A flash of sunlight reflecting off a lake on Saturn's largest moon Titan confirms the presence of liquid in the moon's northern hemisphere.

Scientists have been on the lookout for the glint – known as specular reflection – since the Cassini spacecraft began orbiting Saturn in 2004, but Titan's northern hemisphere had been in winter darkness until the onset of spring this year. While Titan's thick atmosphere blocks out reflections of sunlight at most wavelengths, this image was captured using Cassini's visual and infrared mapping spectrometer.

“This one image communicates so much about Titan – thick atmosphere, surface lakes and an otherworldliness,” says Cassini project scientist, Bob Pappalardo. “It’s an unsettling combination of strangeness yet similarity to Earth. This picture is one of Cassini’s iconic images.”

Large hydrocarbon lakes have been theorized on Titan's surface for many years, and Cassini confirmed the presence of large lakes in both northern and southern hemispheres. Last year, liquid was confirmed in the southern hemisphere's largest lake, but the glint seen in the new image is the first confirmation of liquid in the northern hemisphere, which hosts larger lakes.

“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,” says Katrin Stephan, an associate member of the Cassini visual and infrared mapping spectrometer team. “But we also had to do more work to make sure the glint we were seeing wasn’t lightning or an erupting volcano.”

By processing the image further, and comparing the image to earlier radar and near-infrared-light images the reflection was matched up to the location of the southern shoreline of a lake called Kraken Mare, located 71 degrees north and 337 degrees west, and covering an area an impressive 400,000 square kilometres.

“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,” says Ralf Jaumann, a visual and infrared mapping spectrometer team member who leads the scientists at the German Aerospace Center who work on Cassini. But whereas Earth's hydrological cycle is water-based, Titan's is methane. “These results remind us how unique Titan is in the Solar System. 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.”

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