Cassini’s parting gift: Hexagonal vortex hundreds of kilometers high discovered on Saturn

As the first anniversary of the climax of the Cassini mission approaches, the European Space Agency (ESA) has revealed the probe’s parting gift: evidence of a hundreds-of-kilometers tall, hexagonal vortex at Saturn’s north pole.

NASA’s Voyager mission first discovered the unusual northern polar structure in the 1980s but the Cassini mission afforded a more detailed, multi-wavelength perspective which unveiled the potential scale of this incredible structure – a high-altitude vortex with a hexagonal shape that extends hundreds of kilometers into the planet’s stratosphere.

Saturn’s stratosphere is where the majority of the planet’s weather systems interact.

Cassini could not initially observe the structure due to atmospheric temperatures of -158C (-252 Fahrenheit), some 20 degrees too cold for the probe’s Composite Infrared Spectrometer (CIRS) to view. A research team from the ESA conducted a long-term study of atmospheric conditions on the gas giant, with a particular focus on the unusual dynamics of the northern hemisphere.

The study’s co-author Sandrine Guerlet from Laboratoire de Météorologie Dynamique, France explained that winters on the planet are incredibly long as “one Saturnian year spans roughly 30 Earth years.” As a result, the team could only examine the northern stratosphere from 2014.

“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,” she added.

The researchers also noted that Saturn’s two poles appear to be asymmetric in nature, the northern vortex is not as mature as its southern counterpart and the two display different atmospheric dynamics.

Saturn’s northern hemisphere just passed its summer solstice in May 2017, and is on track to reach its autumn equinox some time in 2024, meaning the researchers can continue to study how this incredible natural megastructure develops over time and how it compares with a broad, warm, high-altitude vortex at Saturn's southern pole.

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“While we did expect to see a vortex of some kind at Saturn’s north pole as it grew warmer, its shape is really surprising,” says Leigh Fletcher of the University of Leicester, UK, lead author of the new study. “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."

The timing of the scientific revelation has proved somewhat bittersweet, however, as noted by Nicolas Altobelli, ESA Project Scientist for the Cassini-Huygens mission.

“The Cassini spacecraft continued to provide new insights and discoveries right up to the very end. Without a capable spacecraft like Cassini, these mysteries would have remained unexplored. It shows just what can be accomplished by an international team sending a sophisticated robotic explorer to a previously unexplored destination – with results that keep flowing even when the mission itself has ended," Altobelli said. 

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