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Astronomers Spy Surface Ice Through Titan's Haze

Titan, Saturn's moon



IMAGE COURTESY OF JET PROPULSION LABORATORY
Imagine Los Angeles on an especially smoggy summer day: the sun's otherwise-intense rays are muted, bounced back and forth off the particles in the air as if in a giant game of pinball. Light that does make its way through the dense atmosphere is unlikely to bounce off the planet's surface and make it out again. And so it goes on Saturn¿s moon Titan, where haze forms an atmosphere ten times as thick as the one on Earth. Because so little light can escape its atmosphere, Titan is shrouded by a nearly opaque curtain that has prevented planetary scientists from learning much about what lies beneath the haze. But work published today in the journal Science provides the clearest picture yet of Titan's surface. The findings indicate that the moon is covered, at least in part, by frozen water.

Previous research had left scientists divided over the composition of Titan's surface. At the temperatures recorded on the moon, water freezes and methane and other compounds that are gases on Earth would instead be liquids. These compounds could form vast oceans or rain that falls to Titan¿s surface and covers the moon with a thick layer of organic sediment, some scientists proposed. Others believed that water ice comprises much of the surface.

In order to probe Titan's landscape, astronomers use discreet wavelengths, or 'windows' of light, to peer through its atmosphere. Earlier studies used no more than four windows of infrared light. To get a refined view of the moon's surface, Caitlin Griffith of the University of Arizona¿s Lunar and Planetary Laboratory and her colleagues utilized eight different wavelengths. Using high-powered infrared telescopes, the team saw highly reflective material on the surface of Titan indicative of water ice. In addition, they did not see evidence for giant swaths of surface covered with organic precipitates.

The presence of ice on the surface could mean that the atmosphere is younger than the moon itself. If this is the case, estimates for how much organic matter should exist (based on the presumption that weather capable of producing organic-rain has existed since Titan formed 4.5 billion years ago) may be too high. Alternatively, Griffith says, it could mean a lot of organic sediment exists but there is enough surface activity--such as wind, rain and flowing streams--to move the sediment around. Although the new results cannot yet resolve these debates, they provide a solid foundation for work that will be done when the Cassini spacecraft reaches Titan in late 2004. Once the craft enters orbit, a probe will be sent down to the moon to relay information back to the mother craft, and then to Earth. "The orbiter will give us images of the surface so we¿ll see for the first time if there are oceans," Griffith says. The mission, she hopes, "will further lift the veil from our view of Titan."

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