‘Towering hexagon’ and auroras: Saturn’s north polar wonders

The already well-known hexagon at Saturn’s north pole is a massive jet stream in the atmosphere, similar to the Polar Jet Stream on Earth, but much larger. A new study shows that a “secondary” hexagon extends above this one for hundreds of miles. Photo Credit: NASA/JPL-Caltech/Space Science Institute

Saturn’s north pole is an incredibly active place, data from NASA’s Cassini mission and the Hubble Space Telescope have shown, with its massive hexagonal weather system and powerful auroras. Now, a new study, published Sept. 3 in Nature Communications, has shown that not only is there the main hexagon in the lower layers of the atmosphere, as already seen, but also a related hexagon pattern above that, towering over the surrounding clouds. This structure is thought to extend hundreds of miles in height. The new images from Hubble, meanwhile, show Saturn’s spectacular auroras in ultraviolet light, fluttering in the upper atmosphere just like ones on Earth, but much more intense.

The hexagon discovery is intriguing, since it appears that the higher-altitude hexagon is massive, hundreds of miles tall. The observations show that it warms as the north pole of Saturn approaches summertime. This high-altitude vortex towers above the surrounding cloud decks, extending into the stratosphere. Like the well-known hexagon formation lying below it, it also has a distinct hexagonal shape.

“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, lead author of the new study.

Closer, detailed view from directly overhead of the main hexagon at Saturn’s north pole, in enhanced color. Image Credit: NASA/JPL-Caltech/SSI/Hampton University

When Cassini started making its observations, with its Composite Infrared Spectrometer (CIRS), the instrument could not peer further up into the northern stratosphere, which had temperatures around -158 degrees Celsius – 20 degrees too cold for reliable CIRS infrared observations. This meant that these higher-altitude regions would be left relatively unexplored for many years.

“One Saturnian year spans roughly 30 Earth years, so the winters are long,” said study co-author Sandrine Guerlet from Laboratoire de Météorologie 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.”

But starting in 2014, Cassini was able to view the region for the first time.

“We were able to use the CIRS instrument to explore the northern stratosphere for the first time, from 2014 onwards,” Guerlet said. “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.”

Brightness temperature maps of Saturn’s north polar stratosphere, from 2013-2017. The images depict indicates the hexagonal boundary to the polar stratospheric hood. Image Credit: Nature Communications

There is also a warm, high-altitude vortex at Saturn’s south pole, but it is not hexagonal. The hexagon phenomenon seems to occur only in the north polar region.

“This could mean that there’s a fundamental asymmetry between Saturn’s poles that we’re yet to understand, or it could mean that the north polar vortex was still developing in our last observations and kept doing so after Cassini’s demise,” said Fletcher.

The hexagons seem weird, but they are basically the Saturnian equivalent of the Polar Jet Stream on Earth – a long-lasting atmospheric wave, the cause of which may be connected to Saturn’s rotation. The “original” hexagon was first seen by the Voyager spacecraft in the 1980s, and then later by Cassini which returned stunning high-resolution images of the phenomenon. The lower, main hexagon also contains an immense hurricane in its center.

According to Linda Spilker, a Cassini project scientist, “The mystery and extent of the hexagon continue to grow, even after Cassini’s 13 years in orbit around Saturn. I look forward to seeing other new discoveries that remain to be found in the Cassini data.”

New composite image of the brilliant auroras at Saturn’s north pole, from the Hubble Space Telescope in 2017/2018. Image Credit: ESA/Hubble, NASA, A. Simon (GSFC) and the OPAL Team, J. DePasquale (STScI), L. Lamy (Observatoire de Paris)

But the hexagons are not the only extraordinary things to be seen at the north pole – there also the brilliant auroras. The Hubble Space Telescope took some new images of the auroras in 2017 in ultraviolet light, using the Space Telescope Imaging Spectrograph (STIS). These observations were coordinated with the “Grand Finale” of the Cassini spacecraft, when it took its fatal deep dive into Saturn’s atmosphere, ending the mission. The images were also taken before and after the Saturnian northern summer solstice, providing the best achievable viewing of the northern auroral region.

The images show high variability of emissions in the auroral displays and highly variable localised features. This variability is due to both the solar wind and Saturn’s rapid rotation rate (only 11 hours). The auroras were also seen to peak at certain times – at dawn and just before midnight. The peak just before midnight had not been observed before, and is thought to be tied specifically to the interaction of the solar wind with the magnetosphere at Saturn’s solstice.

Ultraviolet image of the auroras, taken by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Image Credit: ESA/Hubble, NASA & L. Lamy (Observatoire de Paris)

This wasn’t the first time that Hubble observed Saturn’s auroras; it looked at the ones in the southern hemisphere just after the southern solstice back in 2004 and then in 2009, it was able to see the auroras at both poles simultaneously when Saturn’s rings were edge-on as seen from Earth.

Saturn is already a majestic planet with its beautiful ring system, but now we have an even better view of just how incredible this world really is – a place of towering polar hexagons and mesmerizing auroras. As the saying goes, truth is often stranger than fiction.

This article was first published on AmericaSpace.




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