Scientists who have peered through the smoggy orange haze of Saturn's largest moon, Titan, have discovered that the surface is not entirely covered by liquid and solid organic materials that rain out of the atmosphere. Extensive areas of icy bedrock lie exposed on Titan's surface, they reported in the April 25 issue of Science.
"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," said Caitlin A. Griffith of the University of Arizona Lunar and Planetary Laboratory.
Titan's atmosphere, ten times as massive as Earth's, is primarily nitrogen laced with such poisonous substances as methane and ethane. Titan is thickly veiled by a dense hydrocarbon haze that forms in the high stratosphere as atmospheric 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.
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. UA planetary scientist Jonathan Lunine and others theorize that atmospheric methane is replenished on Titan in a liquid cycle similar to Earth's hydrologic cycle. Others theorize that Titan's methane is produced by geologic activity.
Scientists have measured and modeled the rate of methane photolysis, and from that deduced how much material annually settles out of the atmosphere.
"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," she 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."
Since 1991, Griffith and others have developed and used a technique that allows observers to spectroscopically view the surface at several narrow infrared "windows," or regions between the very thick methane bands. On this project, the team used the United Kingdom Infrared Telescope and NASA's Infrared Telescope Facility, both on Mauna Kea, Hawaii, to observe at eight near-infrared windows.
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.
"Titan's spectra resemble Ganymede's spectrum, dominated by ice features," they report.
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 that Titan has large patches of darker terrain, Griffith noted.
"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."
The new findings are indirectly relevant to the NASA/ESA Cassini mission/Huygens probe to arrive at Saturn in July 2004, Griffith noted. Scientists would like a better idea of how optically thick Titan's haze is, and how bright or dark its surface will be, to calculate camera exposure times. In addition, scientists are fine tuning their questions as they plan the Cassini observations.
Cassini spacecraft instruments include the Visual and Infrared Mapping Spectrometer (VIMS), an experiment headed by Robert H. Brown of the UA Lunar and Planetary Lab. The orbiter-borne instrument will map large chunks of Titan's surface at optimal haze-penetrating, near-infrared wavelengths. Griffith is working with the VIMS science team.
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