Saturn Has Grown a Ten-Sided Storm at Its South Pole, and Nobody Saw It Start

Each side of the southern figure is longer than the diameter of Earth. The whole structure extends through several layers of the atmosphere rather than sitting at one altitude.

Why it matters beyond being strange

Because a polygon that forms while you are watching is an experiment you did not have to run.

The hexagon has been a puzzle for forty years precisely because it was already there. Laboratory work with rotating tanks of fluid has reproduced polygonal jets, and the physics is broadly understood, but a stable feature tells you little about how such things begin, how long they take, and what decides whether the answer is six sides or ten.

A decagon that emerged over two or three years gives planetary scientists the beginning of the process for the first time. Amy Simon of NASA put the question plainly: why did it suddenly form now, when nothing like it had been seen before.

Saturn's south pole has just come out of a long winter and into spring, and the seasonal change in sunlight is the obvious candidate for what changed. Whether that is the cause, and whether the decagon will persist for decades like its northern counterpart or fade, is exactly what the team says it does not yet know. The James Webb Space Telescope is scheduled to look.

The amateur contribution

It is worth noticing who saw it in 2024.

Hubble time is scarce and Saturn is a large, bright, frequently photographed target for amateurs with good telescopes. The undulating band that bridged the gap between Hubble's first hints and its confirmation was recorded by people observing from back gardens and sending images to a shared archive. Planetary astronomy is one of the few fields where that still matters, and this discovery is a clean example of it.

Our earlier reporting on the largest map of a nervous system ever produced, another case of patient observation paying off on a timescale longer than a career, is in the connectome piece. The full description of the decagon is on NASA's Hubble mission site.