A fluorescent glow high in the atmosphere of Titan, Saturn's largest moon, signifies the presence of a gas that astronomers have yet to identify. Data gathered by the Saturn-orbiting Cassini craft during Titan flybys show that the spectral emission is strongest at an infrared wavelength of about 3.28 micrometers. That wavelength is very near one where emissions of methane, a gas prevalent in Titan's atmosphere, are also strong—one reason that emissions from the unknown gas were previously obscured, the researchers note. The glow appears only on the daytime side of the moon at altitudes between 600 and 1250 kilometers , with the largest intensity occurring at an altitude of about 950 km, the team reports online and in a forthcoming issue of Geophysical Research Letters. (Although the gas is present throughout the atmosphere, it doesn't glow on the nighttime side of the moon because the fluorescence is produced only when sunlight strikes the gas molecules.) Detailed analyses reveal that the glow doesn't stem from a problem with Cassini's sensors, and it isn't associated with methane or any of the other hydrocarbons already identified as constituents of Titan's atmosphere. The yet-to-be-identified compound may belong to groups of molecules called polycyclic aromatic hydrocarbons or heterocyclic aromatic compounds, the researchers suggest, because the 3.28-micrometer emission is characteristic of those substances, all of which contain ring-shaped arrangements of atoms as part of their overall structure.