Glint of sunlight confirms liquid in northern lake district of Titan

Cassini scientists had been looking for the glint, also known as a specular reflection, since the spacecraft began orbiting Saturn in 2004. But Titan's northern hemisphere, which has more lakes than the southern hemisphere, has been veiled in winter darkness. The sun only began to directly illuminate the northern lakes recently as it approached the equinox of August 2009, the start of spring in the northern hemisphere. Titan's hazy atmosphere also blocked out reflections of sunlight in most wavelengths. This serendipitous image was captured on July 8, 2009, using Cassini's visual and infrared mapping spectrometer.

"This one image communicates so much about Titan -- thick atmosphere, surface lakes and an otherworldliness," said Bob Pappalardo, Cassini project scientist, based at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "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 harbor liquid on its surface. While data from Cassini have not indicated any vast seas, they have revealed 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 lakes are also larger.

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 10th.

This image shows the first flash of sunlight reflected off a lake on Saturn's moon Titan. The glint off a mirror-like surface is known as a specular reflection. This kind of glint was detected by the visual and infrared mapping spectrometer (VIMS) on NASA's Cassini spacecraft on July 8, 2009. It confirmed the presence of liquid in the moon's northern hemisphere, where lakes are more numerous and larger than those in the southern hemisphere. Scientists using VIMS had confirmed the presence of liquid in Ontario Lacus, the largest lake in the southern hemisphere, in 2008.

The northern hemisphere was shrouded in darkness for nearly 15 years, but the sun began to illuminate the area again as it approached its spring equinox in August 2009. VIMS was able to detect the glint as the viewing geometry changed. Titan's hazy atmosphere also scatters and absorbs many wavelengths of light, including most of the visible light spectrum. But the VIMS instrument enabled scientists to look for the glint in infrared wavelengths that were able to penetrate through the moon's atmosphere. This image was created using wavelengths of light in the 5 micron range.

By comparing the new image to radar and near-infrared light images acquired from 2006 to 2008, Cassini scientists were able to correlate the reflection to the southern shoreline of a Titan lake called Kraken Mare. The sprawling Kraken Mare covers about 400,000 square kilometers (150,000 square miles). The reflection appeared to come from a part of the lake around 71 degrees north latitude and 337 degrees west latitude.

It was taken on Cassini's 59th flyby of Titan on July 8, 2009, at a distance of about 200,000 kilometers (120,000 miles). The image resolution was about 100 kilometers (60 miles) per pixel. Image processing was done at the German Aerospace Center in Berlin and the University of Arizona in Tucson.

Credit: NASA/JPL/University of Arizona/DLR

"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, Tucson, processed the image further, and scientists were able to compare the new image to radar and near-infrared-light images acquired from 2006 to 2008.

They were able to correlate the reflection to 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. It is located around 71 degrees north latitude and 337 degrees west latitude.

The finding shows 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, said Ralf Jaumann, a visual and infrared mapping spectrometer team member who leads the scientists at the DLR who work on Cassini. 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," Jaumann said. "But they also show us that liquid has a universal power to shape geological surfaces in the same way, no matter what the liquid is."

Source: Jet Propulsion Laboratory
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