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.