The surface of Titan, Saturn’s largest moon, may contain frozen water. Caitlin Griffith at the University of Arizona and colleagues at the Institute for Astronomy in Honolulu, the Gemini Observatory and the Université Pierre et Marie Curie measured the infrared spectrum of the moon’s surface and detected the tell-tale characteristics of water ice. This result may overturn the hypothesis that Titan’s surface is completely covered by a thick layer of organic liquids and solids (C A Griffith et al. 2003 Science 300 628).
The atmosphere of Titan is made up of a thick haze, about 800 metres deep, of methane, nitrogen and carbon dioxide. This layer obscures the surface and makes it difficult to detect what lies below. Previous studies focused on a small range of wavelengths but spectral peaks that are characteristic of surface compounds only show up at a larger range of wavelengths.
It is known from the Voyager mission that the atmosphere of Titan becomes more transparent in the near infrared part of the spectrum. Griffith and co-workers have now made multiple measurements between 0.8 and 5.1 μm using the United Kingdom Infrared Telescope (UKIRT) and NASA’s Infrared Telescope Facility (IRTF). They investigated the reflectivity – the fraction of light reflected – of Titan’s surface at certain narrow wavelength ‘windows’ to catch glimpses of the surface where it is not masked by the atmosphere.
The team measured reflectivities at eight separate wavelengths of 0.83, 0.94, 1.07, 1.28, 1.58, 2.0, 2.9 and 5.0 μm. “These values, if taken together, indicate the presence of water ice,” Griffith told PhysicsWeb. In fact, Titan’s spectrum resembles that of Ganymede – Jupiter’s largest satellite – which is dominated by ice features. Below 1 μm, dirty water ice features like those found on many of Jupiter’s other satellites were also seen. Moreover, the reflectivities did not match those of the organic sediments that had been expected.
