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planetary science

Titan, Once a World Apart, Becomes Eerily Familiar

The praise was polyglot, but the sense of it was clear enough: incredible, magnificent, astonishing. The European probe Huygens had blazed into the upper atmosphere of Saturn's big moon Titan, floated down by parachute for two-and-a-half hours—as it snapped pictures, sniffed the air, and checked the weather—and almost miraculously survived a hard landing to taste the surface and return a "wish you were here" view of a truly alien world.

The mission was more than simply a brilliant engineering success. "I was blown away by what I saw," said European Space Agency (ESA) science director David Southwood. "I had wanted to know that there was complexity down there." And complexity he got. What had frustrat-ingly remained an unrecognizable world of broad smears of light and dark, veiled even from the passing Cassini spacecraft by Titan's hazy atmosphere, exploded into sharp details of canyons, riverbeds, plains, rocks, mud, and possible lakes and seas.

Perhaps most astonishing was how familiar it all looked. "I was struck by how similar it looks to what we've seen on a variety of planets," said Huygens descent imager principal investigator Martin Tomasko of the University of Arizona (UA) in Tucson. 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 shock of the familiar crept up on icy-satellite geologist Robert Pappalardo of the University of Colorado, Boulder. "When I first saw the image from the sur face," he recalls, "I scrolled right by it because I thought it was Mars. I was amazed." The rusty orange color later added by the imager team is the cast that sunlight gives the surface as it leaks through Titan's hazy atmosphere; Mars, on the other hand, takes its color from the yellow-brown of oxidized iron. But the "rocks" strewn into the distance of a flat plain (inset, upper left) could at first glance

A blur no longer. The Huygens probe revealed new detail on Titan (center, 60 kilometers across), including drainage channels (inset, lower right) and surface rocks (inset, upper left).

easily be taken for martian. In fact, they are probably water ice, as suggested by spectra taken by Huygens. The 10- to 30-centimeter cobbles are well rounded, as if they've been tumbled in a streambed, and are scattered across the scene as if a powerful current had debouched nearby, spread across a broad valley floor, and dropped the rocks where they're now found. On Earth geologists call that a playa.

Huygens's view of the surface on its way down made it plain that powerful currents have indeed carved the surface of Titan. With 20 times the resolution of Cassini and a view from beneath the obscuring haze, the Huygens descent imager returned a picture

(inset, lower right) that screams fluid flow. The view from 16 kilometers up "looks very much like drainage channels," said Tomasko, with signs of seepage from canyon walls familiar from both Earth and Mars. Collected fluids would run down the dark-floored channels "out to what looks very much like a shoreline" of a dark sea. This and other Huygens images now add credibility to earlier Cassini observations. "We saw what we called 'dark meandering lines' " in Cassini images, says imaging team member Alfred McEwen of UA, but "we weren't ready to call them channels." And Huygens radar team member Ralph Lorenz of UA had pointed out bright, triangular features in the radar images and sug-gested—boldly at the time—that they could be rough, bouldery fans of debris dumped where channeled flows opened onto valley floors.

With so many signs of erosion, "the big question is, are the liquids there now?" McEwen asks. Theoreticians had invoked liquid methane—liquefied natural gas—on the surface to explain the presence of methane in the atmosphere. But Cassini observations had failed to reveal any clear sign of a dark methane ocean, sea, or even lake (Science, 3 December 2004, p. 1676). As much as the canyon-riddled highland draining to a dark, "shore"-lined plain suggested a sea, Huygens found no obvious sign of standing fluids either. It landed in a generally dark area, said Tomasko, that turns out to be a flat plain.

Even so, Huygens may have found the predicted reservoir of liquid methane. Atmospheric chemist Sushil Atreya of the University of Michigan, Ann Arbor, and the gas chromatograph/mass spectrometer team reported that when they gently heated their instrument's sampling inlet after it was driven into the surface on landing, methane was released. And John Zarnecki of the University of Kent, U.K., principal investigator of the surface science package, said that the penetrometer encountered a thin crust before passing through 15 cen-


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