing more methane in the process. Indeed, earthly bacteria have been found to thrive on the kind of complex organics produced in Titan’s atmosphere.
Vital signs
They are hardly unicorns or fire-breathing dragons, but methane-farting microbes would still be a pretty exciting find. They would also explain an oddity of Titan’s atmosphere: the presence of methane in the first place. Methane is destroyed by ultraviolet light, so the atmospheric stock should have been wiped out within about 50 million years. Something must be pumping methane into the atmosphere – and it could be bugs. Equally, it could be an underground reservoir of methane, or one of several more ordinary chemical processes. But if Cassini spotted an imbalance in the carbon isotopes in the methane, it would be a clue that the methane might have originated in living organisms.
“Cassini’s haze-penetrating radar should finally tell us whether there are hydrocarbon pools on Titan’s surface”
Biochemical processes on Earth tend to distinguish between two forms of carbon: carbon-12 and carbon-13. And even quite different forms of life would probably do the same. If the methane in Titan’s atmosphere is the discarded by-product of bugs, it should have a high ratio of carbon-12 to carbon-13. One of Cassini’s instruments, called CIRS, can pick up the slight difference in the wavelength of infrared light emitted by molecules containing the different isotopes. “If we discover that methane on Titan has a higher abundance of carbon-12 to carbon-13 than on Earth and meteorites, it would be a clue that something odd is going on there,” Owen says.
Another line of investigation for Cassini’s sensors is the atmosphere’s nitrogen. Microbes in an ocean might survive by metabolising ammonia and excreting nitrogen, in which case they might have provided the bulk of Titan’s atmosphere. And the ratio of nitrogen isotopes, this time nitrogen-14 and nitrogen-15, would be skewed.
But even if Cassini does pick up peculiar nitrogen and carbon isotopes, they will only be hints of life. “Isotope ratios would be indications, but not full evidence. There are non-biological processes that can affect the isotope ratios too,” Raulin says. Could we find a less ambiguous signature of life? “It’s going to be tough,” Owen says. “We’d have to find something that could not be made under natural conditions. First we have to understand the full suite of chemical reactions on Titan, and then maybe there will be a puzzle that only life could explain.”
Cassini’s remote sensors are not sophisticated enough to give a full picture of the atmospheric chemistry, but the craft carries a passenger that can. In December Cassini will launch Huygens, a wok-shaped probe that is set to enter Titan’s atmosphere on 15 January (see Graphic). On it is a mass spectrometer that will make a detailed chemical analysis of the Titanian air. Even then, it may be difficult to know what to look for. If there is truly exotic life on Titan, based on an unknown biochemistry, its traces might be all but impossible to interpret.
There is one sign, however, that would be almost unambiguous evidence for life. Many molecules that play a role in earthly biology come in two forms that are otherwise identical except that they are mirror images of each other, like left and right hands. Yet life makes and uses only one of the orientations. For example, all but one of our amino acids are left-handed, and sugar molecules are all right-handed. Left-handed sugars can be synthesised in a test tube, but they are indigestible. Aliens might use quite different molecules, but it seems likely that each would be of only one form. Chemists know of no natural, non-biological process that can churn out molecules of just one handedness, so discovering molecules with a particular handedness would be powerful evidence of life.
Lifting the veil
Neither Cassini nor Huygens is capable of dredging hydrocarbon tar on Titan to isolate complex molecules and test their handedness, so we will have to wait for another mission. That may be only 20 or 30 years off, Raulin says.
Before any of this happens, Cassini needs to find out whether those imagined habitats are really there. Its haze-penetrating radar should finally show us whether there are hydrocarbon pools on the surface, as scientists suspect. And by mapping Titan’s topography Cassini will show planetary scientists what kind of tectonic processes might be operating there, so they can work out whether there really is a liquid ocean under the icy crust.
In the meantime, we can dream. Mars is still widely thought to be the likeliest candidate in the solar system for life beyond Earth, and Europa probably comes second. Although microbial life on Titan is unlikely, in some ways it is the most exciting place to look because any bugs that do exist must be truly exotic little beasts.
Could there be anything more than bugs? Is there any chance of Huygens spotting tentacled monsters surfing the giant ethane waves of Titan and feeding on acetylene-eating microbes? It’s not impossible, but it is exceedingly unlikely. Even the enthusiasts think complex, multicellular life is a very long shot. “Complex life on Earth needs more energetic reactions, which are provided by oxygen. Energies at this level are not available on Titan,” McKay says, although he does point out that this opinion is based on a single data point: life on Earth.
Viewed from Titan, the picture might seem different. Earth would be scalding hot, bathed in intense radiation from the sun, and awash with toxic water. Who knows, in the Titanian edition of New Scientist someone might be speculating on the remote possibility of life on Earth. Especially if local astronomers have spotted a small metal meteoroid zooming towards their world, beaming signals back to the inner solar system.
Mysterious moons
EVEN mighty Titan doesn’t overshadow Saturn’s other icy moons. We know of more than 30 moons orbiting the planet, and even among the lesser lights there are plenty of mysteries. On its tour of Saturn, Cassini is using Titan as a hub, changing its trajectory each time it passes and aiming itself at another of Titan’s siblings.
Among those due for a visit is Enceladus, which shines such a brilliant white that astronomers believe its surface is almost pure water ice. Somehow this moon has been frosted over, and it must have happened recently or radiation would have changed the chemistry of the ice and darkened it. The best bet is that geysers or even ice volcanoes erupt on Enceladus, powered by some source of internal heat – probably friction caused by the gravitational effects of nearby Dione flexing the solid insides of Enceladus. The ice volcanoes might even be responsible for Saturn’s faint, foggy E-ring, if they are powerful enough to fling crystals out of the moon’s gravitational grip.
The mystery of Saturn’s third largest moon, Iapetus, is its two-faced nature. One side is bright and the other is a deep red, almost black. It may be that Iapetus has swept up tarry debris blasted off Phoebe, Saturn’s outermost main moon. Or perhaps the darkness came from within. Could methane have erupted from Iapetus and darkened as ultraviolet radiation converted it to more complex hydrocarbons?
Some of Saturn’s smaller moons, which range from a few kilometres to a few tens of kilometres in diameter, also show some pretty strange behaviour. Janus and Epimetheus regularly swap orbits. Others trail after their larger companions, a habit not seen anywhere else in the solar system. Yet others shepherd icy particles, helping to keep some of Saturn’s rings in place. It is possible that Cassini will find new moonlets with unforeseen quirks.
With the exception of Titan, none of Saturn’s moons is considered a plausible site for life, but few of them lack a claim to fame, even small, misshapen Hyperion. Its rotation is thought to be chaotic, scrambled by the influence of Titan’s gravity. Cassini will tell us for sure whether this really is a world where there can be no calendar.