Saturn, the second-largest planet in our solar system, is famous for its spectacular rings. But did you know that these rings are not just smooth and flat? They also have mysterious dark patterns that look like spokes on a wheel. These patterns are called ring spokes, and they are one of the most intriguing phenomena on Saturn.
 |
| This NASA Hubble Space Telescope photo of Saturn reveals the planet's cloud bands anda phenomenon called ring spokes. NASA, ESA, STScI, Amy Simon (NASA-GSFC) |
Ring spokes are transient features that rotate along with the rings. They appear and disappear as they revolve around Saturn. Their ghostly appearance only persists for two or three rotations around Saturn. During active periods, freshly-formed spokes continuously add to the pattern.
Ring spokes were first discovered by NASA's Voyager 2 spacecraft in 1981, when it flew by Saturn and captured stunning images of the ringed planet. NASA's Cassini orbiter also saw the spokes during its 13-year-long mission that ended in 2017. Cassini revealed that the spokes are made of tiny dust or ice particles that are electrostatically levitated above the ring plane.
But what causes these particles to levitate and form spokes? And why do they change in frequency and contrast with Saturn's seasons? These are some of the questions that scientists are still trying to answer.
To shed more light on this mystery, NASA's Hubble Space Telescope has been observing Saturn annually as part of its Outer Planets Atmospheres Legacy (OPAL) program. OPAL is a long-term project that began nearly a decade ago to monitor weather changes on all four gas-giant outer planets: Jupiter, Saturn, Uranus, and Neptune.
Hubble's ultra-sharp vision reveals a phenomenon called ring spokes. This photo of Saturn was taken by NASA's Hubble Space Telescope on October 22, 2023, when the ringed planet was approximately 850 million miles from Earth. Credit: NASA, ESA, STScI, Amy Simon (NASA-GSFC)
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 Saturn equinox, when we'd expect maximum spoke activity, 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.
This year, these ephemeral structures appear on both sides of the planet simultaneously as they spin around the giant world. Although they look small compared with Saturn, their length and width 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 near the equinox on Saturn, the planet and its rings are less tilted away from the Sun. In this configuration, the solar wind may more strongly batter Saturn's immense magnetic field, enhancing spoke formation. The solar wind is a stream of charged particles that flows from the Sun and interacts with the magnetic fields of planets and other bodies in the solar system.
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.
This Hubble Space Telescope time-lapse series of Saturn images (taken on October 22, 2023) resolves a phenomenon called ring spokes appearing on both sides of the planet simultaneously as they spin around the giant world. The video zooms into one set of spokes on the morning (left) side of the rings. Credit: NASA, ESA, STScI, Amy Simon (NASA-GSFC)
Ring spokes are not only fascinating to look at, but they also provide clues about the nature and origin of Saturn's rings. The rings are composed of countless chunks of ice and rock, ranging in size from dust grains to boulders. Some of these chunks may have been captured by Saturn's gravity from passing comets or asteroids, while others may have been formed by collisions or tidal forces within Saturn's system of moons.
By studying the ring spokes, scientists can learn more about the physical and chemical properties of the ring particles, as well as the interactions between the rings and Saturn's magnetic field, atmosphere, and moons. Understanding these processes may help us understand how planets and their rings form and evolve over time.
Saturn is not the only planet with rings in our solar system. Jupiter, Uranus, and Neptune also have rings, but they are much fainter and less structured than Saturn's. Hubble has also observed the rings of these planets, but has not detected any spokes on them. This makes Saturn's spokes even more unique and intriguing.
Hubble's observations of Saturn and its rings are part of a larger effort to explore the outer planets and their diverse environments. Hubble is the only telescope that can observe these planets in visible, ultraviolet, and near-infrared wavelengths, providing a wealth of information that complements other missions such as Juno, which is orbiting Jupiter, and the upcoming Europa Clipper and Dragonfly, which will explore Jupiter's moon Europa and Saturn's moon Titan, respectively.
Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.