December 18, 2009: NASA's Cassini spacecraft has photographed a flash of sunlight reflecting from a lake on Saturn's moon Titan, confirming the presence of liquid hydrocarbons on a part of the moon dotted with many lake-shaped basins.
Cassini scientists had been looking for the glint, also known as a specular reflection, since the spacecraft began orbiting Saturn in 2004. But until recently Titan's northern hemisphere, where most of the lakes are located, had been veiled in winter darkness. Now, however, the seasons are changing and sunlight has returned to the north, allowing Cassini to capture this serendipitous image:
This image, obtained using Cassini's Visual and Infrared Mapping Spectrometer (VIMS), shows the first observed flash of sunlight reflected off a lake on Saturn's moon Titan. Credit: NASA/JPL/University of Arizona/DLR.
The picture, which shows sunlight reflecting from the smooth surface of a liquid on July 8, 2009, was presented today at the Fall meeting of the American Geophysical Union in San Francisco.
"This one image communicates so much about Titan - a thick atmosphere, surface lakes and an otherworldliness, " says Bob Pappalardo, Cassini project scientist, based at NASA's Jet Propulsion Laboratory. "It's an unsettling combination of strangeness yet similarity to Earth. This picture is one of Cassini's iconic images."
Titan, Saturn's largest moon, has captivated scientists because of its many similarities to Earth. Scientists have theorized for 20 years that Titan's cold surface hosts seas or lakes of liquid hydrocarbons, making it the only other planetary body besides Earth believed to have liquid on its surface. While data from Cassini have not indicated any vast seas, they have revealed what appeared to be large lakes near Titan's north and south poles.
In 2008, Cassini scientists using infrared data confirmed the presence of liquid in Ontario Lacus, the largest lake in Titan's southern hemisphere. But they were still looking for the smoking gun to confirm liquid in the northern hemisphere, where the basins are larger and more numerous.
A false-color radar map of putative methane lakes in Titan's northern hemisphere. Credit: Cassini Radar Mapper, JPL, ESA, NASA
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Katrin Stephan, of the German Aerospace Center (DLR) in Berlin, an associate member of the Cassini visual and infrared mapping spectrometer team, was processing the initial image and was the first to see the glint on July 10, 2009.
"I was instantly excited because the glint reminded me of an image of our own planet taken from orbit around Earth, showing a reflection of sunlight on an ocean, " Stephan said. "But we also had to do more work to make sure the glint we were seeing wasn't lightning or an erupting volcano."
Team members at the University of Arizona in Tucson processed the image further. They were able to pinpoint the reflection at the southern shoreline of a lake called Kraken Mare. The sprawling Kraken Mare covers about 400, 000 square kilometers (150, 000 square miles), an area larger than the Caspian Sea, the largest lake on Earth.
By comparing this new image to radar and near-infrared images acquired since 2006, scientists were able to show that the shoreline of Kraken Mare has been stable over the last three years and that Titan has an ongoing hydrological cycle that brings liquids to the surface. Of course, in this case, the liquid in the hydrological cycle is methane rather than water, as it is on Earth.
"These results remind us how unique Titan is in the solar system, " says Ralf Jaumann, who leads the scientists at the DLR who work on Cassini. "They also show us that liquid has a universal power to shape geological surfaces in the same way, no matter what the liquid is."
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