UNRELIABLE weather forecasts are not restricted to Earth. On Saturn鈥檚 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鈥檚 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. 鈥淭his is our first opportunity to look at seasonal change,鈥� says Bonnie Buratti of the . 鈥淎nd it鈥檚 not as simple as it might have been.鈥�
The arrival of the equinox should be accompanied by changes in Titan鈥檚 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掳 south.
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These southern clouds are thought to be caused by convection in Titan鈥檚 atmosphere, driven by the heat of the sun. They were expected to disappear as Titan鈥檚 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. 鈥淭itan鈥檚 meteorology is still active in late summer, which does not agree with climate model predictions,鈥� says Sebastien Rodriguez of the University of Nantes in France, who led the study (Nature, ).
鈥淭itan鈥檚 meteorology is still active in late summer, which does not agree with climate model predictions鈥�
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鈥檚 gravity. Rodriguez鈥檚 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鈥檚 parent planet are somehow disturbing the convection at these points, discouraging clouds from forming.
鈥淚t does look like atmospheric tides are playing some role in cloud formation,鈥� 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. 鈥淚 think the real story here, which we鈥檙e only just beginning to acknowledge within the field, is that Titan鈥檚 atmosphere is much more complicated than we ever imagined,鈥� he says.
