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As Winter Sets In, Titan Continues to Surprise

Imaged by Cassini in January 2012, the swirling vortices in Titan’s southern hemisphere may be a herald of the arrival of summer. Photo Credit: NASA/JPL

More than three decades since the Voyagers beheld the opaque orange haze of its thick atmosphere and invited scientific inquiry into the nature of what – surface, slush or seas – lay beneath, Titan continues to serve up unending surprises and mysteries. Since the summer of 2009, the tilt of its orbit around Saturn has caused its northern and southern hemispheres to effectively exchange seasons…and produce startling climatic change.

With an equatorial diameter of 5,150 km, Titan is bigger than Mercury and a little smaller than Jupiter’s moon Ganymede, making it the second-largest natural satellite in the Solar System. It is also the only moon known to possess a dense atmosphere – a thick, nitrogen-rich soup, laced with methane and various hydrocarbons – and since the arrival of the Cassini spacecraft in July 2004 it has only grudgingly surrendered its secrets.

Titan’s northern hemisphere was in the dead of winter when Cassini arrived, whilst its southern hemisphere was enjoying a relatively balmy summertime. Encircling the north pole was a large, brownish ‘hood’ of particulate haze. This hood was nothing new: the Voyagers had seen it during their encounters. However, since Saturn’s equinox in August 2009, the Titanian north progressed from winter into spring and the south from summer into autumn. Today, three years later, new Cassini results indicate the presence of a high-altitude aerosol haze and a swirling vortex in the southern hemisphere, prompting scientists to speculate that it heralds the long-anticipated start of winter.

It is hoped that the discovery may enable better testing of atmospheric photochemistry and circulation models. Earlier this year, Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) acquired false-colour images of the southern aerosol haze, which lingers some 320 km above the surface. This discovery came as a surprise, said VIMS team member Christophe Sotin of NASA’s Jet Propulsion Laboratory in Pasadena, California, since aerosols were previously unobserved at this altitude and latitude.

Titan’s northern polar hood (top) has been observed by the Voyagers and Cassini, although the onset of summer in the southern hemisphere promises to create similarly dynamic phenomena. Photo Credit: NASA/JPL

Fellow team member Tony Del Genio of NASA’s Goddard Institute for Space Studies in New York has described the vortices as “reminiscent of the open cellular convection that is often seen over Earth’s oceans”…but with one significant exception. “Unlike on Earth, where such layers are just above the surface,” he said, “this one is at very high altitude; maybe a response of Titan’s stratosphere to seasonal cooling as southern winter approaches.”

Although Titan’s northern hood remains present, atmospheric circulation appears to be driving particulate haze from the steadily warming north towards the steadily cooling south. It is hypothesised that heightened levels of sunlight are responsible for generating atmospheric uplift, which may soon precipitate the formation of ethane-bearing clouds. As scientists continue to study changes in the Titanian south, it can be hoped that Cassini’s data will play a crucial role in understanding the climate, energy balance and weather of this strange world.

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