Titan’s seasons make sharp turn

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A true-colour image of the south pole vortex observed in Titan’s atmosphere at about 200–300 km altitude, as seen during a Cassini flyby of Saturn’s largest moon on 27 June 2012. Since equinox in August 2009, the seasons have been changing, becoming spring in the northern hemisphere and autumn in the southern hemisphere. The formation of the vortex over the south pole indicates the effect of the changing seasons on the circulation pattern in Titan’s atmosphere, specifically with cooler air sinking down from warmer, high altitudes. The images were obtained with the Cassini spacecraft narrow-angle camera at a distance of approximately 484,000 kilometres from Titan.

Credits: NASA/JPL–Caltech/Space Science Institute

28 November 2012
Scientists using the international Cassini spacecraft have studied the rapid change in seasons on Saturn’s moon Titan, following equinox in August 2009, which saw the formation of a swirling vortex and a build up of exotic gases at unexpectedly high altitudes.

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Titan is the only other body in the Solar System with a thick nitrogen-rich atmosphere like Earth’s. Titan’s atmosphere also contains methane and hydrogen, with trace amounts of other gases including hydrocarbons that form at high altitudes as a result of reactions with sunlight.

These complex molecules filter down into the lower atmosphere and eventually combine to produce an orange smog.

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A separate layer of haze is found at a much higher altitude of 400–500 km and can be seen at the limb of the moon, apparently detached from the rest of the atmosphere.

This haze was thought to represent the ceiling of Titan’s ‘middle atmosphere’ circulation which extends from pole to pole in one giant cell, but new results from Cassini suggest otherwise.

Artist’s impression of the change in observed atmospheric effects before, during and after equinox in 2009. The Titan globes also provide an impression of the detached haze layer that extends all around the moon (blue).

During the first years of Cassini’s exploration of the Saturnian system, Titan sported a ‘hood’ of dense organic gases (white) in a vortex above its north pole, along with a high-altitude ‘hot spot’ (red). During this time the north pole was pointed away from the Sun.

At equinox both hemispheres received equal heating from the Sun. Afterwards, the north pole tilted towards the Sun, signalling the arrival of spring, while the southern hemisphere tilted away from the Sun and moved into autumn.

After equinox and until 2011 there was still a significant build up of trace gases over the north pole, but the vortex winds had significantly reduced and the hot spot had almost disappeared. Instead, similar features began developing at the south pole, which are still present today.

These observations are interpreted as a large-scale reversal in the single pole-to-pole atmospheric circulation cell of Titan immediately after equinox, with an upwelling of gases in the summer hemisphere and a corresponding downwelling in the winter hemisphere.

This graphic is based on data from the Cassini mission, a partnership among NASA, ESA and the Italian Space Agency.

Credits: ESA/AOES

When Cassini arrived in the Saturn system in 2004, Titan sported a vortex with a ‘hood’ of enriched gas and dense haze high above its north, winter pole. After equinox in August 2009, spring arrived in the moon’s northern hemisphere while the southern hemisphere headed towards autumn.

The change in solar heating was reflected by a rapid reversal in circulation direction in Titan’s single pole-to-pole atmospheric cell, with an upwelling of gases in the summer hemisphere and downwelling in the winter hemisphere.

“Even though the amount of sunlight reaching the south pole was decreasing, the first thing we saw there during the six months after equinox was actually an increase in temperature at altitudes of 400–500 km, as atmospheric gases that had been lofted to these heights were compressed as they subsequently sank into a newly forming southern vortex,” says Dr Nick Teanby from the University of Bristol, UK, and lead author of the study reported in the journal Nature.

“This heating effect is the same one that causes compressed air in a bicycle pump to heat up, and provided the smoking gun that the change in seasons was underway.”

In the months that followed, up to a hundred-fold increase in atmospheric gas concentration was measured over the south pole at the same high altitudes.

Cassini’s instruments found that these gas molecules were sinking through the atmosphere at a rate of 1–2 millimetres per second.

Dr Teanby’s team conclude that for the enrichment and motion to be seen throughout these altitudes, the actual source of the complex gas molecules must be higher still, and that the detached haze layer cannot signal the top of the atmospheric circulation cell.

The new observations instead suggest that these complex haze molecules are produced higher up, but that when they drop down to the 400–500 km level, a change in the character of the haze takes place, perhaps as individual particles clump together.

“It’s impressive to see such dramatic solar-driven seasonal changes on a world where the sunlight is nearly a hundred times weaker than it is on Earth,” adds Dr Teanby.

“Since a year on Titan is nearly 30 Earth years long, for the atmosphere to change over a period of just six months is extremely rapid.”

“Models have predicted this change in Titan’s atmospheric circulation for nearly 20 years, but Cassini has provided the first direct observations of it actually happening,” says Nicolas Altobelli, ESA’s Cassini project scientist.

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