While it may not be a colorful extravaganza like autumn in New England, astronomers have been awaiting the change of seasons on Saturn’s moon Titan with considerable anticipation. The Cassini space probe has been monitoring Titan since 2004, but as the moon’s year lasts 30 Earth years, it has only ever seen one season. The equinox marking the end of summer in the south will come this August. “This is our first opportunity to look at seasonal change,” says [researcher] Bonnie Buratti…. “And it’s not as simple as it might have been” [New Scientist].
Indeed, Cassini‘s observations have revealed some surprises in Titan’s atmosphere. The moon currently has clouds of methane (which sometimes produce methane rain showers) in its southern hemisphere. These southern clouds are thought to be caused by convection in Titan’s atmosphere, driven by the heat of the sun [New Scientist]. But astronomers’ models of Titan predicted that the southern clouds would have disappeared by now as the moon approaches its equinox. Says study coauthor Sebastien Rodriguez: “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” [UPI].
Clouds on Titan are extremely difficult to detect, because the thick stratospheric haze layer hides everything underneath, including any weather events and surface features. The high-altitude haze was discovered by the Voyager probes, which flew by Saturn in 1980-81 and were the only previous spacecraft to return close-range images of Titan before Cassini. Since then, scientists have realized that there are a few wavelength bands in the infrared range that can peer through the thick haze layer [Ars Technica].
The new study, published in Nature, doesn’t mark the end of scientists’ observations of the moon; using the Cassini orbiter, researchers will continue to peer through the haze at least past the equinox to 2010. After that, an extended-extended mission is currently under development, which will hopefully stretch the orbiter’s activities until the solstice in 2017, as the spacecraft is in excellent health and plenty of propellant is left onboard to continue the mission [Ars Technica].
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Image: NASA/JPL/University of Arizona/University of Nantes