NASA's Hubble Space Telescope is helping scientists solve a longstanding mystery about Saturn's rings.
Some new images reveal mysterious shadows known as "spokes" that move along the rings.
"Their ghostly appearance only persists for two or three rotations around Saturn. During active periods, freshly-formed spokes continuously add to the pattern," NASA said in a statement.
Scientists still don't know why the spokes appear, but hope to gain more data during Saturn's upcoming equinox in 2025, when spoke activity is at its peak.
"It's believed the dark spokes are dust particles electrostatically levitated above the ring plane. They seem to change in abundance with Saturn's seven-year-long seasons. And, this may be linked to changes in Saturn's magnetic field, as it is battered by the solar wind," NASA said.
The spokes were captured by the Hubble telescope on October 22, when the ringed planet was approximately 850 million miles from Earth. NASA says the crisp images show that the frequency of spoke apparitions is seasonally driven. 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 Amy Simon, lead scientist of Hubble's Outer Planets Atmospheres Legacy program. "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."
NASA's Voyager 2 first photographed the ring spokes in 1981. NASA's Cassini orbiter also saw the spokes during its 13-year-long mission that ended in 2017. Hubble, which has been orbiting Earth a few hundred miles above the surface since 1990, continues observing Saturn annually as the spokes come and go.
After several decades, no theory perfectly predicts the spokes, but NASA says continued Hubble observations may eventually help solve the mystery.