Strong evidence of lakes on Titan.

Using Earth-based radar to penetrate the thick atmosphere of Saturn's moon Titan, planetary scientists have confirmed that the smog-shrouded moon is unique among known residents of the solar system. The new radar observations suggest more strongly than has any previous study that lakes or oceans of hydrocarbons cover large stretches of the moon's surface.

Cornell astronomer Donald B. Campbell and his colleagues used the 305-meter Arecibo Radio Telescope in Puerto Rico to bathe the Saturnian moon, about 1 billion kilometers distant, with 900 kilowatts of microwaves. Traveling through the moon's atmosphere and bouncing off its surface, the microwaves returned to Earth after a 135-minute round-trip journey.

Each of the radar echoes, received at either the Arecibo telescope or the new Green Bank (W. Va.) Telescope, included a broad, diffuse swath of microwave wavelengths. Within that swath, most of the echoes showed a strong spike, similar to sunlight glinting off an ocean. The shape of that feature, known as a specular reflection, is characteristic of a smooth, liquid surface of hydrocarbon, Campbell's team reports in an upcoming Science.

The researchers found this distinctive signal in about 70 percent of the radar echoes examined during 25 nights of observations in late fall of 2001 and early winter of 2002. Because Titan was rotating slightly, each observation surveyed a strip of Titan some 20 to 90 km long, Campbell says.

Ultraviolet light from the sun breaks down some of the methane and other hydrocarbons in Titan's atmosphere, and theorists have suggested for decades that the breakdown products rain on the surface and form lakes or oceans. A similar organic brew may have resided on the early Earth, just before life gained a foothold.

Results from previous radar-echo studies, as well as observations of near-infrared light reflected from Titan's surface (SY: 9/4/99, p. 152), are consistent with vast pools of liquid hydrocarbons on that moon. But these earlier studies had lower resolution than the current set of experiments does, says Campbell.

The new study is "the result of years of planning and anticipation as Saturn and Titan moved above the local Arecibo [Radio Telescope] horizon beginning in the early 1990s" notes Jonathan I. Lunine of the University of Arizona in Tucson. "Compared to the old [radio telescope] data, this is a great improvement."

"The limits of what can be achieved from Earth have [now] essentially been reached; notes Ralph Lorenz of the University of Arizona in a commentary accompanying the Science article. He says the radar echoes indicate that impact craters filled with liquid hydrocarbons may cover as much as 75 percent of Titan's surface.

Campbell cautions that his team's findings don't prove that Titan has lakes of ethane, methane, or other hydrocarbons. For instance, a similar radar reflection might be received from a smooth surface of solid hydrocarbon or water ice mixed with a few rougher patches, he says.

Lunine agrees that researchers can't rule out smooth solid surfaces.

But definitive data should soon be in hand. Next summer, the Cassini spacecraft will enter orbit about Saturn and, in October 2004, will begin a series of close passes by Titan during which the spacecraft will image the moon with visible-light, near-infrared, and radar detectors.

Then comes the piece de resistance: In January 2005, Cassini will release a probe that will parachute through Titan's haze. If Campbell and other astronomers are right, the probe will make quite a splash.