Cassini mission: Towering hexagonal vortex spotted on Saturn

Scientists of the Cassini Mission have discovered a unique hexagonal vortex on Saturn’s north pole—a towering atmospheric structure of whirling fluids that spans hundreds of kilometres in height.

The Cassini mission—which crashed into the Saturn last year to end its two-decade long journey—spotted the feature emerging at the ringed planet’s northern pole as it nears summertime.

This warm vortex sits hundreds of kilometres above the clouds, in a layer of atmosphere known as the stratosphere, and reveals an unexpected surprise.

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“The edges of this newly-found vortex appear to be hexagonal, precisely matching a famous and bizarre hexagonal cloud pattern we see deeper down in Saturn’s atmosphere,” said Leigh Fletcher of the University of Leicester in the UK.

“Either a hexagon has spawned spontaneously and identically at two different altitudes, one lower in the clouds and one high in the stratosphere, or the hexagon is in fact a towering structure spanning a vertical range of several hundred kilometres,” he said.

Saturn’s cloud levels host the majority of the planet’s weather, including the pre-existing north polar hexagon.

This feature was discovered by NASA’s Voyager spacecraft in the 1980s and has been studied for decades; it is a long-lasting wave potentially tied to Saturn’s rotation, a type of phenomenon also seen on Earth in structures such as the Polar Jet Stream.

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Its properties were revealed in detail by Cassini, which observed it in multiple wavelengths using instruments including its Composite Infrared Spectrometer (CIRS).

However, at the start of the mission this instrument could not peer further up in the northern stratosphere, which had temperatures around minus 158 degrees Celsius—some 20 degrees too cold for reliable CIRS infrared observations—leaving these higher-altitude regions relatively unexplored for many years.

“One Saturnian year spans roughly 30 Earth years, so the winters are long,” said Sandrine Guerlet from Laboratoire de Meteorologie Dynamique, France.

“Saturn only began to emerge from the depths of northern winter in 2009, and gradually warmed up as the northern hemisphere approached summertime,” said Guerlet.

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A strange process at play within Saturn’s atmosphere sped up this warming: as air sank at the north pole, the upper hexagon warmed increasingly quickly, and the transport of air downwards made the abundance of several minor species more concentrated.

“As the polar vortex became more and more visible, we noticed it had hexagonal edges, and realised that we were seeing the pre-existing hexagon at much higher altitudes than previously thought,” said Guerlet.

The presence of a hexagon way up in Saturn’s northern stratosphere, hundreds of kilometres above the clouds, suggests that there is much more to learn about the dynamics at play in the gas giant’s atmosphere.