titan
Source: MARK LEMMON

Titan, Saturn's largest moon, has an atmosphere often compared with that of our home planet, Earth. Among other things, Titan is made up mostly of nitrogen and boasts a wealth of organic material. And now researchers have discovered another Titan trait familiar to us Earthlings: rain. In today's issue of Science, Caitlin Griffith and Joseph Hall of Northern Arizona University and Thomas Geballe of the Gemini Observatory describe an analysis that suggests sparse clouds capable of producing showers over 1 percent of Titan every day. (Four views of the cold orb appear at the right.)

Because Titan's atmosphere is difficult to see through, researchers have collected data and images through narrow spectral "windows" for the past decade. From all these glimpses, only one earlier observation showed evidence of clouds: spectra recorded on September 4 and 5, 1995, hinted at the presence of a hurricane-size system. This time, the researchers recorded repeat observations--collecting 21 infrared spectra--in September 1999 and supplemented these data with five spectra from 1993 and 1997. They found characteristics of rain, sparse clouds and convective plumes, and clues suggesting that latent heat holds more sway over Titan's weather patterns than it does on Earth. Not knowing Titan's terrain, topography, winds or humidity makes it impossible to explain at the moment how these clouds form, the researchers note. But more information about the satellite's atmosphere should emerge when the Cassini spacecraft--which is now nearing Jupiter--swings by in 2004