BENEATH the dense haze that shrouds Saturn鈥檚 giant moon, Titan, is a mysterious surface feature about the size and shape of Australia, say astronomers in the US. 鈥淚t could be an impact crater, or perhaps a mountainous region in the underlying frozen water-ice and rock,鈥� says Peter Smith of the University of Arizona in Tucson, who presented the discovery last month in Washington DC at a meeting of the Division of Planetary Sciences of the American Astronomical Society.
Titan鈥檚 atmosphere is opaque to visible light because it is covered by dense smog. Images of Titan taken by NASA鈥檚 Voyager 1 spacecraft revealed virtually no features. However, at longer, infrared wavelengths, there are a few spectral 鈥渨indows鈥� where the atmosphere is more transparent and permits glimpses of the surface.
Smith and his colleagues used the Hubble Space Telescope to look through these windows and record images of Titan鈥檚 surface. By observing the surface over a 16-day period, the astronomers were able to map virtually all of it.
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In the past, measurements made through these windows, using Earth-based telescopes, have provided only tantalising hints of what this moon鈥檚 surface might look like. Last year, Smith鈥檚 colleague Mark Lemmon and Caitlin Griffith from NASA鈥檚 Ames Research Center in California reported that the hemisphere of Titan visible from Earth at one extremity of its orbit seemed to be brighter than the hemisphere visible at the other extremity. Titan鈥檚 orbit, like that of the Earth鈥檚 Moon, is 鈥渟ynchronous鈥�, with the same face always facing Saturn.
But because Titan is star-like when viewed through telescopes on Earth, the astronomers were unable to determine the size and shape of any surface feature.
Radar images made by Duane Muhleman of the Jet Propulsion Laboratory in Pasadena also suggested that there was a bright area on one of Titan鈥檚 hemispheres. This finding was corroborated by the Hubble results, which show a clear bright feature in the same place on Titan鈥檚 surface.
But the mysterious feature is a headache for theorists. They had thought that Titan鈥檚 surface was covered with a deep ocean of liquid methane and ethane. They suspected this because Titan鈥檚 atmosphere is very rich in methane, and while 鈥減hotochemical鈥� processes should destroy the methane over time, it could be replenished if there was an ocean of it.