What Are These Mysterious Dark ‘Spokes’ on Saturn’s Rings?

A Hubble image highlights the seasonal features, which scientists think could be caused by interactions between the planet’s magnetic field and solar wind

An image of Saturn and its rings
A photo of Saturn taken by the Hubble Telescope last October. Small dark marks called ring spokes are visible on the planet's left side, just inside the widest black band of space between rings. NASA, ESA, STScI, Amy Simon (NASA-GSFC)

A new photo of Saturn from NASA’s Hubble Space Telescope captures mysterious dark blotches that appear on the planet’s rings. Called “ring spokes,” the spots circle Saturn along with its signature bands of icy debris.

Scientists have known about ring spokes for decades, but they’re still not positive where they come from.

“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,” Amy Simon, a NASA planetary scientist and lead scientist for Hubble’s Outer Planet Atmospheres Legacy (OPAL) program, says in a statement from NASA.

The agency’s Voyager 2 space probe first imaged Saturn’s ring spokes in 1981, and the Cassini spacecraft also observed the mysterious phenomenon while studying the gas giant between 2004 and 2017.

But Voyager 2 left the solar system roughly five years ago, and NASA deliberately ended Cassini’s mission by crashing the probe into Saturn in 2017. Now, Hubble has taken up the mantle of studying Saturn’s ring spokes from its perch in low-Earth orbit. The new photo was taken by the space telescope on October 22, 2023.

Cassini Views Spokes in Saturn's Rings [720p]

The spokes disappear around Saturn’s winter and summer solstices, but researchers think they will become more prevalent around the planet’s fall and spring equinoxes. They expect to start spotting more spokes as we get closer to the next autumnal equinox for Saturn’s northern hemisphere, which will occur on May 6, 2025, per a NASA statement from February.

“We are heading toward Saturn equinox, when we’d expect maximum spoke activity, with higher frequency and darker spokes appearing over the next few years,” Simon says in NASA’s statement.

It takes Saturn a little under 30 years to complete an orbit of the sun, so its seasons each last around seven years. Its last equinox was in 2009, around which time Cassini detected the spokes.

During the equinoxes, Saturn’s rings are tilted more toward the sun. Scientists suspect that when the planet is in this alignment, solar wind—a plasma of charged, subatomic particles moving at extremely high speeds from the sun—hits Saturn’s magnetic field more strongly. This creates an electrically charged environment that could lead the spokes to form.

The rings’ tiniest icy particles could become charged and float above the rest of the rings, leading to the appearance of spokes, per NASA’s February statement. Scientists still need to confirm this theory, though.

See Saturn's 'ring spokes' in amazing Hubble Space Telescope time-lapse

Ring spokes only last for a couple of rotations around Saturn, but new spokes are constantly appearing during active periods. While they appear small relative to Saturn’s girth, the structures can actually stretch longer than Earth’s diameter. The spokes can appear either light or dark, according to the European Space Agency (ESA).

Hubble’s OPAL program will continue to observe Saturn as the planet’s next equinox draws nearer. The telescope observes wavelengths of light ranging from ultraviolet to near-infrared from Saturn.

The data could provide further insights into how the spokes are formed and how they work. Studying other gas giants in our solar system, which all have similar halos of debris, could reveal whether their rings also have spokes.

“It’s a fascinating magic trick of nature we only see on Saturn—for now at least,” Simon says in NASA’s February statement.

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