Some researchers have speculated that Titan might harbor microbial life.
"Titan's surface reflectivity looks a lot like that of Jupiter's moon, Ganymede," said Caitlin Griffith of the University of Arizona Lunar and Planetary Laboratory. "This is somewhat surprising because Titan is believed to have a lot of organic gook on its surface."
The new study led by Griffith is detailed in today's issue of the journal Science.
Titan is hard to study, veiled by a dense hydrocarbon haze that forms in the high stratosphere as methane is destroyed by sunlight. The haze is much thicker than Earth's worst city smog, scientists say. Attempts to see through it during the Pioneer and Voyager missions failed.
The by-products of methane molecules destroyed in the sun's ultraviolet light react with other molecules in Titan's atmosphere, forming organic droplets and particulates that fall onto the moon's surface, blanketing the icy bedrock and forming lakes and oceans, Griffith and her colleagues believe.
Jonathan Lunine, also of the university of Arizona, and others theorize that atmospheric methane is replenished on Titan in a liquid cycle similar to Earth's water cycle. Others figure 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 meters of sediments would lie on the surface," Griffith said. "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."
The new study relies on narrow infrared "windows," specific wavelengths of light energy that allow researchers to peer through the haze. The researchers employed the United Kingdom Infrared Telescope and NASA's Infrared Telescope Facility, both on Mauna Kea, Hawaii.
Griffith, Tobias Owen of Hawaii's Institute for Astronomy, Thomas R. Geballe of the Gemini Observatory, John Rayner of Hawaii's Institute for Astronomy, and Pascal Rannou of the Pierre and Marie Curie University in Paris conclude after analyzing surface reflectivity that much of Titan's surface is exposed icy bedrock.
Images from UA planetary scientist Peter Smith, who used the Hubble Space Telescope in 1994 to get the first image of Titan's surface, and images from others since show 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," Griffith said. "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."
The new findings are expected to help scientists fine tune their questions and investigation of Titan, which will be studied up close next year. The Cassini spacecraft, due to arrive at Saturn in July 2004, will send the Huygens probe plunging through Titan's atmosphere, collecting data all the way to the surface.
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