NASA this week announced that it has photographed Saturn’s moon Titan “glowing in the dark.” The images were taken by the Cassini spacecraft while Titan was behind Saturn, in eclipse from the sun. They show a visible glow emanating from both the top of titan’s atmosphere and from deep in its atmosphere. The glow is only a millionth of a watt, and was detected using long exposure photographs.

“It turns out that Titan glows in the dark – though very dimly,” said Robert West, lead author of a recent study in the journal Geophysical Research Letters and a Cassini imaging team scientist at NASA’s Jet Propulsion Laboratory. “It’s a little like a neon sign, where electrons generated by electrical power bang into neon atoms and cause them to glow. Here we’re looking at light emitted when charged particles bang into nitrogen molecules in Titan’s atmosphere.”

Scientists had already studies the phenomenon, known as an airglow, which is caused when atoms and molecules are excited by ultraviolet sunlight or electrically charged particles. In Titan’s case, the airglow is from Titan’s nitrogen molecules, which were excited by X-rays and ultraviolet radiation from the sun. What has surprised scientists is the glow coming from deeper in Titan’s atmosphere.

“This is exciting because we’ve never seen this at Titan before,” said West. “It tells us that we don’t know all there is to know about Titan and makes it even more mysterious.”

The current guess as to the deep-atmosphere glow is that it’s being caused by deeper-penetrating cosmic rays or by light emitted by a chemical reaction deep in the atmosphere.

Scientists are interested in the glow because they are studying the chemical reactions that form the heavy molecules that make up Titan’s haze of organic chemicals. “This kind of work helps us understand what kind of organic chemistry could have existed on an early Earth,” said Linda Spilker, Cassini project scientist.

(Image courtesy NASA/JPL-Caltech/SSI)