Crab's X rays probe Titan.

While observing a rare celestial alignment, astronomers made the first X-ray measurement of the atmosphere of Titan, Saturn's most tantalizing moon. Titan is the only moon in the solar system known to have an atmosphere, and the new study suggests that its atmosphere could be bigger than previous observations had indicated.

Scientists preparing for the robotic Cassini mission, scheduled to begin touring Saturn and its moons this July, are paying close attention to the findings for guidance on the craft's flight plan.

On Jan. 5, 2003, Titan passed in front of an X ray-spewing supernova remnant known as the Crab nebula. During that passage, or transit, some of the Crab's X rays were absorbed by Titan's atmosphere. Koji Mori of Pennsylvania State University in State College and his colleagues captured the transit with NASA's orbiting Chandra X-ray Observatory.

From characteristics of the orbits of Titan and Saturn, Mori's team calculates that the 2003 event is the first time Titan has passed in front of the Crab nebula, which is only 950 years old. This transit is much less common than the upcoming passage of Venus in front of the sun (seep. 247).

The team's study reveals that the upper reaches of Titan's atmosphere, the region that absorbs X rays, may extend as far as 880 kilometers above the moon's surface, Mori's team reports in the June 1 Astrophysical Journal. If the finding holds up, it would indicate that Titan's atmosphere is 10 to 15 percent larger than it was in 1980 when the Voyager 1 spacecraft recorded radio, infrared, and ultraviolet data there.

With the Cassini spacecraft fast approaching Saturn, scientists want to keep it from encountering unexpected drag or torque from Titan's atmosphere. During its 4-year tour, Cassini will skim Titan's atmosphere some 44 times, both to study the moon and to get gravity boosts to help the craft stay on course.

If Titan's atmosphere indeed extends farther than previously measured, Cassini may have to fly at a slightly higher altitude above Titan than currently planned, says mission scientist Roger Yelle of the University of Arizona in Tucson. The newly determined traits of the upper atmospheres won't affect the descent in January 2005 of a packet of instruments that Cassini will release into Titan's atmosphere. Most of the braking action for this delivery to the moon's surface will occur in the lower atmosphere.

Combined with previous studies, which indicate that Titan's lower atmosphere hasn't changed significantly since Voyager 1 flew past the moon, the new findings suggest that Titan's upper atmosphere is bigger, denser, and about 60 kelvins warmer than it was in 1980.

Cassini researcher Darrell F. Strobel of Johns Hopkins University in Baltimore suggests that the upper atmosphere's temperature could be influenced by Saturn's magnetosphere. This vast magnetic bubble shields the planet from bombardment by the sun's wind of charged particles. In 1980, Titan resided within Saturn's magnetosphere, but it may not have been protected in that way in 2003. If Titan was outside the magnetosphere, its atmosphere would have been heated by energetic particles in the solar wind, Strobel says.