The iconic rings of Saturn, while appearing smooth from a distance, have puzzled scientists for over 40 years with the intermittent appearance of peculiar dark smudges dubbed “spokes.” These dusty features spread across the rings briefly before vanishing once again. The Hubble Space Telescope is currently studying these ephemeral spokes as part of an ongoing program monitoring the outer planets.

Though first spotted in 1981 by Voyager 2, the exact cause of these striking marks remains unknown. Their appearance seems connected to Saturn’s seasons, each lasting seven Earth years, as well as its magnetic field. The new Hubble images from October 2022 capture the dark spokes traversing the illuminated rings. The observations are part of the Outer Planet Atmospheres Legacy Program (OPAL), which tracks the spokes’ rotation.

The approaching Saturnian equinox in May 2025 makes now an opportune time for observation. Past equinoxes brought increased spoke activity, with more frequent and darker spokes anticipated in the coming years, according to OPAL principal investigator Amy Simon of NASA’s Goddard Space Flight Center. While the correlation between seasons and spokes is clear, the underlying mechanism behind the phenomenon is still unclear.

Spokes can stretch over vast swaths of the rings, even wider than Earth itself. Leading theories link them to interactions between Saturn’s magnetic field and the solar wind. Yet their unpredictable nature makes them difficult to forecast. Hubble’s gaze in the equinox lead-up may thus prove invaluable for unraveling the mystery.

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