Hubble Spots Saturn's South Pole Decagon Storm

Astronomers, utilizing observations from the NASA/ESA Hubble Space Telescope, have identified a striking new atmospheric phenomenon on Saturn: a ten-sided wave, or decagon, swirling around the planet's south pole. This colossal storm spans an estimated 104,000 miles in diameter, with each straight-looking side stretching approximately 10,000 miles. The discovery marks the geometric counterpart to the well-known six-sided jet stream that has continuously encircled Saturn's north pole for over four decades.

The finding, detailed in a paper published in Science Advances and led by Agustín Sánchez-Lavega of the University of the Basque Country, reveals a decagon centered near 63 degrees south latitude. It appears to be riding a powerful eastward jet stream and is reportedly strengthening. While the precise dimensions are estimated at roughly 104,250 miles across with sides of about 10,425 miles, scientists emphasize that these figures represent a dynamic wave in a moving atmosphere rather than fixed boundaries.

The new decagon is not a surface feature but rather the visible manifestation of a wave embedded within a potent jet stream, extending vertically through multiple atmospheric layers. Evidence for its depth comes from observations using different filters on the Hubble telescope; each filter probes distinct atmospheric depths, causing the feature to appear to shift slightly, indicating a three-dimensional structure. This vertical structure mirrors that of the northern hexagon, which NASA's Cassini mission revealed to be similarly deep-rooted.

A Pattern in Saturn's Atmosphere

Part of the reason the southern decagon remained undetected for so long is Saturn's orbital mechanics and axial tilt. The planet's south pole was largely obscured from Earth's view for nearly a decade, from approximately 2012 to 2023. Although the Cassini mission (2004-2017) observed a warm polar vortex and a cyclone at the south pole during its operational years, it did not record a persistent polygonal jet stream comparable to the northern hexagon.

The initial clues for the decagon emerged from ground-based observations by amateur astronomers, including Trevor Barry in Australia and Jean-Paul Oger. Their coordinated efforts, managed through the Planetary Virtual Observatory Laboratory, suggested a structure at the south pole in 2024 and 2025. These hints prompted the research team to re-examine archived data from the Hubble Space Telescope. They discovered the pattern faintly present in 2023 imagery and becoming more pronounced in subsequent years. These crucial observations were part of the Outer Planet Atmospheres Legacy (OPAL) program, led by Amy Simon of NASA's Goddard Space Flight Center, which provides consistent, yearly portraits of the outer planets to track atmospheric changes.

The existence of both a northern hexagon and a southern decagon suggests that polygon formation is a natural process for Saturn's atmosphere under specific conditions, rather than a singular, unique event. The difference in the number of sides—six versus ten—is significant. It implies that the southern jet stream is configured differently from its northern counterpart, influencing the speed, width, and shear of the atmospheric flow. Laboratory experiments with rotating fluids have long demonstrated the ability to produce similar polygonal patterns, attributed to instabilities within strong circumpolar flows.

Significant questions remain about the decagon's emergence and longevity. Scientists are investigating why the pattern appeared in the 2020s when Cassini's earlier surveys did not detect it. Possible explanations include seasonal variations tied to Saturn's 29-year orbit and the amount of sunlight reaching its poles. Future monitoring by the Hubble telescope will track the decagon's evolution, while the James Webb Space Telescope, with its infrared capabilities, may probe deeper atmospheric layers. The ultimate test will be for numerical models of Saturn's atmosphere to successfully reproduce both the northern hexagon and the southern decagon simultaneously. For now, the discovery of this ten-sided storm represents a significant step in understanding the complex dynamics of the gas giant's atmosphere.