What is at Saturn's north pole?

A hexagon. A cloud pattern with six straight sides and sharp corners, about 30,000 kilometres across - wide enough to fit four Earths - rotating with the planet and apparently stable for at least the forty-odd years since Voyager photographed it in 1981. There is nothing else like it in the Solar System.

The leading explanation is a standing wave in a jet stream. Spin a fluid in a tank fast enough, with a faster-rotating band inside it, and the boundary between them can settle into a polygon - laboratory experiments have produced triangles, squares, hexagons and heptagons depending on the speed. On that reading Saturn's is a resonance rather than a structure, and its six sides are simply which polygon fits the wind speeds it has.

Being reasonable and being confirmed are different things. The lab models reproduce the shape but not everything about the real thing, and there is no agreed account of why it has stayed so sharply defined for decades while the rest of Saturn's weather churns. Inside it sits a hurricane-like storm with a genuine eye about 2,000 kilometres across, with clouds at the eyewall moving at some 500 kilometres an hour, over a pole where there is no ocean for a hurricane to feed on.