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Seasons of change on Titan
Posted: 22 September 2010

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Using some 2,000 Cassini images, planetary scientists are putting together a picture of Titan's seasons, which last seven Earth years.

Thanks to monitoring by the Visual and Infrared Mapping Spectrometer (VIMS), a team of planetary scientists led by Sebastien Rodriguez of the AIM laboratory at the Universite Paris Diderot have extracted around 2,000 images from a total of 20,000 collected since the mission began, to study long-term patterns in Titan's atmosphere.

Comparison of cloud cover between the T43 and T63 flybys. In May 2008 a large cloud caps the north pole. By December 2009 the north pole is cloud free, but a huge cloud system is present at 40 degrees south. Image: NASA/JPL/University of Arizona/University of Nantes/ University of Paris Diderot.

“Even having eliminated 90 percent of the images, we were still left with several million spectra to analyse,” says Rodriguez. “We developed a computer program that picked out the cloudy pixels and we then went back and visually checked the detections to make sure that they were relevant.”

Significant changes were noted between July 2004 and April 2010, which coincides with early summer in the southern hemisphere to spring in the northern hemisphere. Cloud activity has also recently decreased over both poles, having been heavily overcast during late southern summer until 2008, as Saturn approached equinox.

Fractional cloud coverage in TitanŐs atmosphere integrated between July 2004 and April 2010. Black areas are cloud free and yellow are fully covered. Image: NASA/JPL/University of Arizona/University of Nantes/ University of Paris Diderot.

“Over the past six years, we’ve found that clouds appear clustered in three distinct latitude regions of Titan: large clouds at the north pole, patchy cloud at the south pole and a narrow belt around 40 degrees south,” describes Rodriguez. “However, we are now seeing evidence of a seasonal circulation turnover on Titan – the clouds at the south pole completely disappeared just before the equinox and the clouds in the north are thinning out. This agrees with predictions from models and we are expecting to see cloud activity reverse from one hemisphere to another in the coming decade as southern winter approaches.”

Combining the observations with global climate models developed by Pascal Rannou of the Institut Pierre Simon Laplace, allowed the team to evaluate the evolution of the observed cloud patterns over time. They found that, during winter, northern polar clouds of ethane form at altitudes of 30-50 kilometres from a constant influx of ethane and aerosols from higher altitudes. Meanwhile in the southern hemisphere, mid- and high-latitudes clouds are produced by the upwelling of air enriched in methane from the moon's surface.

Now that Cassini's mission lifetime has been extended through to May 2017, scientists will be able to observe Titan from mid-winter to mid-summer in the northern hemisphere. Long-term monitoring of clouds and atmospheric phenomena is crucial for building an accurate picture of the moon's global climate.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!


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