Image: Saturn rings
NASA / Univ. of Colorado
Bright bands in the left part of the image are peaks of a density wave caused by gravitational stirring of the rings by the moon Janus. A smaller density wave on the right is produced by the moon Pandora. The observation was made by watching the light of a star flicker as the rings passed in front of it.
By Senior science writer
updated 11/16/2004 5:01:07 PM ET 2004-11-16T22:01:07

A trio of new observations suggests that several tiny, unseen moons orbit Saturn and control the shape of its rings.

One study found strong evidence for a small moon, likely just a few miles wide, creating patterns at the edge of a gap that are identical to features at another ring boundary generated by a larger, known moon.

In other research, scientists spotted clumps the size of football fields embedded in the rings. Nothing so small has ever been seen before around Saturn, but the two-dimensional images don't reveal whether the clumps are solid objects or cloudlike gatherings of particles. This work also revealed that Saturn's myriad ringlets have surprisingly sharp edges, suggesting they are sculpted by tiny, undiscovered moons.

A third investigation found a halo of oxygen around Saturn, which the researchers think is caused by constant collisions between small objects — referred to as "moonlets" — embedded in the rings.

The studies were presented last week at the annual American Astronomical Society Division for Planetary Sciences meeting in Louisville, Ky.

Hiding in a gap
A few relatively small moons are clearly visible in major gaps between Saturn's rings and had been discovered in Voyager data. With diameters as little as 12 miles (20 kilometers), these moons are known to gravitationally shepherd ring particles as well as maintain gaps between the rings.

But whether or not every one of the dozen or so gaps in Saturn's rings has embedded moons is an important question the Cassini spacecraft is designed to answer. Scientists also do not know how Saturn's rings originally formed or exactly what they're made of.

Carolyn Porco of the Space Science Institute in Boulder, Colo., led a study of Cassini images suggesting one possible new moonlet.

Porco told that she has "circumstantial evidence" for a small moon in the Keeler gap, near the outer edge of Saturn's main rings. The evidence is in the form of "spiky wisps" of material that protrude into the Keeler gap from its outer edge.

The features resemble those generated on the inner edge of the F ring by the shepherding moon Prometheus, Porco said.

The moon has not been seen yet, and Porco suspects it's no bigger than 3 miles (5 kilometers) or so in diameter. She stressed that the size estimate is just a guess, based on the size of the gap — just 22 miles (35 kilometers) wide — the dynamics involved, and the fact that the moon has never been seen.

"It was exciting to see the fingerprints of this moon," she said. "We'll be looking closely for it in future observations."

More moon fingerprints
Another set of observations has helped scientists better understand the size of particles in the rings, also showing that the rings have stark cutoff points with glaring gaps between. This most detailed view ever obtained of Saturn's rings was made by watching light from a distant star flicker as it passed through the rings.

Particles are bunched very closely in individual ringlets, said Joshua Colwell of the University of Colorado at Boulder. At times, the starlight is completely blocked for stretches of 130 feet (40 meters). In other spots, the starlight shone right through. "We see rings go from completely transparent to completely opaque," Colwell said.

Future observations might reveal whether the structures are solid clumps — essentially icy moonlets — or fluffy agglomerations of particles that aren't stuck together.

What's potentially more telling is that the amount of material drops off to zero in as little as 160 feet (50 meters) from the edge of a ringlet.

"The very sharp transitions were not expected," Colwell said in a telephone interview.

It is likely that gravity from a nearby small moon, along with collisions between ring particles, confine the particles in a ringlet, he said.

The high-resolution observation also showed more than 30 density waves in the rings caused by gravitational interactions between moons and ring particles. Density waves have been spotted in the rings before, but not with this resolution.

"This can create a wave in the ring that looks like a ripple in a pond," Colwell said. "The shapes of these wave peaks and troughs help scientists understand whether the ring particles are hard and bouncy, like a golf ball, or soft and less bouncy, like a snowball."

Constant collisions
Another new set of observations shows an immense cloud of oxygen atoms surrounding Saturn.

Scientists think the cloud is replenished when moonlets within the ring system collide, shatter, and release ice particles. Radiation belts around the giant planet bathe the ice particles, which release the oxygen, said the University of Colorado's Larry Esposito, who heads up operation of Cassini's Ultraviolet Imaging Spectrometer.

The finding probably explains observations from earlier this year showing that ring particles are covered in gunk, and that the amount of gunk varies in different locations.

"The fluctuations we see can be explained by the recent destruction of small moons within the rings and by wave action in the rings that dredges fresh material onto the surfaces of the ring particles," Esposito said. "This indicates that the material in the rings is continually recycled from rings to moons and back."

Porco said the study of Saturn's rings and moons could pay off in an unexpected way.

The system is similar to the disk that is typically left orbiting a star after its formation, and out of which planets form. Determining how Saturn's moons and moonlets develop, collide and sometimes end up in odd-shaped orbits could shed light on the formation of planetary systems.

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Photos: Best of Cassini

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  1. Starring Saturn

    This backlit view of Saturn was voted the favorite image to come from the Cassini orbiter - and it has been described as "perhaps the most stunning photograph ever taken." The image, captured on Sept. 15, 2006, shows two faint rings that were discovered by the Cassini team. And at the highest resolution, Earth itself appears as a pale blue dot just to the left of the brightest rings, at about the 10 o'clock position. (NASA / JPL / Space Science Institute) Back to slideshow navigation
  2. Dark rings

    An infrared image from the Cassini orbiter, acquired May 24, 2007, reveals clouds beneath the hazes in Saturn's atmosphere, as well as the unilluminated side of the giant planet's rings. The shadows of the rings fall upon the planet's cloud layer. This image shares the honors as the favorite black-and-white picture from Cassini. (NASA / JPL / Space Science Institute) Back to slideshow navigation
  3. Abstract art

    A Cassini image from May 10, 2006, shows the shaded edge of Saturn's disk, rounded by dark rings seen nearly edge-on. The crescent disk of Titan, Saturn's largest moon, can be seen in the background beyond the rings. This image shares the honors as the favorite black-and-white image from Cassini. (NASA / JPL / Space Science Institute) Back to slideshow navigation
  4. Pearly moons

    Two of Saturn's moons - Tethys and Enceladus - look like pearls backdropped by the planet's disk in this image, captured on July 24, 2007. The thin "string" connecting the pearls is actually the plane of the planet's rings, seen edge-on. The rings cast a dark shadow on Saturn's disk. Two other moons appear in this image, although they can barely be made out at the highest resolution: Hyperion is near the lower left corner of the image, and Epimetheus is the slightest of specks between Tethys and Enceladus. (NASA / JPL / Space Science Institute) Back to slideshow navigation
  5. Saturn from on high

    The Cassini spacecraft provides a high-contrast view of Saturn and its rings, as seen from above. This portrait is actually a mosaic of 36 images taken on Jan. 19, 2007, from about 40 degrees above the plane of the rings. (NASA / JPL / Space Science Institute) Back to slideshow navigation
  6. Shadows on clouds

    Saturn's darkened rings cast shadows on the planet's blue and gold cloud tops, while the moon Dione hangs like a dot in the black sky beyond. This image was taken by the Cassini spacecraft on Feb. 4, 2007, from a distance of about 800,000 miles. (NASA / JPL / Space Science Institute) Back to slideshow navigation
  7. View from below

    Cassini coasts beneath giant Saturn, staring upward at its gleaming crescent and icy rings. A great bull's-eye pattern is centered on the south pole, where a vast, hurricane-like storm spins. This view, obtained on Jan. 30, 2007, looks toward the lit side of the rings from about 26 degrees below the ring plane. (NASA / JPL / Space Science Institute) Back to slideshow navigation
  8. Pastel planet

    Dark and sharply defined ring shadows appear to constrict the flow of color from Saturn's warmly hued south to the bluish northern latitudes. Scientists studying Saturn are not yet sure about the precise cause of the color change from north to south. The different colors may be due to seasonal effects on the atmosphere. The images that went into this mosaic were obtained by the Cassini spacecraft on Feb. 4, 2007. (NASA / JPL / Space Science Institute) Back to slideshow navigation
  9. Ringing success

    This ultraviolet image from the Cassini spacecraft shows the detailed composition of Saturn's outer C and inner B rings from left to right, with the inner B ring beginning a little more than halfway across the image. The general pattern is from "dirty" red particles to the denser ice shown in turquoise as the ringlets spread outward. (University Of Colorado, LASP / AP) Back to slideshow navigation
  10. A is for amazing

    This ultraviolet image shows the A ring, beginning with a 'dirty' interior of red followed by a general pattern of more turquoise as it spreads away from the planet, indicating a denser material made up of ice. The red band roughly three-fourths of the way outward in the A ring is known as the Encke gap. (University Of Colorado, LASP - NASA / Reuters) Back to slideshow navigation
  11. Casting a shadow

    This image taken by Cassini shows the planet Saturn casting a shadow over its rings. (NASA - JPL - Caltech / Reuters) Back to slideshow navigation
  12. Titan revealed

    This is an infrared image of Titan, one of Saturn's moons, mapping the surface hidden beneath the moon's opaque atmosphere. Green areas represent water ice, while yellow areas have higher concentrations of hydrocarbons. The white spot is a methane cloud. (NASA) Back to slideshow navigation
  13. Ripples in the ring

    Scallops in the ring on the left side of this image were likely caused by a Saturnian moon rolling along the edge. One bright ringlet is visible within the gap, and at least one other faint ringlet can be made out. "This is textbook ring physics, right there, in one image," says Cassini imaging team leader Carolyn Porco. (NASA - SSI) Back to slideshow navigation
  14. Wisps in space

    A close-up of one of Saturn's rings shows a wispy pattern of ripples that may have been stirred up by a moonlet's orbit. Such unprecedented views of ring details are possible because of the Cassini camera's resolution. (NASA TV) Back to slideshow navigation
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