Titanic Close-up: Cassini eyes Saturn’s big moon

Using radar to penetrate the thick haze surrounding Saturn’s moon Titan, the Cassini spacecraft has found evidence that the moon’s surface is coated with hydrocarbons. Dark patches in radar images, including a region the size of Lake Tahoe, might be liquid ethane or methane, the Cassini scientists say. These pools might resemble the chemical brew that led to life on Earth billions of years ago.

RADIO-ACTIVE MOON. False-color radar image of a patch of Saturn’s moon Titan taken when Cassini passed within 1,174 kilometers. Sinuous linear features may be cracks in surface ice. T. Farr/NASA/JPL

During press briefings last week from NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., the scientists reported these and other findings from Cassini’s first close passage by Titan on Oct. 26.

The researchers cautioned that the images show only 1 percent of Titan’s surface. Nonetheless, they provide the first detailed look at the moon’s surface, which has intrigued planetary scientists since 1980. That’s when the Voyager spacecraft revealed that Titan has a thick, nitrogen-rich atmosphere that contains methane.

But “for all we knew, the surface of Titan could simply be a geologically dead, cratered place that happened to be surrounded by an atmosphere,” reports Cassini scientist Jonathan I. Lunine of the University of Arizona in Tucson. The data now demonstrate that “Titan is an extremely dynamic and active place, not simply in its atmosphere but on its surface as well,” he says.

By bouncing radio waves off Titan’s surface and timing their return, Cassini imaged a strip of the moon’s northern hemisphere about 200 kilometers wide and 1,200 km long. The radar images show about a dozen dark areas. At least one large, very dark region—dubbed by a researcher’s daughter the “Halloween Cat” because of its shape—may be a hydrocarbon lake, says Charles Elachi, JPL director and team leader for the radar instrument.

Interpreting the radar data is tricky, notes team member Steve Wall of JPL, because several factors can make a region a poor reflector of radio waves. An area may look dark in radar because it is tilted away from the detector, has a smooth surface, or is a good electrical conductor.

During the rest of the mission, the scientists plan to identify the nature of the region with more certainty by examining the same location with several different Cassini instruments, including infrared cameras.

Using the radar instrument in a different way, the scientists also recorded the heat radiated by Titan’s surface at microwave energies. These emissions suggest that Titan is “really covered in organic material,” says Ralph Lorenz of the University of Arizona. The regions that emit the most microwaves are those that are dimmest in visible-light images, a trait consistent with the presence of organic compounds, he says.

Sinuous features that stretch across large parts of the radar images in random directions could be evidence that the surface is cracked like an eggshell, with one or more layers of a solid hydrocarbon floating on top of a hydrocarbon fluid, Lunine suggests.

Bright streaks seen by the craft’s visible-and-infrared-mapping spectrometer could be hydrocarbon-based dust bunnies blown around the surface, glaciers moving slowly, or waves on a hydrocarbon lake, notes Alfred McEwen of the University of Arizona.

Over the next 4 years, Cassini will make 44 more passes by Titan. On Jan. 14, the Huygens probe, now piggybacking on Cassini, is scheduled to parachute through Titan’s haze, making direct measurements of the atmosphere and, if it survives impact, of the character of its landing site.

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