Spring on Titan brings sunshine and patchy cloud

Now, a team led by Sebastien Rodriguez (AIM laboratory -- Universite Paris Diderot) has used more than 2000 VIMS images to create the first long-term study of Titan's weather that includes the equinox, using observational data.

Together with Saturn in its 30-years orbit around the Sun, Titan has seasons that last for 7 terrestrial years. The team has observed significant atmospheric changes between July 2004 (early summer in the southern hemisphere) and April 2010, the very start of northern spring. The images showed that cloud activity has recently decreased near both of Titan's poles. These regions had been heavily overcast during the late southern summer until 2008, a few months before the 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. 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," said Dr Rodriguez.

The team has used results from the Global Climate Models (GCMs) developed by Pascal Rannou (Institut Pierre Simon Laplace) to interpret the evolution of the observed cloud patterns over time. Northern polar clouds of ethane form in the Titan's troposphere during the winter at altitudes of 30-50 km by a constant influx of ethane and aerosols from the stratosphere. In the other hemisphere, mid- and high-latitudes clouds are produced by the upwelling from the surface of air enriched in methane. Observations of the location and activity of Titan's clouds over long periods are vital in developing a global understanding of Titan's climate and meteorological cycle.

High resolution image

This pair of false-color images, made from data obtained by NASA's Cassini spacecraft, shows clouds covering parts of Saturn's moon Titan in yellow. Based on the way near-infrared channels of light were color-coded, cloud cover appears yellow, while Titan's hazy atmosphere appears magenta. The images show cloud cover dissolving from Titan's north polar region between May 12, 2008 (left), and Dec. 12, 2009 (right). The clouds in the second image appear around 40 degrees south latitude, still active late after Titan's equinox.

The data for the images was detected by Cassini's visual and infrared mapping spectrometer in near-infrared wavelengths. Scientists focused on three wavelengths of infrared radiation that were particularly good for observing cloud signatures and assigned them red, green and blue channels. Emissions in the 2 micron wavelength of light, colored red, detect the Titan surface. Emissions in the 2.11 micron wavelength, colored green, detect the lowest part of the Titan atmosphere, or troposphere. Emissions at the 2.21 micron wavelength, colored blue, detect the hazy stratosphere, a higher part of the atmosphere. The clouds appear yellowish because they lit up the channels designated red and green, but not the blue channel.

Credit: NASA/JPL/University of Arizona/University of Nantes/University of Paris Diderot




High resolution image

This graphic, constructed from data obtained by NASA's Cassini spacecraft, shows the percentage of cloud coverage across the surface of Saturn's moon Titan. The color scale from black to yellow signifies no cloud coverage to complete cloud coverage, over a period spanning July 2004 to April 2010. The data for this graphic came from Cassini's visual and infrared mapping spectrometer. Credit: NASA/JPL/University of Arizona/University of Nantes/University of Paris Diderot

Since Cassini reached Saturn, VIMS has acquired more than 20 000 images of Titan. The VIMS instrument consists of two detectors, one that maps in visible wavelengths and the other that maps in infrared, which also gather spectral information about the composition of observed targets. Rodriguez and his colleagues filtered the images to eliminate night-time and distorted views, ending up with around 2000 that were useful for identifying cloud activity. However, with the need to analyse each pixel in each image to identify the spectral properties of clouds, this was too vast a dataset for the team to evaluate manually.

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

In Feburary 2010, the Cassini mission was extended to a few months past Saturn's northern summer solstice in May 2017. This means that Rodriguez and his team will be able to observe the seasonal changes right the way through from mid-winter to mid-summer in the northern hemisphere.

"We have learned a lot about Titan's climate since Cassini arrived in at Saturn but there is still a great deal to learn. With the new mission extension, we will have the opportunity to answer some of the key questions about the meteorology of this fascinating moon," said Rodriguez.

Source: Jet Propulsion Laboratory
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