Spring on Titan brings sunshine and patchy cloud

Titan’s northern hemisphere is set for mainly fine spring weather, with polar skies clearing since the equinox in August last year.  Cassini’s VIMS instrument has been monitoring clouds on Titan continuously since the spacecraft went into orbit around Saturn.  Now, a team led by Sébastien Rodriguez (AIM laboratory – Université 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.  Dr Rodriguez will be presenting the results at the European Planetary Science Congress in Rome on Wednesday 22nd September.

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.

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.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The visual and infrared mapping spectrometer team is based at the University of Arizona, Tucson.

IMAGES

	Left: T43 flyby of Titan – 12 May 2008 – VIMS images a large cloud that caps the north pole of Titan (yellowish tones). Right: T63 flyby of Titan – 12 December 2009 – VIMS still observes a huge cloud system at 40°S (yellowish tones) and the north pole of Titan free of clouds, a few months after the equinox. Credit: NASA/JPL/University of Arizona/University of Nantes/ University of Paris Diderot
	Caption: Fractional cloud coverage in Titan’s atmosphere integrated between July 2004 and April 2010. Black areas are cloud free and yellow are fully covered. Credit: NASA/JPL/University of Arizona/University of Nantes/ University of Paris Diderot

FURTHER INFORMATION

EUROPEAN PLANETARY SCIENCE CONGRESS (EPSC) 2010

EPSC 2010 is organised by Europlanet, a Research Infrastructure funded under the European Commission’s Framework 7 Programme, in association with the European Geosciences Union, with the support of the Italian National Institute for Astrophysics (INAF) and the INAF Institute of Physics of Interplanetary Space (IFSI) in Rome.  EPSC is the major meeting in Europe for planetary scientists. The 2010 programme comprises 48 sessions and workshops covering a wide range of planetary topics.
EPSC 2010 is taking place at the Angelicum Centre – Pontifical University of Saint Thomas Aquinas, Rome, Italy from Sunday 19 September to Friday 24 September 2010.
For further details, see the meeting website:
http://meetings.copernicus.org/epsc2010/

EUROPLANET RESEARCH INFRASTRUCTURE (RI)
Europlanet Research RI is a major (€6 million) programme co-funded by the European Union under the Seventh Framework Programme of the European Commission.
Europlanet RI brings together the European planetary science community through a range of Networking Activities, aimed at fostering a culture of cooperation in the field of planetary sciences, Transnational Access Activities, providing European researchers with access to a range of laboratory and field site facilities tailored to the needs of planetary research, as well as on-line access to the available planetary science data, information and software tools, through the Integrated and Distributed Information Service.   These programmes are underpinned by Joint Research Activities, which are developing and improving the facilities, models, software tools and services offered by Europlanet
Europlanet Project website: http://www.europlanet-ri.eu/
Europlanet Outreach/Media website: http://www.europlanet-eu.org//