Thanks to the international Cassini spacecraft, a team of scientists has studied how the change of season on Titan, Saturn’s largest moon. After the equinox of August 2009, they generated a turbulent vortex in the atmosphere and increased the concentration of exotic gases at high altitude.
Titan is, next to Earth, the only other body in the solar system with a thick atmosphere rich in nitrogen. Titan’s atmosphere also contains methane and hydrogen, and traces of other gases among which hydrocarbons are formed at high altitude through reactions triggered by solar radiation.
These complex molecules leak into the lower layers of the atmosphere, and are sometimes combined resulting in a kind of orange smog.
On the limb of the moon you can see another layer of haze at an altitude even higher, at about 400-500 km above the surface of Titan, which appears to be completely separated from the rest of its atmosphere.
Until recently it was thought that this haze layer could be the upper limit of the flow cell of the ‘middle atmosphere’ of Titan, stretching from pole to pole, but a new study suggests that it is not.
When Cassini arrived at Saturn system in 2004, Titan had a vortex covered by a ‘hood’ of gas enriched by heavy fog and high above the North Pole, which at that time was in winter. Following the August 2009 equinox, spring came to the northern hemisphere of Titan, and came south in the fall.
The difference in the amount of solar radiation received each hemisphere now quickly resulted in the reversal of the direction of movement of the single cell Titan’s atmosphere, causing the upwelling of gases in the atmosphere of the summer hemisphere and the collapse of these in the winter.
“Although the amount of solar radiation reaching the Southern Hemisphere was diminishing, the first thing that we see in the first six months after the equinox was a rise in temperature at an altitude of between 400 and 500 kilometers above the surface Titan. This phenomenon was caused by compression of the gases as they sank into the atmosphere, driven by the new vortex that was beginning to form,” explains Nick Teanby, University of Bristol, UK, and lead author presenting these results in the journal Nature. “This is the same phenomenon that causes the air to warm compress to the bowler of a bicycle, and in this case was conclusive evidence that the change of season had begun.”
In the following months there was an increase of up to one hundred times the concentration of atmospheric gases over the North Pole, at this same level.
Cassini instruments were able to determine which molecules are sinking in the atmosphere at a rate of 1-2 millimeters per second.
The team of Teanby concluded that to which would produce an enrichment of the atmosphere at these altitudes, these complex molecules had to come from a level still higher, so the cloud layer could not be the upper limit of atmospheric circulation cell.
New data suggest that these complex molecules are generated at a height even greater, but when they descend to the altitude of 400-500 kilometers, is a change in their characteristics, probably caused by particle agglomeration.
“It is impressive to see a dramatic change of station on a moon in the solar radiation that is almost one hundred times less intense than on Earth,” adds Teanby. “Given that one year on Titan is almost 30 Earth years, a change from season to six months can be considered as a phenomenon extremely fast.”
“Mathematical models of 20 years ago and predicted this change in Titan’s atmospheric circulation, but Cassini has made the first direct observations of what is happening in reality,” says Nicolas Altobelli, Cassini Project Scientist for ESA.