Four-year study finds no ocean on Titan.

By bouncing radar off the surface of Titan, Saturn's largest moon, scientists have confirmed earlier findings that Titan does not possess a global ocean of hydrocarbons. But the researchers say they still have not ruled out ponds or lakes on the surface.

Ever since the Voyager spacecraft discovered that Titan contains a thick atmosphere that includes methane and other carbon-rich compounds, researchers have speculated that a hydrocarbon ocean might blanket Titan. Such a sea seemed a likely source of the thick atmosphere, according to Jonathan I. Lunine of the University of Arizona in Tucson.

Because the hazy atmosphere of Titan prevents visible light from reaching its surface, astronomers aimed a radar beam at the distant moon and detected the reflected signal received by the Very Large Array (VLA) radiotelescope near Socorro, N.M. The intensity of the reflected signal indicated that Titan does not have a large-scale ocean, because an all-liquid surface would reflect radio waves poorly (SN: 7/1/89, p.5).

Since their initial study, Duane O. Muhleman and Bryan Butler of the California Institute of Technology in Pasadena, Martin A. Slade of the Jet Propulsion Laboratory in Pasadena, and Arie W. Grossman of the University of Maryland in College Park have repeated their measurements over three additional summers, when the Saturn system comes closest to Earth. The researchers reported their studies last month in Munich, Germany, at a meeting of the American Astronomical Society.

Besides verifying their initial results, which suggest that Titan has an icy surface similar to Jupiter's moon Callisto, the scientists detected a puzzling daily variation in the intensity of the radar echoes they received. They suggest the variation may indicate that Titan's surface contains some hydrocarbon tars, and possibly ethane lakes, mixed with large ice patches. Thus, as different areas of the rotating moon face Earth, they would reflect radar waves with greater or less strength.

Grossman and Muhleman also used the VLA to detect thermal radio emissions from Titan. The observed emissions are similar to those expected from frozen water, not a hydrocarbon ocean.

Lunine contends that the team's findings are consistent with a "dirty" sea contaminated by particles of radar-reflecting material or a hydrocarbon sea submerged beneath an icy surface.