Bumper Car Moonlets Crash and Crumble in Saturn’s F Ring

Nothing stands still. Everything evolves. So why shouldn’t Saturn’s kookie, clumpy F ring put on a new face from time to time? 

A recent NASA-funded study compared the F ring’s appearance in six years of observations by the Cassini mission to its appearance during the Saturn flybys of NASA’s Voyager mission, 30 years earlier.

Example of a kink in part of Saturn's F ring. While the ring is held together by the shephard moons Prometheus and Pandora, which orbit just inside and  outside the ring, embedded moonlets are believed responsible for the kinks and clumps. Credit: NASA
A kink in part of Saturn’s F ring. While the ring is held together by the shepherd moons Prometheus and Pandora, which orbit just inside and outside the ring, embedded moonlets are believed responsible for the kinks and clumps. The rings is several hundred kilometers wide. Credit: NASA

While the F ring has always displayed clumps of icy matter, the study team found that the number of bright clumps has nose-dived since the Voyager space probes saw them routinely during their brief flybys 30 years ago. Cassini spied only two of the features during a six-year period.

Scientists have long suspected that moonlets up to 3 miles (5 km) wide hiding in the F ring are responsible for its uneven texture. Kinks and knots appear and disappear within months compared to the years of observation needed changes in many of the other rings.

Saturn's F ring is extremely narrow compared to the historic A, B and C rings. It measure just a few hundred kilometers across. Credit: NASA/Cassini
Saturn’s F ring is extremely narrow compared to the historic A, B and C rings. It measures just a few hundred kilometers across. Credit: NASA/Cassini

“Saturn’s F ring looks fundamentally different from the time of Voyager to the Cassini era,” said Robert French of the SETI Institute in Mountain View, California, who led the study along with SETI Principal Investigator Mark Showalter. “It makes for an irresistible mystery for us to investigate.”

A 2007 artist impression of the aggregates of icy particles that form the 'solid' portions of Saturn's rings. These elongated clumps are continually forming and dispersing. The largest particles are a few metres across.They clump together to form elongated, curved aggregates, continually forming and dispersing. Credit: NASA/JPL/Univ. of Colorado
A 2007 artist impression of small boulder-like chunks of ice that comprise Saturn’s rings. The largest are about 10-12 feet across.They clump together to form elongated, curved aggregates, continually forming and dispersing. Credit: NASA/JPL/Univ. of Colorado

Because the moonlets lie close to the ring and cross through it every orbit, the research team hypothesizes that the clumps are created when they crash into and pulverize smaller ring particles during each pass. They suspect that the decline in the number of exceptionally bright kinks and the clumps echoes a decline in the number of moonlets available to do the job.

So what happened between Voyager and Cassini? Blame it on Prometheus. The F ring circles Saturn at a delicate point called the Roche Limit. Any moons orbiting closer than the limit would be torn apart by Saturn’s gravitational force.

A possible culprit! Prometheus measures 74 miles (119 km) across and orbits the inner edge of Saturn's F ring. Credit: NASA
The culprit? Prometheus measures 74 miles (119 km) across and orbits the inner edge of Saturn’s F ring. Credit: NASA

“Material at this distance from Saturn can’t decide whether it wants to remain as a ring or coalesce to form a moon,” said French.  “Prometheus orbits just inside the F ring, and adds to the pandemonium by stirring up the ring particles, sometimes leading to the creation of moonlets, and sometimes leading to their destruction.”

Every 17 years the orbit of Prometheus aligns with the orbit of the F ring in a way that enhances its gravitational influence. The researchers think the alignment spurs the creation of lots of extra moonlets which then go crashing into the ring, creating bright clumps of material as they smash themselves to bits against other ring material.

Sounds like a terrifying version of carnival bumper cars. In this scenario, the number of moonlets would gradually drop off until another favorable Prometheus alignment.

The Voyagers encounters with Saturn occurred a few years after the 1975 alignment between Prometheus and the F ring, and Cassini was present for the 2009 alignment. Assuming Prometheus has been “working” to build new moons since 2009, we should see the F ring light up once again with bright clumps in the next couple years.

Cassini will be watching.

Saturn’s Fluctuating F Ring

Bright clumps of material spotted within Saturn’s ropy F ring (NASA/JPL/SSI)

Released today, this image acquired by NASA’s Cassini spacecraft shows some interesting structures forming within Saturn’s thinnest but most dynamic ring.

Of Saturn’s countless ring structures the F ring may very well be the most dynamic, if not the most fascinating. Orbiting Saturn just outside the edge of the A ring at a distance of 140,000 km (87,000 miles), the F ring is a hazy, ropy band of fine ice particles that shift, twist and occasionally gather into bright clumps… only to drift apart once more.

The F ring can range in width from 30 to 500 km (20-500 miles), depending on what’s going on in and outside of it.

The image above, originally captured by Cassini on June 28 and released today by the Cassini Imaging Central Laboratory for Operations (CICLOPS), shows a particularly bright clump of material at the outer edge of the F ring, as well as some finer structures and streamers forming within the inner bands. Due to the lighting geometry its thought that the clumps are mostly composed of dusty material.

Detail of the ghostly F ring structures (NASA/JPL/SSI)

The features seen here are likely due to the ring’s interactions with passing shepherd moons — such as the 148-km-wide Prometheus — or with small moonlets embedded within the ring itself. Mostly made of fine particles of dust and ice smaller than those found in smoke, the material orbiting within the F ring is extremely susceptible to external gravitational influences.

Original image scale is 4 km (3 miles) per pixel.

See more images from the entire Cassini mission on the CICLOPS site here (and for a look at more interesting ring dynamics check out these recent Cassini images of my personal favorite moon, Daphnis.)