Saturn’s north pole is a hexagon

A jet stream around Saturn’s north pole has six straight sides. It is about 30,000 kilometres across, and it has kept the shape since Voyager photographed it in 1980.

Voyager’s images of Saturn, looked at years later as map projections, showed a hexagonal feature centred on the north pole. D. A. Godfrey published it in 1988. Each side is a straight run of cloud in a jet stream. The whole figure is nearly 30,000 kilometres across, more than twice the diameter of the Earth, and it rotates roughly with the planet’s interior rather than drifting apart. Cassini watched it for over a decade, in light and in infrared, and the hexagon was still there, with a hurricane nested inside it.

The leading account is a standing wave in the jet: a fluid, given the right speed and the right confinement, can hold a polygonal shape instead of a ring. Laboratories have made polygons in rotating tanks. Nobody built Saturn’s. The surprise is not that a wave can have a wavelength. It is that the wavelength comes out to six, on a scale that swallows planets, and then stays.