2023-05-08 06:47:38
Saturn is one of the gaseous planets traversed by strong storms. Among these storms, “Saturn’s Hexagon” is a hexagonal cloud pattern that rotates continuously above the North Pole of the planet Saturn.
The perfectly straight sides of the hexagon measure approximately 13,800 kilometers… The hexagon is therefore a hurricane about 32,000 kilometers wide. For comparison, the diameter of the Earth is “only” 12,742 kilometers.
This exceptional cyclone changes little in terms of physiognomy in time and space and always resembles a hexagon, unlike the other clouds in the visible atmosphere which change their spatial organization constantly.
The Hexagon of Saturn was first discovered by the two probes of the Voyager program in 1981-1982, but the photos were not of very good quality. He 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 pictures in the infrared until January 2009. When the hexagon faced the Sun, it became observable in visible light, which made it possible to carry out a cyclone video 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 on Saturn and not elsewhere?
Like Jupiter and its red spot, Saturn represents for researchers a giant laboratory of astrophysical fluid mechanics.
Indeed, this very special hexagon must still obey the laws of physics. In general, an astronomical observation must be understood and explained from the angle of physics through a model (made of equations or experiments) to understand the phenomena involved.
Observation instruments in astronomy today give access to complex phenomena (like our hexagon) and to understand them, we need models that take into account the nature of the celestial bodies and the way in which they evolve. These being often gaseous, 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 changes?
Many works on the subject of the Saturnian hexagon have emerged. We can point out approaches of the numerical simulation type and even experimental.
One of the scenarios proposed is the following: Saturn, like Jupiter, is a gaseous planet and its unstable atmosphere is constantly confronted with complex flows comparable to storms, jets, currents and eddies, and this, regardless of the altitude.
And, precisely, the atmospheric flows low altitudes can create whirlpools of different sizes. Here, these outflows would surround a larger horizontal current that blows eastward around Saturn’s North Pole, and which itself consists of several smaller storms.
All these small storms confine the current towards the pole and distort certain jets into a hexagon. So this idea was transformed into a physical model and then simulated – but the simulations formed a geometry with nine sides, 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 faulty.
Another hypothesis is that hexagonal shapes develop where there is a very strong variation in the speed of atmospheric winds 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 at the center and at the periphery. The most common shape was six-sided (hence hexagonal), and shapes with three to eight sides were also produced.
However, these laboratory reproductions are “incomplete”. Indeed, they include vortices stabilizing the edges of the hexagons while that of Saturn is indeed independent of any stabilizing vortex.
The mysteries that produce the hexagon of Saturn are still far from being unveiled… especially since in 2018, a similar structure was observed at 300 kilometers south of the North Pole ! This daunting challenge seems destined to sharpen the creativity of astrophysical fluid dynamics researchers for a long time to come.
This article is produced by The Conversation and hosted by 20 Minutes.
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