A decagon wave around Saturn's south pole.

Saturn has a unique long-lived, nearly stationary hexagon wave close to its north pole that is coupled to an intense eastward jet. Despite decades of observation, no similar wave has been reported so far in the southern hemisphere or elsewhere on Saturn. Ground-based and Hubble Space Telescope (HST) images obtained in 2025 revealed the presence of a decagonal wave at planetographic latitudes ∼58°S to 63°S. HST images in 2023 and 2024 showed vertices with weaker albedo contrasts and linear sides of the decagon, implying an evolving phenomenon. The decagon moves eastward with a velocity of 2.5 meters per second, much slower than the zonal jet at 60.5°S with 116 meters per second, and the longitudes of its vertices oscillate with a period of 32 days and amplitudes of 4.6° to 8.4°. From its properties, the decagon could be a large-scale meandering wave trapped vertically and confined meridionally by the dominant jet curvature. Shallow water model simulations suggest that the decagon may have originated from a spatially periodic disturbance in the jet peak or may have been driven by a dark anticyclonic vortex immediately to the north.