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.

“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.

“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.