At last count, there were 170 moons in Earth's solar system. Of all these satellites, only Titan, the largest moon of Saturn, has a substantial atmosphere. Titan, like Earth, has a nitrogen-rich atmosphere. Earth's atmosphere is 78% nitrogen; Titan's is 98% nitrogen. Titan's climate includes a change of seasons like Earth. But there are a number of factors that make Titan's seasons much different. Titan is 10 times farther from the sun than Earth and therefore receives 100 times less solar energy. With its dense atmosphere and distant location, Titan experiences a much slower change in weather than here on Earth.
Cassini Observes Titan's Atmosphere
NASA's Cassini mission has been observing the clouds on Titan since July 2004. The distribution of the observed clouds has matched the scientists' predictions of global circulation models. However, the time frame is different than what what they were expecting.
Titan's seasons are quite long, lasting more than seven Earth years. Currently, Titan is headed toward its autumnal equinox in August 2009. Astronomers were expecting that clouds in its southern hemisphere should have faded away by now. Yet images from Cassini show that there are still many clouds in the south polar region.
"Titan's clouds don't move with the seasons exactly as we expected," says Sebastien Rodriguez of the University of Paris Diderot, who collaborated 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, Indian summer is a period in late fall when abnormally warm, dry air occurs because low-pressure systems are blocked in the winter hemisphere. But on Titan, Indian summer is a different situation. Scientists think the slow temperature changes at the surface and low atmosphere on Titan may be responsible for its unexpected warm, wet, and cloudy late summer.
Image of Late Summer Clouds on Titan
An infrared image taken by Cassini shows an active area of cloudburst taken on March 26, 2007. Climate models had predicted that the clouds should have faded away by 2005. Scientists observed more than 200 clouds between July 2004 and December 2007 and will continue to observe the long-term changes during Cassini's extended mission, which runs until the fall of 2010.
Source: NASA/Cassini
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