July 19, 2006
Cassini Radar Sees Earth-like Features On Saturn’s Moon Titan
New radar images from NASA's Cassini spacecraft reveal geological features very similar to Earth on an Australia-size, bright region on SaturnÃ´s moon Titan.
In one radar strip over 4,500 kilometers (2,796 miles) long, Cassini provided a virtual goldmine, telling the region's complex geological story.
Radar images show that the region, named Xanadu, is surrounded by darker terrain, reminiscent of a free-standing landmass. At the western edge of Xanadu, dark sand dunes give way to land cut by river networks, hills and valleys. These narrow river networks flow onto darker areas, which may be lakes.
A crater formed by the impact of an asteroid or by water volcanism is visible. More channels snake through the eastern part of Xanadu, ending on a dark plain where dunes, abundant elsewhere, seem absent. Mountains, roughly the height of the Appalachian Mountains, crisscross Xanadu.
"We could only speculate about the nature of this mysterious bright country, too far from us for details to be revealed by Earth-based and space-based telescopes. Now, under Cassini's powerful radar eyes, facts are replacing speculation," said Dr. Jonathan Lunine, Cassini interdisciplinary scientist at the University of Arizona, Tucson. "Surprisingly, this cold, faraway region has geological features remarkably like Earth."
Titan is a place of twilight, dimmed by a haze of hydrocarbons surrounding it. The radar instrument can see through the haze to the surface. It bounces radio signals off Titan's surface and times their return. In radar images, bright regions indicate rough or scattering material, while a dark region might be smoother or more absorbing material, possibly liquid.
Xanadu was first discovered by NASA's Hubble Space Telescope in 1994 as a striking bright spot seen in infrared imaging. When Cassini's radar system viewed Xanadu on April 30, 2006, it found a surface modified by winds, rain, and the flow of liquids. At Titan's frigid temperatures, the liquid cannot be water; it must be methane or ethane.
"Although Titan gets far less sunlight and is much smaller and colder than Earth, Xanadu is no longer just a mere bright spot, but a land where rivers flow down to a sunless sea," said Lunine.
Observations by the European Space AgencyÃ´s Huygens probe that Cassini carried to Titan, and by NASA's Voyager spacecraft strongly hint that both methane rain and dark orange hydrocarbon solids fall like soot from Titan's dark skies. On bright Xanadu, liquid methane might fall as rain or trickle from springs. Rivers of methane might carve Xanadu's channels and carry off grains of material to accumulate as sand dunes elsewhere on Titan.
"This land is heavily tortured, convoluted and filled with hills and mountains," said Dr. Steve Wall, the Cassini radar team's deputy leader at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "There appear to be faults, deeply cut channels and valleys. Also, it appears to be the only vast area not covered by organic dirt. Xanadu has been washed clean. What is left underneath looks like very porous water ice, maybe filled with caverns.
In the 1980s, it took the Shuttle Imaging Radar to discover subsurface rivers in the Sahara. Similarly, if it hadn't been for the Cassini radar, we would have missed all of this. We have a newly discovered continent to explore, just like the early explorers of America," said Wall.
Cassini will view Titan on July 22, this time exploring the high northern latitudes. In the next two years the orbiter will fly by Titan 29 times, nearly twice as many encounters as in the first half of Cassini's four-year prime mission. Twelve of the planned flybys will use radar.