A Huygens image of Titan's surface suggests island-like objects and fluid flow.

A World Apart

Dick writes about Earth and planetary science for Science magazine.

It's been a week since the European probe Huygens landed on Saturn's moon Titan, and researchers are still breathlessly trying to make sense of all of the data. At a press conference today in Paris, Huygens scientists announced more stunning findings from a mission that has been far more than just a brilliant engineering success.

Perhaps the most astonishing thing about the pictures Huygens has beamed back is how familiar Titan looks. In particular, this moon of rock-hard ice, organic goo, and liquefied natural gas bears a striking resemblance to deserts like the Mojave and to Mars. The rocks are probably water ice, as suggested by spectra taken by Huygens. The 10- to 30-centimeter cobbles are well-rounded and are scattered across the scene as if a powerful current had debouched nearby.

Huygens's view of Titan on its way down made it plain that powerful currents have indeed carved the moon's surface. The view from 16 kilometers up "looks very much like drainage channels," said Huygens descent imager principal investigator Martin Tomasko of the University of Arizona (UA), Tucson. Collected fluids would run down the dark-floored channels "out to what looks very much like a shoreline" of a dark sea.

Theorists had invoked liquid methane--liquefied natural gas--on the surface to explain the low percentage of methane in the atmosphere. But Cassini observations had failed to reveal any sign of a dark methane ocean, sea, or even lake (Science, 3 December 2004, p. 1676). Huygens found no obvious sign of standing fluids either. "We don't think we see liquid," said Tamasko, "but there's plenty of evidence of fluid flow."

Methane seas may yet turn up, but Titan already would seem to have all the parts of a "methylogical cycle" that is like the hydrologic cycles of Earth and ancient Mars. Titan's atmosphere contains methane and photochemically produced ethane--analogous to Earth's water vapor--that condense into hydrocarbon clouds. Some clouds must rain onto the surface to erode the channels, and the rain would presumably also pick up the dark photochemical goo that settles from the haze layer. That would explain the dark stain on canyon floors and outwash plains.

Cautioning that other parts of Titan or other seasons might be less desert-like, team members were eager to forge ahead. "We can now dream of rovers," said project scientist and mission manager Jean-Pierre Lebreton.

Related site
Cassini-Huygens homepage

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