Titan: no global ocean, maybe some seas.

Titan: No global ocean, maybe some seas

One of the more intriguing questions about the planets asks whether a hydrocarbon ocean covers Saturn's big moon Titan, which would then become the first liquid surface besides Earth's waters found anywhere in the solar system. After studying Titan by radar, a group of researchers has concluded that although no global Titanian ocean seems to exist, smaller seas remain a possibility.

Some scientists have suggested Titan may be covered with a layer of liquid methane and ethane more than a mile deep. But the radar results, indicating Titan has a far more varied surface than previously believed, rule out a hydrocarbon ocean blanketing the moon, says Duane O. Muhleman of the California Institute of Technology in Pasadena.

On June 3, 4 and 5, Muhleman and graduate students Arie W. Grossman and Bryan Butler, together with Martin A. Slade of Jet Propulsion Laboratory in Pasadena, took part when NASA bounced a radar beam off Titan from the Deep Space Network tracking antenna in Goldstone, Calif. Listening to the beam's echo was a network of 27 electronically linked antennas near Socorro, N.M., collectively known as the Very Large Array. The experiment was possible because NASA has equipped the array to receive a radio frequency that Goldstone can transmit.

On the first and third days of the study, the radar echoes from Titan were so weak -- about 2 to 4 percent -- that the researchers could barely distinguish them from the background noise. A deep methane-and-ethane ocean of the type proposed for Titan would have very low reflectivity, and its smoothness would be likely to reduce the echo strength even more. However, Titan turns about 23 [deg.] on its axis every 24 hours, exposing a different side to the radar beam each day, and the echo on the middle day was far too strong -- 10 to 20 percent -- to have come from such an oceanic surface. Instead, the group suggests, the echoes on the middle day are more like those from Venus.

"We concluded that these differences in reflectivity are real and that we were seeing evidence for surface variability," says Muhleman, who now calls Titan "the most interesting radar target in the solar system." In fact, the group concludes that the face of Titan is "at least as variable as the surface of Mars," some parts of which show reflectivity variations that are just as high. Muhleman notes that the weak Titan echoes could still have come from ethane oceans, although such an ocean "is certainly not 'universal,' and there may very well be some dry land."

Regardless of whether Titan has oceans, the Voyager 2 spacecraft found in 1980 that the pressure of the bottom of Titan's atmosphere is about 1.3 times Earth's, with a surface temperature of -290[deg.]F. Higher up, it appears to be warmer, but in the bottom 15 miles or so, some scientists suggest that methane and ethane rains may fall.

Next year, NASA hopes to nearly double the 360,000-watt power of the Goldstone transmitter, which could then produce more detailed radar returns. NASA and the European Space Agency hope to get a much closer look with the imaging radar of the planned Cassini mission, to be launched in 1995 to radar-map Saturn and Titan in 2002.