Hopes of finding hydrocarbon oceans on Saturn's smoggy moon, Titan, appear to be dashed, scientists report in Nature.
Titan's giant lakes of hydrocarbon may have evaporated
The moon's atmosphere is thick with methane and ethane, prompting speculation that lakes or oceans of these chemicals may sit on the surface.
The Huygens that landed on Titan sent back images suggesting possible shorelines and rivers.
But an extensive search for tell-tale infrared reflections has now revealed no sign of lakes or seas on Titan.
Scientists who made the measurements using the Keck II telescope in Hawaii suggest the flat surfaces previously spotted on Titan are more likely to be solid and dry.
"We infer mechanisms that produce very flat solid surfaces, involving a substance that was liquid in the past but is not in liquid form at the locations we studied," Robert West of the Jet Propulsion Laboratory in Pasadena, US, and his colleagues wrote.
However, the latest observations were focused entirely on Titan's southern hemisphere. It is just possible the northern region may still contain pools of liquid organic material.
"I would not say that the surface is devoid of liquid methane," lead researcher Dr West said.
Scientists believe Titan's smoggy atmosphere may be similar to that of the primordial Earth and studying it could provide clues to how life began.
Early radar studies showed that Titan was covered with pools of methane - a flammable gas on Earth but liquid on Titan because of the intense atmospheric pressure and cold.
Scientists thought they detected lakes, but that now seems unlikely
The Cassini space craft, which arrived at Saturn last year on a mission to study the ringed planet and its many moons, also observed intriguing liquid-like features. Since it neared the moon in 2004, it has detected dark, river-like channels.
But Cassini's visible and infrared cameras have failed to find the reflections expected from the surface.
There could be several reasons why the mysterious moon has thrown up such conflicting messages.
"At one time, maybe a liquid water and ammonia mix flowed onto the surface and froze," Dr West told the New Scientist. "That could be smooth on the scale of radar but rough on the scale we see."
Alternatively, Titan's rivers and lakes of hydrocarbon may have evaporated, leaving flat plains of organic material.
A third possibility is that organic particles in Titan's atmosphere settled onto the surface and were blown into low-lying areas, leaving smooth lake-like surfaces.