Icy Titan

photo: This painting shows the Huygens probe descending through some high clouds in Titan's murky atmosphere. ESA
photo: April 28, 2003

Astronomers peer through the haze enveloping Saturn's largest moon and find evidence of water ice on Titan's surface.In November 1997, NASA and the European Space Agency launched the Cassini spacecraft and its Huygens probe on its way to explore Saturn and its large, mysterious moon, Titan. Five and a half years later, we're still not sure what the Huygens probe will find when it descends through Titan's atmosphere to the moon's surface. But now, astronomers have taken a peek through some small spectral "windows" to learn that Huygens may land on a large region of icy bedrock when it touches down in January 2005.

Since its discovery in 1655, Titan has remained a mystery to scientists who have wondered what lies beneath the moon's thick, hazy atmosphere. Even after the Pioneer and Voyager spacecraft flew past Saturn in the late 1970s and early '80s, Titan appeared to be little more than a foggy, orange orb.

Still, scientists have managed to learn a few things about Titan's atmospheric cloak. Ten times as massive as Earth's atmosphere, it is primarily made of nitrogen and methane. Incoming ultraviolet light from the sun destroys methane molecules, leading to the formation of organic compounds, which scientists suspect produce an organic "rain" from Titan's methane clouds. Researchers have estimated that, if this process has been ongoing for the entire 4.6 billion years of Titan's existence, about 800 meters of liquid and solid sediment should blanket the surface, perhaps forming huge lakes or oceans.However, a team headed by University of Arizona planetary scientist Caitlin Griffith reports that not all of Titan's surface appears to be covered by these dark precipitates. Using the United Kingdom Infrared Telescope and NASA's Infrared Telescope Facility atop Mauna Kea in Hawaii, Griffith and her colleagues took advantage of several narrow, haze-piercing, infrared "windows" tucked among the strong methane bands of Titan's spectrum to study the moon's surface. They detected features characteristic of water ice, and the results reminded them of one of Jupiter's large, icy moons. "Titan's spectra resemble Ganymede's spectrum, dominated by ice features," they write in the April 25 issue of Science.



photo: This image of Titan is a product of observations taken with the Palomar 200-inch telescope, JPL adaptive optics system, and Cornell-built PHARO near-infrared camera. It shows dark and light surface features on Titan's leading hemisphere and possible clouds on the moon's southern limb.

"This is somewhat surprising because Titan is believed to have a lot of organic gook on its surface," Griffith adds.

The results may make sense in relation to Hubble Space Telescope and ground-based observations of Titan's surface. Since 1994, these near-infrared observations have revealed large, dark and light patches on Titan's surface.

"It's not clear what the darker material is, but one possibility is that it is these organic liquids and sediments," Griffith states. "The images, taken together with our results, suggest that organic stuff is moved around on the surface in such a way as to expose [bright] bedrock ice."