For centuries, stargazers have dreamed of finding Earth-like worlds elsewhere in the universe. The Italian monk Giordano Bruno was burned at the stake in 1600 for pushing the idea; Galileo was convinced that alien beings lived on Jupiter; and in the late 1700s, Sir William Herschel, who discovered Uranus, wrote of his certainty that the moon must be inhabited. At the end of the 19th century, canals on Mars were the rage, and well into the 20th, serious scientists imagined swampsand rain forests on Venus.
None of these lush idylls exist, but the dream of life on other worlds lives on. That helps explain why Titan, Saturn’s largest and most exotic moon, has planetary scientists so excited. Over the past two years, a pair of intrepid probes—Cassini, which has orbited Saturn since July 2004, and that satellite’s envoy, Huygens, which plummeted onto Titan’s surface in January 2005—have discovered features that are uncannily reminiscent of Earth. Titan, a world bigger than Mercury, has mountains, volcanoes, dunes, riverbeds, and rainstorms. To cap it all,scientists announced in January that Cassini’s powerful imaging radar had uncovered a system of more than 75 lakes, some up to 40 miles across, dotting the north polar region.
There’s one big difference. At almost one billion miles from the Sun, Titan has a surface temperature that hovers around minus 290degrees Fahrenheit. As a result, all those familiar-looking features are constructed from decidedly unfamiliar materials. The mountains are made primarily of rock-hard water ice; the dunes are most likely ice granules coated with hydrocarbons; volcanoes probably belch methane and ammonia, and methane fills the lakes, evaporates to form clouds, and rains back down to carve out river channels.
Yet deeper similarities also exist. Titan’s chemistry, replete with organic molecules, may be a facsimile of the early Earth’s, but stuck in a deep freeze. Life as we know it couldn’t exist on Titan—but if most of life’s needs exist there, they might exist on balmier worlds around other stars. Each new Titan discovery seems to bolster the old faith that the universe is full of worlds that mirror our own. “It’s extremely exciting, extremely surprising,” says geologist and Cassini researcher Ellen Stofan, of University College London. “It’s been so much fun, like Christmas morning.”
Titan’s central mystery has long been its methane-rich atmosphere. Incoming solar radiation should have shattered these buoyant methane molecules long ago. The fact that thick haze still blankets Titan suggests that the methane is replenished constantly by evaporation from some liquid reservoir. Indeed, the controllers of the Huygens probe were prepared for a splash when it parachuted down near Titan’s equator. Instead it landed with a thump. Huygens did spot what looked like a lake bed near its landing site, with apparent feeder channels—riverbeds, in essence—snaking into it. But they were bone-dry. Maybe, scientists postulated, methane doesn’t stay on the surface most of the time, but only seeps out periodically from a subsurface source.
Then Stofan found those northern lakes, which suggest that Titan is the only other place in the known universe with Earth-like bodies of liquid on its surface. “Titan turns out to be a very complicated place,” says Cassini team member Jonathan Lunine, of the University of Arizona’s Lunar and Planetary Laboratory. “Trying to understand the whole surface by going to just one place is as misleading as trying to do the same thing on Earth.”
Close-up exploration of other locations on Titan’s surface isn’t anoption. The now-frozen Huygens probe can go no further; as expected,its power supply ran out less than two hours after it landed. ButCassini continues to aim its instruments at Titan as it periodicallyglides past it. Because the probe sweeps very close to the moon, itsradar can illuminate only one narrow strip of Titan at a time. “It’s abit like uncovering a mummy,” Lunine says. Not until last July didCassini fly over and scan Titan’s arctic region—a place especiallyintriguing to scientists because it is colder than the equator andtherefore is a region where methane is more likely to condense and raindown to the ground.
Sure enough, Cassini’s radar showed 75 or more dark spots of varioussizes dotting the northern Titanic landscape. The spots appear blackbecause the radar beams that strike there are not scattered back towardthe spacecraft; rather, they bounce cleanly off into space. Thatsuggests that these areas are extraordinarily smooth—a powerfulindication that they are liquid. (A liquid naturally assumes a flatsurface; a smooth solid area is very rare in nature.) The perimeters ofthe dark areas are irregular, as would be expected if fluid had flowedinto a natural basin, and the transition from the dark flat areas tothe much brighter surrounding regions is abrupt, as though a smoothliquid were lapping up against rough, dry land. Finally, many of thelakes are fed by what look like river channels.
Titan’s lakes may form one component of a vast system in whichmethane circulates throughout the planet, much as water does on Earth.“We were hoping to see huge reservoirs,” says Lunine, “because that’swhat you need to maintain methane in the atmosphere for billions ofyears.” In fact the lakes are small, so he is convinced that most ofTitan’s methane lies under the surface, mixed with water and ammonia,which keeps the water from freezing.
When methane seeps up from underground, it may give rise to othereerily Earthlike features. A spectacular 90-mile-long, nearly mile-highmountain range south of Titan’s equator is almost certainly the resultof tectonic activity—the movement of crust driven by subsurface heat.Cassini scientists speculate that mushy ice mixed with methane welledup from below when two crustal plates pulled apart, as happens atmidocean ridges on Earth. The ice froze into mountains, while themethane eventually evaporated. Methane could also have been belchedinto the atmosphere by volcanic action. Cassini’s radar identified atleast one probable volcano—a raised, circular feature—and the orbiter’sinfrared camera spotted a fan-shaped form spreading away from thevolcano, which might well be a hardened flow of that same mushysubsurface ice.
In Titan’s atmosphere, too, methane occupies the same ecologicalniche that water does on Earth: It condenses out and falls on thesurface as rain. Recent models indicate that the rain comes in twoforms: a constant, light drizzle over most of the surface, adding up totwo inches or so of precipitation per year, and occasional cloudburststhat carve out river channels and fill the lakes, only to evaporateagain when temperatures rise.
Finally, hurricane-force winds modify Titan’s surface, eroding thefrigid land and creating dunes that stretch in parallel lines forhundreds of miles. They’re made not of sand but of ice or frozenhydrocarbons or even of a strange, mile-high fluffy ethane-smog mixthat one planetary scientist has dubbed “smust.”
As exciting as these discoveries are, they are only the first hintsof what Titan really looks like; a series of flybys in 2007 should fillout the picture. The dearth of impact craters, for example, suggeststhat Titan has a relatively young surface. That fresh-faced look is amystery, says Lunine. “Is it tectonics?” he wonders. “Is it mostlyburial by organics? Is the crust so thin that features relax away overtime?”
If NASA coughs up the money to extend Cassini’s mission beyond itsformal 2008 end date, these questions could be laid to rest within thenext few years. Then a world that’s so tantalizingly Earthlike yet socompletely alien could finally start to make sense.
