Spoke Formation in Saturn’s Rings Confirmed by Hubble Observations

Credit: NASA/JPL-Caltech/SSI/Mike Malaska.

Saturn’s rings, composed of dust and ice, are remnants of earlier planetesimals that catastrophically broke up as they entered Saturn’s gravitational field. Microscopic dust particles in the rings are electrically charged and are affected by Saturn’s magnetic field, generating phenomena known as “spokes” during Saturn’s fall and spring equinoxes. Spokes appear as roughly perpendicular disruptions in the ultraviolet (UV) and near-infrared (NIR) wavelengths of Saturn’s rings and look like wheel spokes revolving around Saturn.

A team led by Amy Simon from NASA Goddard Space Flight Center confirmed and documented current spoke activity. Using Hubble Space Telescope observations, they found that spoke formation is constrained to Saturn’s main B ring at approximately 1.8 Saturn radii, where micron-sized dust particles are suspended in orbit by electrostatic forces from neighboring particles and Saturn’s global magnetic field. This is consistent with previous observations from the Voyager mission in 1980-1981. Saturn’s magnetic field, however, is affected by solar wind as it orbits the Sun, and these annual variations cause observable movement of charged dust particles. The team found that the UV and NIR wavelengths where spokes are detectable are slightly blue-shifted, meaning they have marginally shorter wavelengths than the background spectra. This variation is observed across the spokes’ entire UV and NIR wavelength range. Interestingly, spoke detection is highly dependent on solar incidence angle, where the angle between the rings and incoming sunlight must be between ±20° to be detected.

Spoke activity began in 2021 and is expected to continue to increase until it reaches its most active phase around 2025, when it will then decrease for approximately four years. Observing and learning about these ring features surrounding Saturn will further illuminate their makeup and temporal variability. READ MORE