Astronomers got to know it thanks to Voyager 2, when the spacecraft flew over the north pole of Saturn in 1981. The gigantic six-sided storm was again seen by Cassini-Huygens, starting in 2004: the spacecraft recorded this whirlwind hundreds of times , but it was never known who was responsible for its peculiar shape – until now. The cause of the Saturn Hexagon may be in the depths of the planet’s atmosphere.
Like other gas giants, Saturn has bands of atmospheric circulation, revealed by different colored bands: the lightest are cloud formations originating from rising hot gas streams; dark ones are descending gases. Winds can reach 1,800 km / h at the planet’s equator.
In its flights over the planet, Cassini-Huygens revealed that the spiral bands of clouds deeply penetrate the planet’s atmosphere, descending more than six thousand kilometers. The data sent by the probe before it disintegrated showed that the pressure in Saturn’s atmosphere can reach 100,000 bars; Sunlight reaches only the region in the upper atmosphere, where the pressure is only 1 bar (the same at sea level on Earth).
Giant 3D storm
Using this and other information sent by Cassini, Harvard University researchers created a new atmospheric model of the planet in 3D. The results showed that thermal convection (when the heat rises and the cold descends, generating a cyclical movement) in the depths of the atmosphere would generate gigantic polar winds; the interaction between different cyclones, anticyclones and latitudinal flows in different directions would create the hexagon.
The starting point for planetary scientist Rakesh Yadav and geophysicist Jeremy Bloxham, authors of the study, was the phenomenon known as Rossby waves. These waves, of planetary proportions, are permanent patterns of a planet’s atmosphere and depend on the distribution between the elements on the surface such as earth and water, the distance between the planet and the Sun, the movements of rotation and translation, the inclination of the planetary axis etc.
Among the proposed ideas, one was to apply this concept used on Earth to Saturn. In the laboratory, researchers have managed on several occasions that the Rossby waves take the hexagonal pattern like those of Saturn’s north pole.
A model for Saturn
Yadav and Bloxham, however, believed that this model did not match the conditions of the planet, since, on Earth, it is easily scalable: we know where the surface of the planet is. Saturn tells us another story, since we ignore how the planet’s internal atmosphere behaves.
The final model simulates a shell that encompasses the outermost 10% of Saturn’s radius. Cassini has sent countless data about the highest layers of the planet’s atmosphere, but almost nothing is known about what is going on towards the surface. For this reason, the study’s authors call this model a “proof of concept”.
“A similar scenario can be imagined for the planet, in which the hexagonal shape of the storm is supported by six adjacent large vortices, hidden by the most chaotic thermal convection in the shallower layers of the atmosphere”, say the authors in the work now published in Proceedings of the National Academy of Sciences of the United States of America (PNAS).
For Yadav and Bloxham, the new model should help to improve studies not only of Saturn’s atmosphere but also of other gas planets – Jupiter, mainly.