With a dense atmosphere of nitrogen and methane, Titan is the only moon in our Solar System that shares Earth-like characteristics in climate. But Titan’s climate, receiving one hundred times less sunlight at ten times Earth’s distance from the Sun, operates at a much slower pace. The seasons on the distant moon last more than seven Earth years, and the motion of its clouds is slow and deliberate.

We’ve had a good look via the Cassini spacecraft at the movement of those clouds, some two hundred of them being examined between July 2004 and December 2007 in a study of global circulation patterns. Summer changes to fall at the equinox in August of this year. We’re at a time when the circulation models say clouds in the southern latitudes should have already disappeared, but it’s clear from the Cassini imagery that many clouds remained as late as 2007.


Image: This infrared image of Saturn’s moon Titan shows a large burst of clouds in the moon’s south polar region. These clouds form and move much like those on Earth, but in a much slower, more lingering fashion, new results from NASA’s Cassini Spacecraft show. Titan’s southern hemisphere still shows a very active meteorology (the cloud appears in white-reddish tones) even in 2007. According to climate models, these clouds should have faded out since 2005. Credit: NASA/JPL/University of Arizona/University of Nantes.

Sebastien Rodriguez (University of Paris Diderot), who has been working with the Cassini visual and infrared mapping spectrometer team, has this to say about the phenomenon:

“Titan’s clouds don’t move with the seasons exactly as we expected. 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 forecast by the models.”

Rodriguez’ comment reminds this jazz buff of the lush Woody Herman tune (with lyrics by Johnny Mercer) called ‘Early Autumn.’ I’ve always loved the Stan Getz version of this standard, but I’m listening to Jo Stafford’s rendition as I write, enjoying the juxtaposition of a jazz classic and imagery from an exotic moon around a ringed world. One of the pleasures of digital music is being able to pop up favorites on a whim.

Cassini’s extended mission will run until the early autumn of 2010, which will offer plenty of opportunities to monitor climate change on Titan — the spacecraft makes its next flyby of the moon on June 6. So expect to learn much more about sluggish weather on Titan, including whether it’s the result of a slow rate of temperature change at the surface and in the low atmosphere. Until then, I can’t help thinking that an autumn that lingers is something Johnny Mercer would have appreciated.