Cloudy but changeable. That is the outlook for Saturn's huge moon Titan, the only moon in the Solar System to have a thick atmosphere. The workings of that atmosphere are still poorly understood, but on page 575 of this issue, Griffith et al. report direct evidence of variable cloud cover. Our attempts to learn about Titan, particularly its lower atmosphere and surface, have proceeded by fits and starts. When the Voyager 1 spacecraft approached Saturn in 1980 and made the first spatially resolved observations of Titan, its cameras showed a body shrouded in an opaque, brownish haze that blocked the view of the surface at visible wavelengths. Information about Titan's surface and lower atmosphere was limited to spectra in the middle infrared, where a window near 18.5 μm allowed the surface temperature to be determined, and radio-occultation soundings, which yielded vertical profiles of temperature and pressure.
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