Saturn is frequently called “the jewel of the solar system”. The origins of that moniker may have been forgotten to history, but the reason is rather apparent. Saturn, of course, has beautiful gigantic rings, twice as wide as the planet itself. Its spectacular golden colour is beautiful, making it a delightful sight through a telescope. Saturn has an additional strange and wonderful feature to it; a massive hexagon at the north pole, adorning the top of the planet.
The hexagon was first discovered in 1988 by researchers looking at the 1980 and 1981 Voyager 1 and 2 photos. They observed the distinctly-geometric shape in the otherwise longitudinally-coloured planet. Unfortunately, the technology did not exist at the time to observe it further after the probes moved on. Its continuing existence was confirmed by Cassini, the NASA-ESA probe that is still in orbit around the planet today. The hexagon is massive, each side 32 000 km wide—larger than the diameter of the Earth.
The cause of the consistent hexagonal pattern is still not known, but several theories have been proposed. A study out of Oxford University suggested that interesting repeating patterns could form if the wind speeds changed quickly as one progressed from the pole to the equator. Under certain conditions relating to properties like the viscosity of the fluid, the hexagonal pattern will occur. The specificity of parameters required to make this phenomenon occur could explain why similar hexagons are not observed at Saturn’s south pole, or on Jupiter’s poles.
A second theory was proposed in 2015. This one, based on computer simulations, suggests that a jet stream near the pole might form a hexagonal pattern due to perturbations. This theory is backed up by the fact that it correctly predicts the hexagon will rotate at about the same rate as the planet itself, and by the fact that there is a jet stream bounding the shape.
Regardless of the origins of the hexagon, it has begun to exhibit another surprising property: colour change. In 2012, Cassini took colour photos using its wide-angle camera, clearly showing that the hexagon was a dark blue hue. This contrasts with the golden hue of the rest of the planet. However, photographs taken this year show that the area inside the hexagon has changed to gold, much more in-line with Saturn’s typical colouration.
The prime theory put forward by NASA to explain this change is that it indicates the passing of seasons on Saturn. Saturn has a 29-Earth-year orbit and a 27° orbital tilt, comparable to the 23° tilt that gives Earth its seasons. Saturn is approaching its summer solstice in May 2017, the time of year when the northern hemisphere receives the most sunlight. NASA suggests that the growing sunlight is causing photochemical reactions in Saturn’s atmosphere. The resulting haze is gold in colour, causing a change in the hexagon’s appearance. As the planet orbits around the sun, it will near its winter solstice. The associated reduction in sunlight will cause the created hazes to not be replenished, and the hexagon will revert to its blue colour.
NASA is being certain to clarify that their photochemical hypothesis is just that: a hypothesis. They also put forward alternate explanations, like other changes in the solar heating of Saturn, or changes in wind patterns that alter atmospheric circulation.
It would be interesting to continue to watch the evolution of the hexagon. However, Cassini’s mission is almost over. On November 29, the craft will perform a flyby of the moon Titan that will put it into a death-orbit. Over the next year, Cassini will make close flybys of Saturn’s rings before entering the planet’s atmosphere on September 15, 2017. This destruction will occur so that the probe is not left to drift where it desires. If this were not done, it could crash into and contaminate one of the icy moons around Saturn that could be harbouring alien life.
This story shows the important of long-term observations of the other bodies in the solar system. Without them, the hexagon would have been discovered, but the colour change would not have been. Hopefully, knowledge that amazing findings like this one can only be found by dedicated orbiter craft will encourage more long-term missions, especially to planets like Uranus and Neptune where their long orbital periods make any seasonal changes difficult to observe.
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