UNRELIABLE weather forecasts are not restricted to Earth. On Saturn’s giant moon Titan, the seasons should now be turning, but summer in its southern hemisphere is defying predictions by refusing to end.
The , 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 Bonnie Buratti of the . “And it’s not as simple as it might have been.”
The arrival of the equinox should be accompanied by changes in Titan’s cloud cover. Currently the north pole has a thin shroud of cirrus cloud, probably made of fine crystals of frozen ethane, while substantial methane clouds can be seen in the southern hemisphere: a clump around the south pole and a thin temperate band about 40° south.
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These southern clouds are thought to be caused by convection in Titan’s atmosphere, driven by the heat of the sun. They were expected to disappear as Titan’s southern summer draws to a close, before reappearing in the northern hemisphere soon after.
However, the latest analysis of Cassini observations shows that the southern clouds have not budged. “Titan’s meteorology is still active in late summer, which does not agree with climate model predictions,” says Sebastien Rodriguez of the University of Nantes in France, who led the study (Nature, ).
“Titan’s meteorology is still active in late summer, which does not agree with climate model predictions”
Rodriguez suggests that the surface of Titan may be holding on to the heat of summer longer than anticipated, perhaps because a porous layer acts as an effective insulating blanket. But he concedes that this remains guesswork until the composition and structure of the surface are better known.
The observations also turned up another surprise: the clouds seem to be shaped in part by Saturn’s gravity. Rodriguez’s team found unexpected gaps in the temperate cloud belt at two points, one on the side of the moon facing towards Saturn and the other facing away. They suspect that atmospheric tides driven by Titan’s parent planet are somehow disturbing the convection at these points, discouraging clouds from forming.
“It does look like atmospheric tides are playing some role in cloud formation,” says of Lowell observatory in Flagstaff, Arizona. Roe believes that the temperate clouds are also feeding off some source of methane on the surface, perhaps a volcano or geyser. “I think the real story here, which we’re only just beginning to acknowledge within the field, is that Titan’s atmosphere is much more complicated than we ever imagined,” he says.
