Moonlets created and destroyed in a ring of Saturn

September 9, 2014 by Preston Dyches, SETI Institute
Cassini spied just as many regular, faint clumps in Saturn's narrow F ring (the outermost, thin ring), like those pictured here, as Voyager did. But it saw hardly any of the long, bright clumps that were common in Voyager images. Credit: NASA/JPL-Caltech/SSI

There is an ongoing drama in the Saturnian ring system that causes small moons to be born and then destroyed on time scales that are but an eyeblink in the history of the solar system. SETI Institute scientists Robert French and Mark Showalter have examined photos made by NASA's Cassini spacecraft and compared them to 30 year-old pictures made by the Voyager mission. They find that there is a marked difference in the appearance of one of the rings, even over this cosmologically short interval, a difference that can be explained by the brief strut and fret of small moons.

"The F is a narrow, lumpy feature made entirely of water ice that lies just outside the broad, luminous rings A, B, and C," notes French. "It has bright spots. But it has fundamentally changed its appearance since the time of Voyager. Today, there are fewer of the very bright lumps."

The bright spots come and go over the course of hours or days, a mystery that the two SETI Institute astronomers think they have solved.

"We believe the most luminous knots occur when tiny moons, no bigger than a large mountain, collide with the densest part of the ring," says French. "These moons are small enough to coalesce and then break apart in short order."

The F ring is at a special place in the , at a distance known as the Roche limit, named for French astronomer Edouard Roche who first pointed out that if a moon orbits too close to a planet, the difference in gravitational tug on its near and far side can tear it apart. This happens at a distance dependent on the mass of the planet, and in the case of Saturn, happens to be at the location of the F ring. Consequently, material here is caught between the yin and yang of forming small moons, and having them pulled apart. The moons in question are typically no more than 3 miles (5 km) in size, and consequently can come together quickly.

This chaotic region is given additional stir by Prometheus, a moon that's roughly 60 miles (100 km) in size that orbits just inside the F ring. Every 17 years, Prometheus aligns with the F ring in a way that emphasizes its gravitational influence on the ring's particles, precipitating the formation of the mini-moons, or moonlets.

"These newborn moonlets will repeatedly crash through the F ring, like bumper cars, producing bright clumps as they careen through lanes of material," says Showalter. "But this is self-destructive behavior, and the moons – being just at the Roche limit – are barely stable and quickly fragmented."

This scenario can explain the rapid variation in the number of bright clumps in the F ring, but is it true? If the periodic influence of Prometheus is causing the waxing and waning of the clumps, then there should be an increase in their prevalence over the next few years, a prediction that the astronomers will be checking with Cassini data.

In addition to the drama of moons that come and go over less than a human lifetime, studies of the ring system give insight into how solar systems in general are built.

"The sort of processes going on around Saturn are very similar to those that took place here 4.6 billion years ago, when the Earth and the other large planets were formed," notes French. "It's an important process to understand."

This research was published in the online edition of the journal Icarus on July 15, 2014.

Explore further: A ghostly 'ladder' in Saturn's F ring

More information: Robert S. French, Shannon K. Hicks, Mark R. Showalter, Adrienne K. Antonsen, Douglas R. Packard, "Analysis of clumps in Saturn's F ring from Voyager and Cassini," Icarus, Volume 241, October 2014, Pages 200-220, ISSN 0019-1035, dx.doi.org/10.1016/j.icarus.2014.06.035 . Preprint: arxiv.org/abs/1408.2548

Related Stories

A ghostly 'ladder' in Saturn's F ring

June 3, 2013

Saturn's F ring is certainly a curious structure. Orbiting the giant planet 82,000 kilometers above its equatorial cloud tops, the F ring is a ropy, twisted belt of bright ice particles anywhere from 30-500 km wide. It can ...

The rings on the planet go 'round and 'round...

December 24, 2012

Recently I posted an image of two of Saturn's shepherd moons, Pandora and Prometheus, captured by Cassini in a face-off across the spindly F ring. Now here's a much wider-angle view of the gas giant's rings, seen by Cassini ...

Saturn's fluctuating F ring

November 20, 2012

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

NASA Cassini images may reveal birth of a Saturn moon

April 15, 2014

(Phys.org) —NASA's Cassini spacecraft has documented the formation of a small icy object within the rings of Saturn that may be a new moon, and may also provide clues to the formation of the planet's known moons.

Recommended for you

Archaeologists discover Incan tomb in Peru

February 16, 2019

Peruvian archaeologists discovered an Incan tomb in the north of the country where an elite member of the pre-Columbian empire was buried, one of the investigators announced Friday.

What rising seas mean for local economies

February 15, 2019

Impacts from climate change are not always easy to see. But for many local businesses in coastal communities across the United States, the evidence is right outside their doors—or in their parking lots.

Where is the universe hiding its missing mass?

February 15, 2019

Astronomers have spent decades looking for something that sounds like it would be hard to miss: about a third of the "normal" matter in the Universe. New results from NASA's Chandra X-ray Observatory may have helped them ...

The friendly extortioner takes it all

February 15, 2019

Cooperating with other people makes many things easier. However, competition is also a characteristic aspect of our society. In their struggle for contracts and positions, people have to be more successful than their competitors ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

cantdrive85
1 / 5 (1) Sep 10, 2014
What's truly amazing is how dynamic Saturn's ring system appears to be.

http://vimeo.com/40234826

Note the asteroid "bouncing" off the rings at :48.
yep
not rated yet Sep 10, 2014
Cool video!

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.