The continuous monitoring of Titan’s atmosphere by the Cassini mission, which has been exploring the Saturnian system since July 2004, is starting to reveal seasonal changes in the atmospheric circulation and shedding new light on the global climatology of the biggest moon of the ringed planet.

In a study that's being published in this week's issue of Nature, planetary scientists examined over 10,000 images captured by the Visible and Infrared Mapping Spectrometer instrument onboard Cassini, taken between July 2004 and December 2007. The study was conducted by an international team led by Sebastien Rodriguez of the University of Nantes in France. They spotted over 200 individual cloud activities in their data and analyzed the clouds’ spatial distribution, temporal variance, and spectral characteristics. Some of these individual events, including rain, have been previously reported, but the new report is the first time the global climatology of the satellite was examined with the intent of identifying seasonal changes in Titan’s atmosphere. It's the sort of study that's enabled by the vast observational data that the spacecraft has accumulated over the years.

Clouds on Titan are extremely difficult to detect, because the thick stratospheric haze layer hides everything underneath, including any weather events and surface features. The high-altitude haze was discovered by the Voyager probes, which flew by Saturn in 1980-81 and were the only previous spacecraft to return close-range images of Titan before Cassini. Since then, scientists have realized that there are a few wavelength bands in the infrared range that can peer through the thick haze layer, and the first map of the surface was made in 1994 when the Hubble Space Telescope peered through one of these so-called spectral windows.

Modeling the invisible

Even with the advances in observational techniques, the studies of Titan’s atmospheric circulation have been largely derived from climate models prior to the arrival of Cassini, as the observational data completely lacked spatial resolution and temporal coverage. Even with today’s state-of-the-art technologies, almost all clouds on Titan remain undetectable from ground- or space-based telescopes. So, scientists applied what little they knew about the moon—solar intensity, atmospheric composition, the rotation rate, and the orbital parameters—to predict the atmospheric conditions.

One of the most significant findings from these modeling efforts is that, with its day 15 times longer than on Earth, the slow rotation rate of Titan should cause its atmospheric circulation to differ radically from that of Earth. On Earth, the solar heat received near the equator is transported poleward through a large scale circulation system called the Hadley cells, which start at the equator and extend to about the 35 degree latitudes, making for two large symmetric convection cells in each hemisphere.

On Titan, scientists predict that the slow rotation rate weakens the Coriolis effect, which limits the latitudinal extent of the Hadley circulation on Earth. This means that, on Titan, a single Hadley cell can straddle the equator and transport the heat directly from the summer hemisphere to the winter hemisphere. As a result, Titan is expected to have one gigantic pole-to-pole convection cell. To date, this prediction, originally made in 1995, has not been confirmed.

Giving models a reality check

The new study is the first to compare the observed global cloud patterns to the models' predictions. Right now, Titan is in late northern winter and approaching an equinox in August 2009, when the northern hemisphere spring (and southern autumn) will begin; i.e., the summer and winter hemispheres will switch. So, if the predicted pole-to-pole Hadley circulation actually exists on Titan, the direction of the circulation should reverse around the time of the equinoxes. The scientists are looking for early signs of this circulation reversal in the data collected so far by Cassini.

In the new report, Rodriguez et al. show that many of the clouds observed by Cassini are consistent with the climate model predictions. As predicted, clouds are seen where the ascending point of the Hadley circulation should be in the southern mid-latitudes. Cumulus clouds are also observed at the south pole (still in summer), where the pole-to-pole circulation should drive methane storms. Stable clouds are found around the north polar region, where the stratospheric ethane should condense in the cold winter polar night.

On the other hand, the timing of the cloud events predicted by the models seem to be a little off. The models indicate that the cloud activities in the southern (summer) hemisphere should be weakening as the equinox approaches. Although the frequency of the southern hemisphere cloud events show hints of a decline, the south polar clouds should have completely disappeared by now according to the models' predictions, so the observed behaviors are different from the predicted gradual decline. The models also predict the presence of mid-latitude clouds in the northern (winter) hemisphere, but none are observed.

The discrepancies between predictions and observations are important clues that should improve our understanding of Titan’s atmospheric circulation. The authors reason that the timing of the drop in southern hemisphere cloud activity is a hint that the climate model does not yet incorporate all the seasonal heating effects the real system has. This points us to what should be the next steps in understanding what drives the atmospheric circulation on Titan. The absence of northern mid-latitude clouds also suggest that the pole-to-pole Hadley cell is more efficient than previously thought at transporting heat from the summer hemisphere to the winter one.

The plans for the Cassini orbiter, which is currently in its first extended-mission phase, will have it continue its mission at least past the equinox to 2010. After that, an extended-extended mission is currently under development, which will hopefully stretch the orbiter's activities until the solstice in 2017, as the spacecraft is in excellent health and plenty of propellant is left onboard to continue the mission.

With all the weather activities we are familiar with, including haze, cloud and rain, Titan makes an interesting case study for comparisons with Earth. The basic physics revealed through studying Earth should also be applicable to Titan, and what we learn from Titan must, of course, also apply to Earth. The continuing Cassini mission demonstrates that Titan is an excellent laboratory to challenge our understanding of the physics of climate.

The images used for this article are derived from a large animated GIF prepared by Cassini scientists that reveals the clouds and surface features of Titan as it rotates. Caution: that link will load a 9MB animation.

Nature, 2009. DOI: 10.1038/nature08014