NASA

A frigid globe covered in a wet sand of organic compounds and sculpted by a liquid, most likely methane--that is the picture of Saturn's moon Titan emerging from data collected by the Huygens probe, which touched down on its surface in January. Nature published the first analyses in a series of papers published online yesterday.

Although conditions on Titan's surface are a frosty -179 degrees Celsius, it is the only body in the solar system other than Earth that has a nitrogen-rich atmosphere. The second largest moon in the solar system--behind only Jupiter's Ganymede--Titan also resembles Earth in having a complex weather cycle, landscapes carved by liquid flow, and volcanic activity, albeit of a freezing cold variety. It may in fact resemble our planet in the earliest stages of its existence.

But Titan is also clearly an exotic world swaddled in a dense layer of smog and frozen in a primitive, hydrogen-rich state. The methane and nitrogen that constitute its atmosphere form various aerosols in the sunlight--giving the moon its orange aspect--that then drift down through the atmosphere as alien snow leaving a soft covering as much as one kilometer thick on the surface, which is littered with water ice instead of rocks.

Huygens collected data throughout its two-and-a-half-hour descent to the surface and for more than an hour after it landed. The probe's various systems included instruments to measure the atmosphere, wind, aerosols, gases and the surface itself. But the joint NASA / European Space Agency mission has raised as many questions as it has answered. Among these new mysteries is the question of how Titan continues to hold such large quantities of methane in its atmosphere when chemistry suggests that the gas should disappear within 100 million years. A vast reservoir of methane must exist somewhere within Titan that replenishes its atmosphere regularly.

Of course, one thing that can produce a lot of methane is life, but the particular carbon isotopes the probe found on Titan make that unlikely. Still, even if Earth and Titan don¿t appear to share life, the family resemblance remains strong. In fact, on its descent to the Titanian surface, Huygens detected ionic disturbances in the atmosphere, suggesting that Titan, like Earth, may experience lightning during summer storms.