The NASA Hubble Space Telescope photographed Saturn on October 22, 2023, when the planet with its rings was approximately 850 million miles away from Earth. Hubble’s ultra-sharp vision revealed a phenomenon called ring spokes. Saturn’s spokes are transient features that rotate along with the rings. Their spectral appearance lasts only for two or three rotations around the planet.

During active periods, newly formed spokes continuously add to the pattern. In 1981, NASA’s Voyager 2 first captured images of the ring spokes. NASA’s Cassini orbiter also observed the spokes during its 13-year mission, concluding in 2017.

Hubble continues to observe Saturn annually, while the spokes come and go. This cycle has been captured by Hubble’s Outer Planets Atmospheres Legacy program, initiated almost a decade ago to monitor yearly climate changes on all four outer gas giants. Hubble’s sharp images show that the frequency of spoke appearances is determined by the season, first appearing in Opal data in 2021, but only on the morning, or left side, of the rings.

Long-term monitoring has revealed that both the number and contrast of spokes vary with Saturn’s seasons, which, like Earth, is tilted on its axis and experiences approximately seven-year seasons. “We are heading towards Saturn’s equinox, when we would expect the maximum spoke activity, with increased frequency and the appearance of darker spokes in the coming years,” said Amy Simon of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and chief scientist of the Opal program.

This year, these fleeting structures appear simultaneously on both sides of the planet as it rotates around the giant planet. Although they appear small compared to Saturn, their length and width can exceed the Earth’s diameter. “The most accredited theory is that the spokes are linked to Saturn’s powerful magnetic field, with a kind of solar interaction with the magnetic field generating the spokes,” Simon stated.

When Saturn is near equinox, the planet and its rings are less inclined relative to the Sun. Hubble’s image reveals how the solar wind can more strongly impact Saturn’s immense magnetic field, thus favoring spoke formation. Planetary scientists believe that electrostatic forces generated by this interaction cause dust or ice to levitate above the ring to form spokes, although after several decades, no theory can precisely predict the spokes.

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