The unique atmosphere of Saturn's moon Titan could have been created by comets blasting gases out of its icy crust.
Titan is the only moon in the solar system with much atmosphere, and the origin of its nitrogen-rich air is a puzzle. There are several theories: volcanic activity may have belched it out, or sunlight may have broken up a primordial atmosphere's ammonia molecules. But these suggestions assume that the young Titan was a warm world, whereas measurements by the Cassini spacecraft imply that Titan has always been fairly cold.
The latest idea is that the atmosphere was created 3.9 billion years ago in a period known as the late heavy bombardment, when comets swarmed through the solar system. "Huge amounts of cometary bodies would have collided with outer icy satellites, including Titan," says Yasuhito Sekine of the University of Tokyo, Japan.
To mimic the effects of such high-speed impacts, Sekine and his colleagues fired projectiles into a mixture of ammonia and water ice similar to Titan's crust. The impacts converted some of the ammonia into nitrogen gas, and Sekine's team calculates that ancient comet impacts could have liberated enough nitrogen to build Titan's atmosphere.
Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1147
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