In 1984, Voyager 2 flew past Saturn, taking astonishing photographs of the gas giant in resolutions we had never seen before. As well as spotting undulations in the planet's rings – the result of a then undiscovered ring-shepherd moon – the probe captured images of strange dark "spokes" in Saturn's rings.
These features have since been captured by Cassini, and repeatedly photographed by NASA's Hubble Space Telescope as part of Hubble's Outer Planets Atmospheres Legacy (OPAL) program to monitor the weather on the solar system's gas giants.
The dark spokes are not a continuous feature of the rings, but remain there for two or three rotations around Saturn. Though they look small, they can be bigger in width and length than Earth's diameter. Monitoring of the rings by Hubble revealed that the frequency of the spokes varies with Saturn's long seasons, appearing more often in Saturn's summer.
"We are heading towards Saturn equinox, when we'd expect maximum spoke activity," OPAL program lead scientist Amy Simon of NASA's Goddard Space Flight Center told NASA, after Hubble captured new views of the spokes in October 2023, "with higher frequency and darker spokes appearing over the next few years."
So what is causing the spokes?
"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," Simon explained.
Saturn takes 29.4 Earth years to rotate around the Sun. Due to a similar tilt to Earth, that means long winters and summers for the respective sides of the planet, lasting around seven years. When the rings are tilted more towards the Sun, the hypothesis is that Saturn's magnetic field is bombarded by the solar wind, helping more spokes to form.
We still don't know for sure how the spokes are formed, but scientists suspect that the interaction between the solar wind and the magnetic field generates electrostatic, levitating ice or dust above the rings to form the spokes.