spaceSpace and Physics

Something Just Put A Dent In One Of Saturn's Rings


Robin Andrews

Science & Policy Writer

Whodunnit? The dent in Saturn's F ring is clear to see. NASA/JPL-Caltech/Space Science Institute

We may have an extraterrestrial vandal on our hands, ladies and gentlemen. A team of scientists working with NASA’s Cassini space mission has noticed that there’s a “dent” in one of Saturn’s remarkably beautiful rings.

It appears the dent (also termed blob or malformation) is in the gas giant’s F ring. With a radius of 140,220 kilometers (87,130 miles), it’s the outermost discrete ring of the planet, and one of the most active in the Solar System – scientists have observed features within it changing in a matter of only a few hours.


So where on Earth – or on Saturn – did this deformity come from? There are numerous possibilities, including a passing asteroid, a rogue moon, or perhaps some unforeseen disruption within the icy, dusty ring itself.


All of Saturn's rings in view. The F-ring is the thing that's white, bright and closest to the browner, teal-colored inner rings. NASA/JPL-Caltech/Space Science Institute

Within the image, the moon Pandora can be seen hiding within the bottom-right column. This lurking satellite, just 80 kilometers (50 miles) across, certainly looks suspicious, and scientists in the past have wondered what it may be doing to the F ring. Some have thought that it uses its gravitational influence to keep not just additional particles away from the ring, but to also maintain the shape of the ring by orbiting closely nearby.

It could be possible that it managed to pick up a larger particle from outside of the ring and fling it towards it, or it may have drifted a little too close and moved a larger particle from the ring itself. However, recent studies and observations have led many to conclude that its gravitational effect on the F ring has been overstated.


Enter Prometheus. This potato-shaped moon of Saturn also drifts and orbits on the inner edge of the narrow F ring, just as Pandora orbits slightly outside of it. Prometheus is known as a shepherd moon, in that it uses its gravitational influence to clear the space near the F ring, while keeping the ring itself largely in shape.

However, Cassini observations have led many to think that Prometheus, thanks to its chaotic orbit, can also create gravitational “knots,” which ultimately allow Prometheus to hijack particles from the ring and launch them away. This, NASA thinks, is the reason behind the new dent in the F ring: Prometheus has stolen something from it.

Cassini has actually seen hundreds of these dents or “ring jets” since it began orbiting Saturn in 2004. In the past, these jets have taken on some twisted and rather aesthetically pleasing shapes, featuring braided and rippled structures. This latest example occurred around the time the photograph was taken on April 8, and more recent images this month show that it’s beginning to smooth out.content-1466423553-f-gif.gif

There is one more twist to the tale: Pandora may not be guilt-free after all. The only reason Prometheus is able to disrupt the F ring is because it has a wobbly orbit. This perturbed orbital path is most certainly caused by the gravitational pull of Pandora, so Pandora in effect is an accessory to the crime.


Ultimately, this photograph is another striking piece of evidence showcasing what an active and dynamic place Saturn’s rings are. An increasing body of evidence is highlighting just how fragile and ephemeral they are, too.

For example, a recent study revealed that these rings, contrary to popular scientific opinion, aren’t actually billions of years old. It’s likely that they were formed when multiple moons smashed into each other around the time the non-avian dinosaurs were in decline, around 100 million years ago. That means that Stegosaurus roamed the world long before Saturn was adorned with its most visually resplendent feature.

Gif in text: Pandora (left) versus Prometheus (right): The latter's strong gravitational effects on the F ring can be seen. NASA/JPL/Space Science Institute


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