Alien world: This Cassini false-colour mosaic shows Titan's north polar region. Approximately 60 per cent of the moon's pole, above 60 degrees north latitude, is now mapped with radar. About 14 per cent of that is covered by what is interpreted as liquid hydrocarbon lakes. Features appearing darkest to the radar, which are thought to be liquid, are shown in blue and black, and the radar-bright areas likely to be solid surface are tinted brown.
Credit: NASA
SYDNEY: Newly assembled radar images from the Cassini space probe are providing a detailed picture of the hydrocarbon lakes and seas on the poles of Saturn's moon Titan.
Approximately 60 per cent of Titan's north polar region (north of 60o latitude) has been mapped by Cassini's radar. About 14 per cent of the mapped region is covered by what scientists believe are lakes filled with liquid methane and ethane.
The latest image from U.S. space agency NASA was created by stitching together a mosaic of radar images from seven Titan flybys over the last year and a half. At least one of the pictured lakes is larger than North America's Lake Superior, which is the largest freshwater lake on Earth.
Methane and ethane rain
"This is our version of mapping Alaska, the northern parts of Canada, Greenland, Scandinavia and Northern Russia," says Rosaly Lopes, Cassini radar scientist at NASA's Jet Propulsion Laboratory in California. "It's like mapping these regions of Earth for the first time."
Lakes and seas are very common at Titan's high northern latitudes where winter is now underway. Scientists say it rains methane and ethane there, filling the lakes and seas. These liquids also carve meandering rivers and channels on the moon's surface.
Now Cassini is moving into unknown territory: the south pole of Titan. On October 2, the spacecraft executed a flyby in which a prime goal was the hunt for southern lakes. Lopes explains: "We wanted to see if there are more lakes present there and, sure enough, there they are, three little lakes smiling back at us."
It is now summer at Titan's south pole. A season on Titan lasts nearly 7.5 years, one quarter of a Saturn year, which is 29.5 years long. Monitoring seasonal changes in the lakes will help scientists understand the processes at work there.
"Karstic erosion"
They are already making progress in understanding how the lakes may have formed. On Earth, lakes fill low spots or are created when the local topography intersects a groundwater table. Lopes and her colleagues think that the depressions containing the lakes on Titan may have formed by volcanism or by a type of "karstic erosion" which leaves a depression where liquids can accumulate. Karstic lakes are common on Earth. For example, in parts of Minnesota and central Florida there are hundreds of such lakes.
"The lakes we are observing on Titan appear to be in varying states of fullness, suggesting their involvement in a complex hydrologic system akin to Earth's water cycle. This makes Titan unique among the extra-terrestrial bodies in our solar system," added Alex Hayes, a graduate student who studies Cassini radar data at the California Institute of Technology in Pasadena.
"The lakes we have seen so far vary in size from the smallest observable, approximately 1 square kilometre, to greater than 100,000 square kilometres, which is slightly larger than the Great Lakes in the Midwestern U.S.," Hayes says. "Of the roughly 400 observed lakes, 70 per cent of their area is taken up by 'seas' greater than 26,000 square kilometres."
with NASA
