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 - 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 eastwards with a velocity of 2.5 m/s relative to System III radio-rotation period, much slower than the zonal jet centered at latitude of 60.5°S that has a peak velocity of 116 m/s. Simultaneously, the position in longitude of its vertices oscillate relative to the mean motion with an average period of 32 days and amplitudes of 4.6°-8.4°. The decagon could be a quasi-geostrophic Rossby wave trapped vertically and confined meridionally by the dominant jet curvature with an intrinsic phase speed of -113 ms^-1. Saturn’s new decagon offers a rare opportunity to probe polar jet dynamics and the processes that create stable polygonal flows in giant planets, including Jupiter’s polar cyclones arranged in polygonal patterns in a weak wind field beyond 80° latitude.