Since its discovery 40 years ago, Saturn’s mysterious hexagon has left scientists scratching their heads. But a recent computer simulation designed by researchers at Harvard may provide some clues about what’s forming this unusual marvel.
The high-speed, six-sided jet stream was first discovered in the early 1980s by NASA’s Voyager flybys, but because of the spacecraft's tilted trajectory, these early images showed only a fraction of the hexagon. When the Cassini spacecraft arrived at Saturn in 2004 to get a better look, the hexagon had crept into winter’s shadow. But even in the dark, Cassini’s thermal camera was able to capture the first full images of the North Pole.
It showed a nearly stationary feature extending some 100 kilometers below the clouds. Initially, scientists speculated that this hexagonal feature might be driven by a nearby storm, but images taken by Cassini ruled that explanation out, showing that the storm had disappeared.
By late 2012, Saturn’s North Pole was no longer in winter, and the sun illuminated the massive vortex at the hexagon’s center. It had an eye about 50 times larger than an average hurricane on Earth and wind speeds clocking in at 500 kph. That’s roughly double the force of Hurricane Katrina.
How has Saturn been keeping this raging hexagon going for so long? Well, one answer might lie in the atmospheric dynamics of planets.
Like Saturn, Earth has polar vortices and jet streams. But Earth’s oceans, landmasses, and shallow atmosphere cause drastic temperature changes that prevent jet streams from settling into long-lived, symmetrical shapes.
Saturn, on the other hand, is a giant ball of gas with no solid surface and a far more uniform composition than Earth’s. In those conditions, it’s likely that a jet stream could get comfortable and settle in.
When Cassini ended its mission in 2017, it collected ultra-close images of the planet’s rings and clouds, while its mass spectrometer measured the composition of its atmosphere.
Using data taken during Cassini’s 13-year mission, researchers from Harvard set out to answer how Saturn’s turbulent conditions could give rise to stable, geometric jets. To find this out, the team built a 3-D computer simulation of Saturn’s hexagon to study its behavior. What they built was something similar to what we see on Saturn. Their computer model showed deep thermal convection, with heat transferring between the planet’s outer layers — sort of like a boiling pot of water. This deep convection led to the formation of three large cyclones near Saturn’s poles, and an eastward moving jet. Working together, the simulated cyclones and the jet combined to create a nine-sided central vortex.
While not a hexagon, this simulated geometric feature looked curiously similar to the polar vortex that’s found on Saturn. It was also found to extend way down into the atmosphere, like thousands of kilometers deep.
So, why didn’t the simulation generate a hexagon exactly like the one on Saturn? Well, it’s hard to account for everything that’s going on in the atmosphere and this simulation didn’t get to run long enough for all the features of the polygon to evolve.
The team plans to refine their simulation in hopes of creating a hexagon that’s also stable. Still, their recent study shows that the convection within Saturn’s atmosphere is enough to produce polygonal jets, which brings us a whole lot closer to solving this 40-year-old mystery of how Saturn’s hexagon has been able to hang around for so long.