It is time for mysterious spokes to look in Saturn’s rings

The Hubble Space Telescope recently captured the appearance of multiple asymmetrical “spokes” rising above Saturn’s rings, marking an upcoming change of season for the ringed gas giant. The spokes are made of charged particles of ice that curve up and away from the rest of the rings. Researchers aren’t sure what exactly is causing the spokes, but they suspect it has something to do with the planet’s strong magnetic fields.

This isn’t the first time the spokes have been observed. They were discovered by the Voyager missions in the 1980s, and more recently the Cassini spacecraft has captured the phenomena up close. Since the end of the Cassini mission in 2017, observations of the Saturn system have mostly been made remotely using Hubble and other ground-based telescopes. Hubble’s latest observations, recorded last September, mark the beginning of a new spoke cycle.

We now know that the spokes are a seasonal phenomenon. Like Earth, Saturn cycles through a year with four seasons based on its tilt, although it takes about thirty Earth years to complete a cycle due to Saturn’s more distant orbit. The spokes typically appear around the equinox, when the rings face the sun, and fade as the summer or winter solstices approach.

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Saturn will reach its autumnal equinox on May 6, 2025. Recently photographed by Hubble, the spokes are among the first of the season, and they are becoming more frequent as the equinox approaches.

A composite of multiple Hubble Space Telescope images showing the recently observed spokes to the left of Saturn’s rings. Image credit: NASA, ESA, Amy Simon (NASA-GSFC). Image processing: Alyssa Pagan (STScI).

The leading theory to explain the spokes (which sometimes look more like blobs than the radial lines we traditionally associate with spokes) has to do with Saturn’s magnetic field. The solar wind constantly interacts with the magnetic field, creating auroras on Saturn just like those we see at Earth’s poles. A similar effect could explain the spokes.

When the rings are aligned edge-to-edge with the Sun, the smallest of the icy particles that make up Saturn’s rings can pick up an electrical charge that momentarily flings them up and away from the rest of the ring’s material.

Dark colored spokes as seen by Cassini in 2010 during the previous spoke season. Photo credit: NASA/JPL/SSI.

“Despite years of excellent observations by the Cassini mission, the exact start and duration of the spoke season is still unpredictable, much like the prediction of the first storm during hurricane season,” said NASA planetary scientist Amy Simon.

It is unclear whether a similar phenomenon occurs in the rings around Uranus or Neptune, which are fainter and more distant.

To better understand the spokes and other seasonal variations in Saturn’s atmosphere, researchers need to collect decades of data so that the system’s long-term changes can be documented. To continue building such a dataset in Cassini’s absence, NASA’s Outer Planet Atmospheres Legacy (OPAL) program regularly monitors the ringed planet with Hubble and will continue to do so for as long as the venerable space telescope is capable.

Learn more:

“Hubble captures the start of a new spoke season at Saturn,” NASA.

Featured image: This Hubble Space Telescope image shows two smeared spokes on the left, the first of the season. Image credits: NASA, ESA and Amy Simon (NASA-GSFC); Image processing: Alyssa Pagan (STScI).

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