The Great Lakes of Titan
The Cassini spacecraft, using its radar system, has discovered very strong evidence for hydrocarbon lakes on Titan. Dark patches, which resemble terrestrial lakes, seem to be sprinkled all over the high latitudes surrounding Titan’s north pole.
|Two radar images were acquired by the Cassini radar instrument in synthetic aperture mode on July 21, 2006. This image centered near 80 degrees north, 92 degrees west measures about 420 kilometers by 150 kilometers (260 miles by 93 miles). Credit: NASA/JPL|
Scientists have speculated that liquid methane or ethane might form lakes on Titan, particularly near the somewhat colder polar regions. In the images, a variety of dark patches, some with channels leading in or out of them, appear. The channels have a shape that strongly implies they were carved by liquid. Some of the dark patches and connecting channels are completely black, that is, they reflect back essentially no radar signal, and hence must be extremely smooth. In some cases rims can be seen around the dark patches, suggesting deposits that might form as liquid evaporates. The abundant methane in Titan’s atmosphere is stable as a liquid under Titan conditions, as is its abundant chemical product, ethane, but liquid water is not. For all these reasons, scientists interpret the dark areas as lakes of liquid methane or ethane, making Titan the only body in the solar system besides Earth known to possess lakes. Because such lakes may wax and wane over time, and winds may alter the roughness of their surfaces. Repeat coverage of these areas should test whether indeed these are bodies of liquid.
The lakes are most likely the source of hydrocarbon smog in the frigid moon’s atmosphere. Finding the source of the complex soup of hydrocarbons in Titan’s atmosphere has been a major goal for the Cassini mission and is a significant accomplishment.
Numerous well-defined dark patches resembling lakes are present in radar images of Titan’s high latitudes taken during a July 22 flyby. At Titan’s frigid temperatures, about minus 180 degrees Celsius, the liquids in the lakes are most likely methane or a combination of methane and ethane.
"This is a big deal," said Steve Wall, deputy radar team leader at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. "We’ve now seen a place other than Earth where lakes are present."
|Cassini’s powerful radar eyes have uncovered a geologic goldmine in a region called Xanadu on Saturn’s moon Titan. The geologic features include river channels, mountains and hills, a crater and possible lakes. |
This area of Titan has been in winter’s shadow since before Cassini arrived, and the spacecraft had not flown over it before. During the flyby, Cassini’s radar spotted several dozen lakes as small as 0.6 miles wide, with some nearly 20 miles wide. The biggest lake is about 62 miles long and may be only partly wet.
"What we see is darker than anything we’ve ever seen elsewhere on Titan. It was almost as though someone laid a bull’s-eye around the whole north pole of Titan, and Cassini sees these regions of lakes just like those we see on Earth," said Larry Soderblom, Cassini interdisciplinary scientist at the U.S. Geological Survey, Flagstaff, Ariz. "Titan has turned out to be like a musical crescendo — each pass is more exciting than the last."
Titan has not yielded its secrets easily because the dense smoggy atmosphere makes it very difficult to obtain good visible images. Radar can penetrate the smog and obtain clear images.
Dark regions in radar images generally mean smoother terrain, while bright regions mean a rougher surface. Some of the new radar images show channels leading in or out of a variety of dark patches. The shape of the channels also strongly implies they were carved by liquid.
Some of the dark patches and connecting channels are completely black — they reflect back essentially no radar signal, which means they must be extremely smooth and might contain liquid. In some cases rims can be seen around the dark patches, suggesting deposits that might form as liquid evaporates.
Scientists had predicted, but had no confirmation until now, that pools of liquid were contributing to the high concentration of methane and other hydrocarbons in Titan’s atmosphere.
|A network of river channels is located atop Xanadu, the continent-sized region on Saturn’s moon Titan. This radar image was captured by the Cassini Radar Mapper on April 30, 2006. Credit: NASA/JPL|
"We’ve always believed Titan’s methane had to be maintained by liquid lakes or extensive underground ‘methanofers,’ the methane equivalent of aquifers. We can’t see methanofers but we can now say we’ve seen lakes," said Jonathan Lunine, Cassini interdisciplinary scientist at the University of Arizona, Tucson.
Since lakes come and go with the seasons, they wax and wane over time, and winds might alter the roughness of their surfaces. Repeat coverage of these areas is expected to provide more information on these lakes. By passing over a lake in a different direction, Cassini may see the effect of prevailing winds in the changing brightness of the lake surface. On later passes toward the end of its prime mission, Cassini might see changes in the shape or size of lakes as winter yields to spring in the northern hemisphere.
Cassini’s next flyby is on September 7. In October, Cassini’s radar will look even closer to the north pole, searching for more lakes and mapping more of the polar region covered by these features.