The Colour-Changing, Hexagonal Vortex On Saturn Just Surprised Scientists Again

Saturn’s hexagon from Cassini (Photo: NASA/JPL-Caltech/Space Science Institute)

Sitting atop Saturn’s north pole is one of the Solar System’s most striking weather patterns: A rotating, colour-changing hexagon slightly wider than Earth. A new study shows there’s another hexagon directly on top of the first one — and that’s weird.

Saturn’s hexagon has remained visible to any probe that’s visited the planet, including Voyager, Hubble and Cassini, though its structure is something of a mystery. In 2016, astronomers noticed that the hexagon had changed in hue, from blue to gold. New observations continue to show that there’s a lot that scientists don’t know about Saturn and its hexagon.

“We’re seeing that it goes straight up into the stratosphere a long, long way. That’s unexpected,” Glenn Orton, one of the study’s authors and a physicist at NASA’s Jet Propulsion Laboratory, told Gizmodo. “I’m supposed to be wise about the dynamics, but I’m at a loss as to how it could maintain itself that high.”

The scientists set out to study the North Polar Stratospheric Vortex (NPSV). This is a swirling feature that forms during Saturn’s spring and summer in the stratosphere, the layer above the cloud tops, and disappears during the winter. The NPSV brings warmer temperatures and the increased presence of hydrocarbons, molecules made from hydrogen and carbon like methane.

But Cassini’s Composite Infrared Spectrometer demonstrated “a significant surprise,” according to the paper published this week in Nature Communications.

The NPSV had a hexagon-shaped boundary that looked exactly like the well-known persisting hexagon in the lowest level of Saturn’s atmosphere. Somehow, during Saturn’s equivalent of spring and summer, the process that creates its hexagon appears to influence the molecules far above its cloud tops and causes the hexagon to extend 300km upward.

Temperature maps of Saturn’s northern stratosphere demonstrate a clear hexagonal shape emerging that matches the hexagon on the planet’s surface. (Graphic: Fletcher et al, Nature Communications 2018)

Scientists think that the well-known hexagon structure comes from Rossby waves, larger-scale movements of the entire structure of the planet’s gas, rather than the motion of the gas molecules themselves, like Earth’s meandering jet stream, caused by the planet’s rotation.

As to how the newly discovered, higher-up hexagon forms, either it’s forming separately from the lower hexagon, or somehow there’s interaction between two layers of Saturn’s atmosphere that produces an enormous single hexagon structure.

The two layers normally can’t communicate, and waves travelling between the two would mean that one hexagon wouldn’t quite align with the other — and yet, the observations show aligned hexagons. Perhaps the Rossby waves are just strong enough to leave an imprint on the stratosphere.

There’s still a lot to learn about the vortex. The Composite Infrared Spectrometer on board Cassini couldn’t study the planet’s north pole until the temperatures were warm enough, as northern winter ended (a Saturn year lasts 30 Earth years). Observations began in 2013 and ended when Cassini plunged into the planet in 2017. The vortex will continue evolve until the planet’s autumn equinox in 2024, according to a press release from the European Space Agency.

Astronomers have known about Saturn’s hexagon for a shorter period of time than, say, Jupiter’s Great Red Spot. “It’s possible we haven’t observed it long enough to see change,” Orton said, “So we’re gonna just keep looking.”

[Nature Communications]

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