Hubble finds a 10-sided wave around Saturn's south pole

NASA's Hubble Space Telescope has recorded a 10-sided atmospheric wave around Saturn's south pole. The feature appears in Wide Field Camera 3 frames taken between 2023 and 2025 for the Outer Planet Atmospheres Legacy programme. NASA posted the analysis on 2 September. Astronomy Picture of the Day carried the composite on 8 September.

Saturn's north pole has worn a hexagon since Voyager first saw it in the early 1980s. That six-sided jet has held for more than 40 years. Cassini, which orbited from 2004 to 2017, saw no matching long-lived shape at the south pole. Hubble's later frames fill that gap. Agustin Sánchez-Lavega of the University of the Basque Country, Amy Simon of NASA Goddard and Michael Wong of the University of California, Berkeley, are the named science leads. Alyssa Pagan processed the colour composite.

The decagon is a vertical structure. It runs through more than one layer of the southern atmosphere. In a single-filter frame the ten sides stand out as a dark inner ring. An X on the published image marks a patch Hubble did not capture. The vertices move back and forth on a 32-day cycle while the whole pattern drifts slowly eastward.

The working idea is a wave. Fast air away from the pole meets slower air closer to the pole. The boundary locks into a standing shape. At the north pole that shape has six sides. At the south pole, in this decade, it has ten. Unstable winds or a nearby anticyclone may have set the southern count. The papers released with the image do not close that choice.

OPAL has watched Jupiter, Saturn, Uranus and Neptune for more than ten years. The programme's value here is the baseline. Because Cassini saw nothing like this, the decagon is either new since 2017 or it was too faint or too intermittent for Cassini's viewing geometry. Hubble's confirmation that the shape is already present in 2023 frames means it is at least three years old. That is short beside the northern hexagon and long enough to treat as a standing feature rather than a one-season swirl.

Ground telescopes contributed supporting images across 2023 to 2025. The Hubble frames remain the ones that resolve the sides. A 10-sided wave is harder to pick out than a hexagon. The extra vertices sit closer together. Processing choices matter. The single-filter product is the cleanest public view of the geometry.

What happens next is a stability test. If the decagon is still there in the 2026 and 2027 OPAL visits, Saturn will have two polar polygons of different order at the same time. If it breaks up, the south pole returns to the less ordered state Cassini recorded. Either result is usable. The northern hexagon has been a fixed problem in planetary fluid dynamics for four decades. A southern decagon that can appear and vanish would be a different problem: a polygon that is allowed, not required.

The 8 September APOD date puts the picture in front of a general audience on the morning the feature is easiest to explain: a planet that already had a famous six-sided jet has grown a ten-sided one at the opposite pole, and the older spacecraft never saw it.