With blistering heatwaves in the U.S. and devastating floods in Europe and Asia, perhaps we can take some comfort in knowing the solar system has places with far more inhospitable weather than Earth. Take Saturn's moon Titan. It's the only rocky world we know of that may have something like Earth weather, with oceans and precipitation, only in this case the suspected oceans are ethane. On Titan, a poorly-understood photochemical smog layer always blocks out about 90 percent of the faint sunlight reaching the distant moon. Fine droplets of methane and ethane, perhaps laced with hydrogen cyanide and acetylene, are thought to perpetually fall to the cloud-shrouded surface.
But the details of Titan's bizarre weather have been elusive. Although the dense smog permeates most of the atmosphere, it is not uniformly distributed. There is a hemispheric dichotomy in its density and a mysterious, distinctive upper layer of smog in its stratosphere. Discovered by Voyager in 1980, this "detached layer" has resisted explanation — until now.
In a paper published August 22nd in Nature, Pascal Rannou and Frederic Hourdin (University of Paris) and Christopher P. McKay (NASA-Ames Research Center) report on a new global circulation model for Titan that seems to solve most of the mysteries. It involves a seasonal transport pattern that changes over the course of Saturn's 30-year-long trek around the Sun.
Titan's global circulation apparently begins with an upwelling of warmer air from the summer hemisphere, especially in the polar area. This air moves at high altitude toward the winter pole where it eventually cools and descends, returning at low altitude toward the sunward pole. As the airmass conveyor belt moves, it builds up a layer of smog at the top of the stratosphere as ultraviolet light dissociates methane and molecular nitrogen. As Titan's seasons change during the course of Saturn's orbit, the circulation pattern reverses.
The smog provides a tracer of the atmospheric circulation. Particles grow larger and descend slowly over time, so that as the airflow reverses direction with the change of seasons, the smog layer folds on itself, coming back lower down than it was the first time. This leaves a gap in between the upper, detached layer and the layers below.
McKay explains that as new haze forms, the strong upper-atmosphere wind is "carrying it to the pole so fast that it doesn't have time to fall down to the layer below it."
"People have been trying to [explain this] since Voyager, so it is a very nice result," says Caitlin Griffith (University of Arizona), a specialist in planetary atmospheres. Her paper in Science two years ago provided observational evidence for a methane-based cycle of evaporation and precitation at Titan's surface.
The new modelling work comes close on the heels of another significant step in understanding Titan's enigmatic climate, in this case, the lower atmosphere. A three-dimensional circulation model, described by Tetsuya Tokano (Institute of Space Simulation, Germany) and Fritz Neubauer (Institute of Geophysics and Meteorology, Germany) in the August issue of Icarus, revealed that Titan has a unique, tidally-driven wind system that dominates circulation in its lower level, the troposphere. The Saturnian tides have little effect on the stratospheric circulation patterns, according to this model.
Griffith says that it is usually hard to determine upper-atmosphere circulation patterns in planetary atmospheres, but Titan's smog provides a useful tracer. That means that when the Cassini spacecraft arrives at Saturn in 2004, not only will we get direct measurements of wind profiles on Titan from the Huygens probe, but it should be possible to monitor circulation over time through direct imaging, allowing long-term correlation with the new models so they can continue to be refined.