The Cassini of NASA sees the polar hexagon of Saturn

The legacy of Cassini further highlights the mysterious mystery of the northern polar hexagon of Saturn.

Cassini view on the Saturn hex of 2012.
NASA / JPL-Caltech / Institute for Space Science.

One of the strangest features of the solar system is the hexagon that adorns the polar region of Saturn. First spotted by the Voyager 1 spacecraft during its historic flight over in 1980, this feature is unique among the planets of the solar system.

Now, a new study published in Nature: Communications suggests that the hexagon – a model found in the troposphere of Saturn – could influence the appearance of a similar structure in the stratosphere, more than 300 kilometers away (200 miles) above.

The polar hexagon of Saturn moving.
NASA / JPL-Caltech / Institute for Space Science / University of Hampton

The results come from NASA's Cassini spacecraft, which arrived in orbit around Saturn in July 2004 and completed its mission with atmospheric entry and disposal a year ago on September 15, 2017. Scientists have carried out key measurements with the spectrometer (CIRS) from 2014.

"While we were expecting to see any vortex at Saturn's North Pole as it warmed up, its shape is really surprising," said Leigh Fletcher (University of Leicester, UK) in a recent statement. spontaneously and identically at two different altitudes, where the hexagon is in fact an imposing structure that extends over several hundred kilometers.

Seasons on Saturn

Saturn revolves around the Sun once every 29.5 years, so its "seasons" last a little over seven years. The winter in the northern hemisphere ended in 2009, a year that marked both the spring equinox and the beginning of Cassini's two-year equinox mission.

But Cassini's CIRS instrument needed (relatively) higher temperatures to perform stratospheric observations on the north hex; at the end of the winter, temperatures hovered around -158 ° C, 20 ° below what CIRS has to do to make observations. The beginning of the Nordic summer on Saturn offered a last chance for observations towards the end of Cassini's mission.

Nordic polar hexagon / luminosity maps of Saturn from 2013 to 2017, showing the emerging stratospheric component.
NASA / JPL-Caltech / University of Leicester / GSFC / L.N. Fletcher et al. 2018

A new system emerges

In 2014, as the northern hemisphere entered in the summer, Cassini began to survey the pole's stratosphere. Researchers soon realized that they were seeing the irregular edges of a stratospheric hex, reflecting the now-familiar pattern in the troposphere. This unexpected discovery is in contradiction with what the researchers expected to find: on Earth or on Saturn, the wind speed changes considerably higher in the atmosphere, making the presence of cloud structures in the atmosphere unlikely. long service life extending over several layers. A cloudy structure such as the north polar hexagon should remain trapped in the lower troposphere.

So, how does the outline of the hexagon persist higher? A possible mechanism proposed by researchers is a phenomenon called evanescence, where a rotting wave has just enough energy to persist in the upper atmosphere.

"We can use background atmospheric conditions (especially temperature and wind gradients) to determine if a wave is trapped or is free to propagate vertically," says Fletcher. "As long as the two regions where the hexagon can spread are not too far apart, a sufficient amount of information from the troposphere could actually reach the stratosphere, which is why we see the hexagon at two heights. "

If the polar hex passes from the troposphere to the stratosphere, it means that this structure crosses hundreds of kilometers in height. But we still do not know if this is really the case: "The hexagon is probably continuous, from the troposphere to the stratosphere, but there is a part of the atmosphere between the two where CIRS is unable to provide direct evidence of the hexagon. Spilker (NASA-JPL). "Future atmospheric modeling will be helpful to help explain what is observed."

TThere was no hexagon at the South Pole, nor at the top of the clouds, nor above, when it was observed at the beginning of Cassini's mission during the austral summer. . The North Saturn jet stream (at 78 ° N latitude) appears to be more unstable than its southern counterpart, perhaps contributing to its unique hexagonal appearance.

Observe the North Hexagon

Admittedly, it's a bit frustrating that Cassini ended his exploration of Saturn as things got interesting in the north. But even with the departure of Cassini, future observations of this phenomenon will always be possible with Hubble and his next infrared successor, the James Webb space telescope.

Saturn of 2017, pictured from the observatory of the Pic du Midi, with the polar hexagon clearly visible.
Damian Peach

The Nordic polar hexagon even arrives on astrophotos of the Earth today. Indeed, it is interesting to note that no observations of Saturn noted the hexagon before its discovery in the 1970s, although such an observation was possible thanks to observers using large optics. The rings of Saturn – and the planet itself – slope up to 27 ° from our line of sight, alternating between each hemisphere every 14 to 15 years. The rings were slanted to their widest last year in 2017 with the Northern Hemisphere visible, and they returned to board in 2025.

One thing is certain: it will take time before returning to Saturn, perhaps when / if the mission of the Titan helicopter explorer is unfolding. But for the moment, we can marvel that Cassini continues to produce a treasure trove of fascinating scientific discoveries.