Hubble Watches ‘Spoke Season’ on Saturn

This Hubble Space Telescope photo of Saturn was taken on October 22, 2023, when the ringed planet was approximately 850 million miles from Earth. Hubble imaging is so sharp it resolves a phenomenon called ring spokes appearing on both sides of the planet simultaneously as they spin around the giant world. In 1981, NASA’s Voyager 2 first photographed the ring spokes. NASA’s Cassini orbiter also saw the spokes during its 13-year-long mission that ended in 2017. Hubble continues observing Saturn annually as the spokes come and go. This cadence has been captured by Hubble’s Outer Planets Atmospheres Legacy (OPAL) program that began nearly a decade ago to annually monitor weather changes on all four gas giant outer planets. Hubble’s crisp images show that the frequency of spoke apparitions is seasonally driven, first appearing in OPAL data in 2021 but only on the morning (left) side of the rings. Long-term monitoring show that both the number and contrast of the spokes vary with Saturn’s seasons. Saturn is tilted on its axis like Earth, and has seasons lasting approximately seven years. “We are heading towards maximum spoke season, so we’d expect to see more of what we’re seeing now, with higher frequency and darker spokes appearing over the next few years,” said the OPAL program lead scientist, Amy Simon of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The spokes are transient features that rotate along the ring plane. Their ghostly appearance only persists for two or three rotations around Saturn. During active periods, freshly-formed spokes continuously add to the pattern. These ephemeral structures can stretch longer than Earth’s diameter! “The leading theory is that spokes are tied to Saturn’s powerful magnetic field, with some sort of solar interaction with the magnetic field that gives you the spokes,” said Simon. When it’s summer or winter on Saturn, the planet and its rings are tilted more toward the Sun. In this configuration, the solar wind may more strongly buffet Saturn’s immense magnetic field, enhancing spoke formation. Planetary scientists think that electrostatic forces generated from this interaction levitate dust or ice above the ring to form the spokes, though after several decades no theory perfectly predicts the spokes. Continued Hubble observations may eventually help solve the mystery. The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.