Scientists have peered through the smoggy orange haze of Saturn's largest moon, Titan, and seen icy bedrock exposed on the surface.
By Dr David Whitehouse
BBC News Online science editor
The observations reveal a surface that is not entirely covered by liquid and solid organic materials that rain out of the atmosphere, as was thought.
Titan may have hydrocarbon oceans
"Titan's surface reflectivity looks a lot like that of Jupiter's moon Ganymede. This is somewhat surprising because Titan is believed to have a lot of organic gook on its surface," says Caitlin Griffith, of the University of Arizona Lunar and Planetary Laboratory, US.
Titan will be visited by a probe next year that will parachute down to its surface sending back pictures.
Worse than city smog
Titan's atmosphere is about 10 times as massive as Earth's and is primarily nitrogen laced with methane and ethane.
It forms a dense hydrocarbon haze high in the moon's stratosphere as methane is destroyed by sunlight. The haze is much thicker than Earth's worst city smog.
It was impenetrable to cameras aboard the Pioneer and Voyager spacecraft that flew by the Saturn system in the late 1970s and early 1980s.
But what makes Titan's atmosphere almost opaque results in a rich chemistry on the surface.
Smog shrouds the moon
The by-products of methane molecules destroyed in the Sun's ultraviolet light react with other molecules in Titan's atmosphere, forming organic substances that fall on to the moon's surface, blanketing the icy bedrock and forming lakes and oceans of hydrocarbons.
University of Arizona planetary scientist Jonathan Lunine and others theorise that atmospheric methane is replenished on Titan by a liquid cycle similar to Earth's hydrologic cycle. Others suggest that Titan's methane is produced by geologic activity.
"Assuming that Titan's atmosphere has existed over the moon's 4.6-billion-year lifetime, 800 metres of sediments would lie on the surface," says Dr Griffith.
"So one might ask whether the surface is covered with the liquid and the solid sediments, such that we can't see the ice and rock that exist beneath."
Windows of light
Looking at Titan through an infrared "window" of light that can reach the surface, or regions between the very thick methane bands, the astronomers conclude that much of Titan's surface is exposed icy bedrock.
"Titan's spectra resemble Ganymede's spectrum, dominated by ice features," they say.
In 1994, the Hubble Space Telescope obtained its first images of Titan's surface, and images from others since show that Titan has large patches of darker terrain.
"It's not clear what the darker material is, but one possibility is that it is these organic liquids and sediments. The images, taken together with our results, suggest that organic stuff is moved around on the surface in such a way as to expose bedrock ice."
In July 2004, the Cassini spacecraft will arrive at Saturn and go into orbit. It will also drop a probe on to Titan.
Cassini will also map large chunks of Titan's surface at optimal haze-penetrating, near-infrared wavelengths.
The latest Titan research is published in the journal Science.