Did Comets Create the Atmosphere of Saturn’s Moon, Titan?

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Saturn's moon, Titan, has been considered a “unique world in the solar system” since 1908 when, the Spanish astronomer, José Comas y Solá, discovered that it had an atmosphere, something non-existent on other moons. One of Saturn's 60 moons, Titan is the only moon in the solar system large enough to support an atmosphere.


Titan is the only moon in the solar system with a substantial atmosphere, and the origin of its nitrogen-rich air is a mystery. A new theory is that the atmosphere was created 3.9 billion years ago in a period known as the late heavy bombardment, when armadas of comets zipped 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.

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 concluded that ancient comet impacts could have liberated enough nitrogen to build Titan's atmosphere.

Titan's dense, nitrogen-methane atmosphere responds much more slowly than Earth's atmosphere, as it receives about 100 times less sunlight than Earth. Seasons on Titan last more than seven Earth years. Its clouds form and move much like those on Earth, but in a much slower, more lingering fashion.

Physicists from the University of Granada and University of Valencia, analyzing data sent by the Cassini-Huygens probe from Titan, have “unequivocally” proved that there is natural electrical activity on Titan. The world scientist community believes that the probability of organic molecules, precursors of life, being formed is higher on planets or moons which have an atmosphere with electrical storms.

Scientists with NASA's Cassini mission have monitored Titan's atmosphere for three-and-a-half years, between July 2004 and December 2007, and observed more than 200 clouds. They found that the way these clouds are distributed around Titan matches scientists' global circulation models. The only exception is timing — clouds are still noticeable in the southern hemisphere while fall is approaching.

"Titan's clouds don't move with the seasons exactly as we expected," said Sebastien Rodriguez of the University of Paris Diderot, in collaboration with Cassini visual and infrared mapping spectrometer team members at the University of Nantes, France. "We see lots of clouds during the summer in the southern hemisphere, and this summer weather seems to last into the early fall. It looks like Indian summer on Earth, even if the mechanisms are radically different on Titan from those on Earth. Titan may then experience a warmer and wetter early autumn than forecasted by the models."

On Earth, abnormally warm, dry weather periods in late autumn occur when low-pressure systems are blocked in the winter hemisphere. By contrast, scientists think the sluggishness of temperature changes at the surface and low atmosphere on Titan may be responsible for its unexpected warm and wet, hence cloudy, late summer.

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The Daily Galaxy via Nature Geoscience, DOI: 10.1038/ngeo1147

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