Titan -- one of two moons in the solar system known to have a significant atmosphere -- has long baffled scientists because it is surrounded by a thick blanket of nitrogen and methane. Scientists have speculated that the atmospheric methane probably came from seas of liquid methane and ethane.

But telescopes and orbiting spacecraft have yet to turn up evidence of a global ocean of methane on Titan.

In the study, scientists using the Keck II telescope in Hawaii failed to see reflections of sunlight that would indicate a body of liquid on the frozen moon during several viewings in 2003 and 2004, said Robert West of the Jet Propulsion Laboratory in Pasadena.

Results appear in today's issue of the journal Nature (www.nature.com).