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SATURN’s cloud-shrouded moon Titan is struggling to keep its secrets. French
astronomers who have peered through the mist using ground telescopes say they
have spotted polar caps and lofty mountains covered with methane ice. But an
American researcher says that images from the Hubble telescope paint a different
picture.

Athena Coustenis of the Paris-Meudon Observatory and her colleagues used a
telescope adapted to pick up infrared light from Titan. They discovered a wide,
bright region stretching one-sixth of the way around the moon’s equator, with
two or three even brighter spots dotted along it. “I think it could be a
mountain, a plateau with peaks,” says Coustenis. She believes the brightness is
caused by methane ice caps.

Coustenis has also spotted bright regions close to Titan’s poles, which she
suspects are polar caps, again of methane ice. The northern one is twice as
bright as its southern sibling. The differentiated surface rules out a
hydrocarbon ocean, which was once thought to cover Titan, she says. Instead, her
team’s observations suggest a surface mostly covered by water ice and
hydrocarbon lakes, Coustenis told the European Geophysical Society meeting in
Nice, France, last month.

Ralph Lorenz, a planetary scientist at the University of Arizona, agrees that
Titan’s large bright area could be mountainous. “I’m actually a big fan of
mountains on Titan,” he says. However, he doesn’t think they could be tall
enough for a topping of methane ice. Methane rainfall could brighten them by
washing away a dark coating of hydrocarbons, he says.

Lorenz is more sceptical about the possibility of polar caps on Titan. He
thinks that images from the Hubble Space Telescope suggest that Coustenis may
just be picking up regions of cloud. “I’d say that basically the data is not yet
good enough to be making strong conclusions,” he says.