The Cassini spacecraft captured this breathtaking view of Saturn on May 4th 2014 but I just noticed it today.
Cassini captured this view at a distance of approximately 2 million miles (3 million kilometers) from Saturn using a near infrared filter. Cassini was high above the ring plane and this is a view of the planet we never get from Earth. From our position near the centre of the solar system Saturn (and most of the planets beyond Earth) look fully illuminated all of the time.
One of the features that immediately catches the eye is that hexagon shape around the north pole of Saturn. The hexagon at Saturn's north pole isn't a new feature. It was seen in images taken by the Voyager 1 and 2 probes back in the 1980s.
Here are some closer Cassini views of it:
For some sense of scale: each side of the hexagon is a bit wider than the Earth.
Saturn is a rapidly rotating gas giant planet. How can a regular and seemingly long-lived feature like this arise in the atmosphere of Saturn? Astronomers don't have the definitive explanation yet although there is experimental evidence from laboratories on Earth which might give a clue.
The video shows an experiment to simulate conditions that might lead to a regular structure being set up in the atmosphere of Saturn. They built a cylindrical tank capable of varying the fluid flow within concentric regions inside. A hexagon appeared at the chaotic boundary between fluids moving at very different speeds. A number of vortices formed in the region separating the fluid flow and distributed themselves evenly around the pole at centre. Why a hexagon? Actually the experimenters could fine tune the spin rates to produce a hexagon but they could also generate other regular polygons too. You can see some them here.
The experiment showed how stability and order could arise from chaotic conditions induced by large differences in wind speeds at different latitudes on Saturn. There are still questions to be answered. For instance why is there no comparable feature at the south pole of Saturn? There is a huge, long-lived storm at the south pole, but no polygonal structure.
I love trawling through the raw image section of the Cassini website. It can lead to serious distraction no matter if they're images of the rings, the moons or Saturn itself. But I'm aware that this mission won't last forever; Cassini's time is running out and sometime in 2017 NASA scientists will place it in a final series of orbits which will send it crashing into Saturn.
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Dr Adrian Jannetta. Amateur astronomer, maths teacher and science enthusiast.