Crab throws light on Titan's atmosphere

A RARE alignment of Saturn's enigmatic moon Titan with the Crab nebula has allowed astronomers to see X-rays flooding through the moon's atmosphere. The observations, which hint that Titan's atmosphere may extend much further out than expected, could help NASA mission controllers fine-tune their plans for the Cassini spacecraft, which is on its way to Saturn. The Crab nebula, in the constellation Taurus, is the splattered remains of a star that people on Earth saw exploding in AD 1054. Today high-energy electrons still zing around the magnetised debris, generating X-rays and making the Crab one of the brightest sources of X-rays in the sky. On 5 January 2003, a team led by Hiroshi Tsunemi of Osaka University in Japan, used the Chandra X-ray Observatory to witness Saturn's largest moon, Titan, crossing the face of the Crab. Astronomers calculate that this was probably the first time this had happened since the nebula formed in the Middle Ages. The chance alignment allowed the team to image X-rays from the Crab that had penetrated Titan's atmosphere. Titan is the only moon in the solar system with a thick atmosphere, which at its surface, is even denser than Earth's. NASA's Voyager 1 spacecraft measured the structure of Titan's atmosphere at altitudes below 500 kilometres and above 1000 kilometres when it flew past the moon in 1980, but Chandra's X-ray measurements gave astronomers their first chance to probe the region in between. Now the team has analysed the results.

In a paper due to appear in The Astrophysical Journal they will report that Titan's atmosphere may be up to 15 per cent more extended than when Voyager 1 flew by. This could be because the atmosphere is slightly warmer than it was in 1980, as Saturn is now 70 million kilometres closer to the sun. Mission planners for the Cassini spacecraft, due to enter Saturn's orbit in July, are assessing the results to see if they need to tweak the planned route. "Cassini is scheduled to make many passes through the upper atmosphere of Titan as part of its tour of the moons," says David Burrows who analysed the Chandra results. "If the atmosphere really is thicker than people have assumed, it could affect the amount of atmospheric drag on the spacecraft and they may have to modify their plans." It may also be necessary to alter the trajectory of the European Space Agency's Huygens probe, which is piggybacking on Cassini and will be released into Titan's atmosphere.

A camera on Huygens will capture more than 1100 images as it descends through the atmosphere, while five other instruments will sniff out its composition. ESA project scientist, Jean-Pierre Lebreton calls the new work a "wonderful observation" and says his team is checking to see what impact it might have on the probe's descent. Astronomers are on tenterhooks to find out what Huygens beams back. Titan's atmosphere, rich in nitrogen and methane, is thought to have a composition similar to Earth's atmosphere 4 billion years ago, before life evolved. "Titan provides us with an opportunity to travel back in time," says Tsunemi's colleague Koji Mori of Penn State University, who hopes the probe might provide clues to how terrestrial life emerged.

by Hazel Muir

New Scientist issue: 17 April 2004

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