• A hexagonal cloud pattern is constantly rotating over the north pole of the planet Saturn, according to our partner The Conversation.
  • Like Jupiter and its red spot, Saturn represents for researchers a giant laboratory of astrophysical fluid mechanics.
  • This analysis was conducted by Waleed Mouhali, professor and researcher in Physics at the École centrale d'électronique (ECE Paris).

Saturn is one of the gaseous planets traversed by strong storms. Among these storms, the "hexagon of Saturn" is a hexagonal cloud pattern that rotates permanently over the north pole of the planet Saturn.

The perfectly straight sides of the hexagon measure about 13,800 kilometers... France is therefore a hurricane about 32,000 kilometers wide. For comparison, the diameter of the Earth is "only" 12,742 kilometers.

This exceptional cyclone evolves little in terms of physiognomy in time and space and always resembles a hexagon, unlike other clouds in the visible atmosphere which change their spatial organization permanently.

Saturn's hexagon was first discovered by the two probes of the Voyager program in 1981-1982, but the photos were not of very good quality. It was studied again by the Cassini-Huygens mission in 2006.

As on Earth, the poles face the sun only in certain seasons (in summer for the North Pole for example); The rest of the time, they are plunged into darkness... especially since, on Saturn, a season lasts about seven years.

Thus, Cassini was only able to take infrared photos until January 2009. When the hexagon faced the Sun, it became observable in visible light, which made it possible to make a video of the cyclone and also to complete the information that feeds the work of astrophysicists, with more complete optical spectra, in the visible and infrared.

Why is the only hexagon-shaped hurricane over Saturn and not elsewhere?

Like Jupiter and its red spot, Saturn represents for researchers a giant laboratory of astrophysical fluid mechanics.

Indeed, this particular hexagon must still obey the laws of physics. In general, an astronomical observation must be understood and explained from the point of view of physics through a model (made of equations or experiments) to understand the phenomena involved.

Observation instruments in astronomy today provide access to complex phenomena (such as our hexagon) and we need, to understand them, models that take into account the nature of celestial bodies and how they evolve. These are often gaseous, so we speak of "fluid mechanics".

The recent development of astrophysical fluid mechanics is essentially linked to that of numerical simulation, which makes it possible to explore situations never observed in the laboratory or in space: for example, what conditions are necessary to observe a hexagonal cyclone? How would the cyclone react if the wind direction changed?

Many works on the subject of the Saturnian hexagon have emerged. We can point out approaches such as numerical and even experimental simulations.

One of the proposed scenarios is as follows: Saturn, like Jupiter, is a gaseous planet and its unstable atmosphere is constantly confronted with complex flows similar to storms, jets, currents and eddies, regardless of altitude.

And, precisely, low-altitude atmospheric flows can create eddies of different sizes. Here, these flows would surround a wider horizontal current that blows eastward around Saturn's North Pole, which itself consists of several storms smaller in size.

All these small storms confine the current to the side of the pole and deform some hexagon jets. So this idea was turned into a physical model, then simulated – but the simulations formed a nine-sided geometry, instead of the 6 observed. On the other hand, the stability of this geometry proves that the mechanism envisaged, without giving the observed result, is not necessarily defective.

Another hypothesis is that hexagonal shapes develop where there is a very strong variation in atmospheric wind speeds at certain latitudes in Saturn's atmosphere. Similar regular shapes could be created in the laboratory by rotating a fluid in a circular tank at different speeds in the center and periphery. The most common shape was six-sided (hexagonal), and three- to eight-sided shapes were also produced.

OUR "SATURN" DOSSIER

However, these laboratory reproductions are "incomplete". Indeed, they include vortices stabilizing the edges of hexagons while that of Saturn is indeed independent of any stabilizing vortex.

The mysteries that produce Saturn's hexagon are still far from being revealed... especially since in 2018, a similar structure was observed 300 kilometers south of the North Pole! This major challenge seems destined to sharpen the creativity of researchers in astrophysical fluid dynamics for a long time to come.

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This article is produced by The Conversation and hosted by 20 Minutes.

  • Sciences
  • Video
  • Saturn
  • Solar system
  • Hurricane
  • Astronomy
  • Physics
  • Astrophysics