A new Saturnian hexagon high above the clouds

Saturn's North Polar Hexagon

Using data captured by the Cassini spacecraft, scientists working at the University of Leicester have recently detected a high-altitude hexagonal feature, towering high above the clouds.

As part of a long-term study, they discovered a North Polar Stratospheric Vortex (NPSV). This newly discovered detail, abundant in hydrocarbons, could be influenced by a much larger feature bearing the same shape: the North Polar Hexagon (NPH). The vortex rises incredibly high into the atmosphere, possibly escalating to a height of 300 km above Saturn’s clouds.

“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,” says Leigh Fletcher of the University of Leicester, UK, lead author of the new study. “We simply need to know more. It’s quite frustrating that we only discovered this stratospheric hexagon right at the end of Cassini’s lifespan.” The Cassini mission ended in September 2017 with a controlled de-orbit into Saturn’s cloud tops.

Saturn's North Polar Stratospheric Vortex

We have known about the existence of a hexagonal feature at Saturn’s North Pole since Voyager 2 captured the first image of this unique feature back in 1981. In 2004, the Cassini-Huygens mission  embarked on a journey around Saturn that gave rise to many new discoveries. But Cassini had to wait five long years before the elusive North Polar Hexagon revealed itself once again.

A year at Saturn is almost 30 Earth years, meaning the seasons last around 7.5 years each. Cassini arrived at the Saturnian System when the northern climes were in the grip of winter. So Cassini had to wait until northern spring was well under way before acquiring its first glimpse of the NPH. These first images hinted at a pale blue-green, dynamic structure, shaped by Saturn’s jet stream. 

The cold temperatures of winter lingered well after the North Polar Hexagon had come back into view, meaning that multi-wavelength data capture was still not possible – with the region being too cold for infrared imaging. However, in 2014, temperatures had crept up enough to allow for the first infrared spectroscopy using Cassini’s Composite Infrared Spectrometer (CIRS) instrument.

Saturn's Hexagon as summer solstice approaches

By May 2017, with the Hexagon in full view, the colour had changed to golden hues of hazy yellow with just the central polar vortex keeping its blue-green tinge. This marked the summer solstice on Saturn. Between October 2014 and August 2017, CIRS was able to image the vortex which mapped a steady warming of Saturn’s stratosphere. 

“We were able to use the CIRS instrument to explore the northern stratosphere for the first time, from 2014 onwards,” commented co-author Sandrine Guerlet from Laboratoire de Météorologie Dynamique, France. “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.”

“Saturn’s northern hexagon is an iconic feature on one of the most charismatic members of the Solar System, so to discover that it still holds major mysteries is very exciting,” says Nicolas Altobelli, ESA Project Scientist for the Cassini-Huygens mission


© Melanie Davies 2018

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