Exotic Chemistry on Frigid Titan

PLANETARY EXPLORATION

The first detailed glimpse of Saturn's moon Titan comes a week after the European Space Agency's Huygens probe plunged through Titan's thick nitrogen-and-methane atmosphere.

The world revealed by the probe bears a striking geological resemblance to Earth--except that on Titan, where the temperatures hover around –180 C, methane plays the role of water, raining out of the atmosphere to feed lakes and streams. And instead of mineral rocks, Titan has water ice and a complex hydrocarbon "dirt."

"There's lots of evidence for very familiar Earth-like processes, but with very exotic materials," says Martin G. Tomasko, a professor of planetary sciences at the University of Arizona, Tucson.

The Huygens probe spent seven years riding to Saturn onboard the Cassini spacecraft and reached the moon on Jan. 14. The probe survived a two-and-a-half-hour descent to Titan's surface and continued transmitting data for 73 minutes after its landing. Scientists are now plowing through the chemical data collected by Huygens' six instruments.

The Huygens team has deduced that the probe arrived on Titan just after a methane rainstorm, coming to rest beside a large lake bed fed by eroded channels. The warm probe settled into the moon's gooey, icy organic "topsoil," releasing a burst of gaseous methane from liquid just below the surface.

As the Huygens probe fell, its mass spectrometer determined that nitrogen is dominant in the upper atmosphere but gives way to methane lower down. Once the probe was on the ground, the instrument recorded a 30% jump in methane levels, suggesting that the gas was coming out of the ground. "That means there's liquid methane very near the surface," says Tobias C. Owen, an astronomer at the University of Hawaii. "This is a planet where liquids are right there."

The predominance of liquid methane surprised scientists, who expected to see perhaps higher concentrations of liquid ethane. Previous models had suggested that photochemical processes would convert atmospheric methane to other molecules such as ethylene and acetylene, which would then collect on the surface in large pools.

Evidence for a photochemical "smog fallout" exists in the form of dark material that has collected in channels and drained into lake beds. These lake beds are mostly dry right now. "The pools gradually dry out and liquid sinks into the surface," Tomasko explains. "Liquid is just underneath the surface as if it rained not long ago."

Huygens also captured images of a series of short, stubby branches indicative of a methane spring system, along with streaky white patterns that could be upwellings of water ice.

In addition, the probe measured argon in Titan's atmosphere. Surprisingly, it detected only one isotope--argon-40, which is produced by the decay of radioactive potassium in primordial rocks in Titan's core. Other argon isotopes, expected relics from Titan's early atmosphere, were missing. "We find these [isotopes and other] noble gases in our atmosphere, on Venus, and on Jupiter, but we cannot find them on Titan," Owen says. "Surely there's an interesting clue there about how Titan formed."

Numerous mysteries remain about the chemistry of methane and other organics on Titan. For example, Owen asks, how does the methane replenish itself after making photochemical smog? "There must be some source of methane inside Titan," he says.