Despite the moon's extreme cold--its surface temperature is -180 degrees Celsius--Titan is similar to Earth in many ways. Like our planet, Titan has a thick atmosphere composed mainly of nitrogen. Another significant constituent is methane, which condenses at low temperatures and appears to play the same meteorological role on Titan as water does on Earth. Scientists had long speculated that Titan might have clouds of methane and lakes or seas of liquid hydrocarbons. To test this hypothesis, the European Space Agency built the Huygens probe, which for seven years journeyed to Saturn attached to NASA's Cassini orbiter. The two craft separated as they approached Titan, and Cassini relayed Huygens's data to Earth after the probe parachuted through the satellite's haze, which had obscured previous attempts to observe the surface.
One of the mission's first surprises was that this haze extended so far down. Titan's main haze layer is at least 200 kilometers thick; the probe's view of the surface did not clear until Huygens was less than 20 kilometers above the ground. At that same altitude the onboard gas chromatograph and mass spectrometer detected an abrupt jump in the amount of methane in the atmosphere. Together the findings suggest the presence of methane clouds, with the droplets possibly condensing around the haze particles. (These hydrocarbon solids are believed to come from the breakdown of methane in Titan's upper atmosphere caused by the sun's ultraviolet radiation.)
Evidence of methane rainfall came from Huygens's images of the surface, which showed sinuous, branching channels extending from relatively bright highlands to a tarry plain. The contrasts in brightness suggest that precipitation may have washed dark hydrocarbon deposits off the icy highlands and into the channels. But some of the channels are short and stubby, leading scientists to speculate that liquid methane may also be flowing from underground springs.
The probe's landing, though, offered the strongest indications of liquid methane on Titan. As Huygens hit the surface at five meters per second, a device called a penetrometer--essentially a spring-loaded stick on the underside of the craft--measured the force of impact and found the ground's resistance to be like that of wet sand with a brittle crust. Three minutes after landing, Huygens detected a 30 percent rise in the abundance of methane; the heat of the probe's instruments had apparently vaporized the liquid hydrocarbon in the top few centimeters of Titanian soil.
Researchers will spend the next several months analyzing the data and devising theories to explain the observations. "Titan is living up to our expectations," says Jonathan I. Lunine, a University of Arizona scientist on the Cassini-Huygens team. "It's as interesting as we'd hoped it would be." Tobias C. Owen, an atmospheric scientist at the University of Hawaii's Institute for Astronomy, warns that theorists must be careful about generalizing the results from a single landing site; it may not be possible, for example, to estimate the frequency of methane rainfall on Titan or the total amount of liquid methane on the surface. But continuing observations by the Cassini orbiter, which scanned Titan's surface near the landing site in a flyby in February, may help investigators make sense of Huygens's remarkable findings.