Another important clue about Titan’s lakes comes from microwaves emitted by the lakes and by surrounding terrain. Cassini’s microwave detectors have sensed stronger radiation—which implies more heat—coming from the lake surfaces than from the adjacent territory, which is presumably made of water ice. “So the whole thing hangs together most simply if what we’re looking at is liquid,” says Lunine. That wet stuff is probably a mix of methane and ethane, another organic molecule that has been detected in Titan’s atmosphere, since pure methane would evaporate too quickly. “If NASA extends the mission a couple of years, as Titan’s northern hemisphere begins to warm up in spring, we should see the shape of the lakes change as methane begins to evaporate,” Lunine says.

pink-150.jpgA pink organic haze suffuses
the atmosphere of Titan (above).
The green band and blob at the
left of the image are probably
clouds of methane, which occas
-ionally condenses and falls to
the surface as rain.

Titan’s lakes may form one component of a vast system in which methane circulates throughout the planet, much as water does on Earth. “We were hoping to see huge reservoirs,” says Lunine, “because that’s what you need to maintain methane in the atmosphere for billions of years.” In fact the lakes are small, so he is convinced that most of Titan’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 other eerily Earthlike features. A spectacular 90-mile-long, nearly mile-high mountain range south of Titan’s equator is almost certainly the result of tectonic activity—the movement of crust driven by subsurface heat. Cassini scientists speculate that mushy ice mixed with methane welled up from below when two crustal plates pulled apart, as happens at midocean ridges on Earth. The ice froze into mountains, while the methane eventually evaporated. Methane could also have been belched into the atmosphere by volcanic action. Cassini’s radar identified at least one probable volcano—a raised, circular feature—and the orbiter’s infrared camera spotted a fan-shaped form spreading away from the volcano, which might well be a hardened flow of that same mushy subsurface ice.


In Titan’s atmosphere, too, methane occupies the same ecological niche that water does on Earth: It condenses out and falls on the surface as rain. Recent models indicate that the rain comes in two forms: a constant, light drizzle over most of the surface, adding up to two inches or so of precipitation per year, and occasional cloudbursts that carve out river channels and fill the lakes, only to evaporate again when temperatures rise.

Finally, hurricane-force winds modify Titan’s surface, eroding the frigid land and creating dunes that stretch in parallel lines for hundreds of miles. They’re made not of sand but of ice or frozen hydrocarbons or even of a strange, mile-high fluffy ethane-smog mix that one planetary scientist has dubbed “smust.”

As exciting as these discoveries are, they are only the first hints of what Titan really looks like; a series of flybys in 2007 should fill out the picture. The dearth of impact craters, for example, suggests that Titan has a relatively young surface. That fresh-faced look is a mystery, says Lunine. “Is it tectonics?” he wonders. “Is it mostly burial by organics? Is the crust so thin that features relax away over time?”

If NASA coughs up the money to extend Cassini’s mission beyond its formal 2008 end date, these questions could be laid to rest within the next few years. Then a world that’s so tantalizingly Earthlike yet so completely alien could finally start to make sense.

map-550.jpg(Click on image to enlarge)

A mosaic of infrared images gleaned from the Cassini spacecraft, now orbiting Saturn, were knit together to yield a global digital map of Titan. The white, bright terrain dubbed Xanadu is an Australia-size region pelted by methane rain and crisscrossed by mountains, rivers, and porous, possibly cavernous, water ice. In January 2005, the Huygens probe landed in the western part of the dark area labeled Shangri-La­, a vast plain named for the fictional Tibetan paradise. This former sea is bounded by the high grounds of Adiri to the west and Dilmun to the north. Titan’s northern polar regions, which appear blank in this map, will be captured by Cassini over the next few years as the moon nears its vernal equinox in 2009.