Bright Clumps in Saturn Ring Now Mysteriously Scarce

A map of Saturn's F ring from 2006 shows one of the few bright, extended clumps (indicated by a green box) seen during six years of observation by Cassini.

Compared to the age of the solar system -- about four-and-a-half billion years -- a couple of decades are next to nothing. Some planetary locales change little over many millions of years, so for scientists who study the planets, any object that evolves on such a short interval makes for a tempting target for study. And so it is with the ever-changing rings of Saturn.

Case in point: Saturn's narrow, chaotic and clumpy F ring. 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. The study team found that, while the overall number of clumps in the F ring remained the same, the number of exceptionally bright clumps of material plummeted during that time. While the Voyagers saw two or three bright clumps in any given observation, Cassini spied only two of the features during a six-year period. What physical processes, they wondered, could cause only the brightest of these features to decline sharply?

While a variety of features in Saturn's many rings display marked changes over multiple years, the F ring seems to change on a scale of days, and even hours. Trying to work out what is responsible for the ring's tumultuous behavior is a major goal for ring scientists working on 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."

The researchers hypothesize that the brightest clumps in the F ring are caused by repeated impacts into its core by small moonlets up to about 3 miles (5 kilometers) wide, whose paths around Saturn lie close to the ring and cross into it every orbit. They propose that the diminishing number of bright clumps results from a drop in the number of these little moonlets between the Voyager and Cassini eras.

As for what might have caused the moonlets to become scarce, the team has a suspect: Saturn's moon Prometheus. The F ring encircles the planet at a special location, near a place called the Roche limit -- get any closer to Saturn than this, and tidal forces from the planet's gravity tear apart smaller bodies. "Material at this distance from Saturn can't decide whether it wants to remain as a ring or coalesce to form a moon," French said. 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 such a way that its influence is particularly strong. The study team thinks this periodic alignment might spur the creation of many new moonlets. The moonlets would then crash repeatedly through the F ring, like cars in a Hollywood high-speed chase, creating bright clumps as they smash across lanes of ring material. Fewer clumps would be created as time goes by, because the moonlets themselves are eventually destroyed by all the crashes.

As with any good scientific hypothesis, the researchers offer a way to test their ideas. It happens that the Voyager 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. If the moon's periodic influence is indeed responsible for creating new moonlets, then the researchers expect that Cassini would see the F ring return to a Voyager-like number of bright clumps in the next couple of years.

"Cassini's continued presence at Saturn gives us an interesting opportunity to test this prediction," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, who was not involved in the study. "Whatever the result, we're certain to learn something valuable about how rings, as well as planets and moons, form and evolve."

The study by French and colleagues was published in the online edition of the Journal Icarus on July 15, 2014.

Image credit: NASA/JPL-Caltech/SSI

Possible New Moon Forming Around Saturn

The disturbance visible at the outer edge of Saturn's A ring in this image from NASA's Cassini spacecraft could be caused by an object replaying the birth process of icy moons.

The image is adapted from one in a paper in the journal Icarus, reporting the likely presence of an icy body causing gravitational effects on nearby ring particles, producing the bright feature visible at the ring's edge. The object, informally called "Peggy," is estimated to be no more than about half a mile, or one kilometer, in diameter. It may be in the process of migrating out of the ring, a process that one recent theory proposes as a step in the births of Saturn's several icy moons.

This image is a portion of an observation recorded by the narrow-angle camera of Cassini's imaging science subsystem on April 15, 2013. The bright feature at the edge of the A ring is about 750 miles (about 1,200 kilometers) long.

This view looks toward the illuminated side of the rings from about 53 degrees above the plane of the rings. It was obtained from a distance of approximately 775,000 miles (1.2 kilometers) from Saturn, with a sun-Saturn-spacecraft, or phase, angle of 31 degrees. The scale is about 4 miles (about 7 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Note: For more information, see NASA Cassini Images May Reveal Birth of a Saturn Moon and Possible New Moon Forming Around Saturn.

Wavy F-Ring

The constant change in Saturn's wavy, wiggly F ring is on display in this set of images obtained by NASA's Cassini spacecraft. The images show a view looking directly down onto the ring with the planet removed from the center. The radial distance from the center of the F ring has been exaggerated by a factor of 140 to make the wiggles and other radial structures more visible.

The regular patterns here primarily occur because of the gravitational pull of the moon Prometheus, which averages about 53 miles or 86 kilometers in diameter. Prometheus can create channels and snowballs as they part and push together icy F ring particles. These regular wiggles are about 65 miles (100 kilometers) from top to bottom.

Other processes are also at work. In the top left image, the object known as S/2004 S 6 – which may be a moonlet or just a dense clump – has produced an extended trail of material that can be seen at the top left of the image. A close-up of this event can be seen in PIA08863. In some of the images (bottom row, middle and right image), a bright spiral ring is visible. This was caused by an object, possibly S/2004 S 6, colliding with the main F ring and producing a trail of particles that subsequently sheared right around the planet. These events give the F ring its multi-stranded appearance.

Cassini's narrow-angle camera obtained the images in the top row on December 23, 2006; February 27, 2007 and March 17, 2007 (left to right). In the bottom row, the images were obtained on April 18, 2007; January 7, 2008 and February 24, 2008.

The F ring is the outermost of Saturn's main rings, with a radius of about 87,129 miles (140,220 kilometers).

Photo credit: NASA/JPL-Caltech/SSI/QMUL

Unexpected Chevron Structure on the Edge of Saturn's B-Ring

The outer edge of Saturn's B ring exhibits an unexpected feature in this movie made from images captured by NASA's Cassini spacecraft. The images were obtained early in the planet's equinox "season" -- the period leading up to and away from August 11, 2009 when the Sun was over the planet's equator and lit the rings exactly edge on.

The B ring is shown at the top of the frame. The Cassini Division, a low-density region that separates the A and B rings, dominates the middle of the frame. The inner A ring is at the bottom.

It is apparent in the movie that the outer B ring edge location varies with time. For a more detailed view and explanation for this behavior, see PIA12794.

But, about halfway through, an unusual, 20,000-kilometer-long (12,000-mile-long), chevron-shaped structure can be seen moving along the B ring edge. Higher resolution images, taken during equinox, have shown that this region is the site of vertical structures, as tall as 3.5 kilometers (2.2 miles), whose existence was betrayed by long shadows (see PIA11668).

Cassini imaging scientists found that this chevron feature is one of two sites at the B ring's outer edge that does not follow any of the three newly discovered rotating patterns distorting the ring's edge or another pattern previously known to be caused by the moon Mimas. They have found instead that it orbits Saturn as would an independently orbiting body. As a result, scientists conclude that these are likely sites of massive bodies, or moonlets, embedded near the ring's edge but independently orbiting Saturn. In the particular region shown in this movie, the moonlets are likely big enough to cause ring material streaming past them to be excessively compressed and thrown vertically as a result. The moonlets themselves can't be seen.

This interpretation is supported by Cassini's previous discovery of a moonlet embedded in this region of the B ring (see PIA11665). The imaged moonlet, whose size is estimated at 300 meters (1,000 feet) across, was found only because it was betrayed by the shadow it cast during Saturn's August 2009 equinox period.

This view looks toward the southern, sunlit side of the rings from about 4 degrees below the ring plane.

The movie is a concatenation of 39 images taken about 2 minutes, 40 seconds apart, over the span of 1 hour, 40 minutes. The images, taken on February 25, 2009, were re-projected into the same viewing geometry.

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The images were taken in visible light with the Cassini spacecraft narrow-angle camera. The view was obtained at a distance of approximately 822,000 kilometers (511,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 154 degrees. Image scale is 5 kilometers (3 miles) per pixel.

Video credit: NASA/JPL/Space Science Institute

Note: A continuous loop video of the chevron structure can be found here.

Vertical Structures in Saturn's B-Ring

Vertical structures, among the tallest seen in Saturn's main rings, rise abruptly from the edge of Saturn's B ring to cast long shadows on the ring in this image taken by NASA's Cassini spacecraft two weeks before the planet's August 2009 equinox.

Part of the Cassini Division, between the B and the A rings, appears at the top of the image, showing ringlets in the inner division.

In this image, Cassini's narrow angle camera captured a 1,200-kilometer-long (750-mile-long) section arcing along the outer edge of the B ring. Here, vertical structures tower as high as 2.5 kilometers (1.6 miles) above the plane of the rings -- a significant deviation from the vertical thickness of the main A, B and C rings, which is generally only about 10 meters (about 30 feet).

Cassini scientists believe that this is one prominent region at the outer edge of the B ring where large bodies, or moonlets, up to a kilometer or more in size, are found. It is possible that these bodies significantly affect the ring material streaming past them and force the particles upward, in a "splashing" manner.

This image and others like it (see PIA11669) are only possible around the time of Saturn's equinox, which occurs every half-Saturn-year, or about every 15 Earth years. The illumination geometry that accompanies equinox lowers the Sun's angle to the ring plane and causes structures jutting out of the plane to cast long shadows across the rings. The "season" of equinox allows shadows to appear on the rings in the months before and after equinox, but the actual equinox occurred August 11, 2009, as the Sun shone directly edge-on to the ring plane.

This view looks toward the southern, sunlit side of the rings from about 32 degrees below the ring plane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 26, 2009. The view was acquired at a distance of approximately 336,000 kilometers (209,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 132 degrees. Image scale is 2 kilometers (1 mile) per pixel.

Photo credit: NASA/JPL/Space Science Institute

Long Spiky Shadows

Vertical structures in the variable outer edge of Saturn's B ring cast shadows in these two images captured by NASA's Cassini spacecraft shortly after the planet's August 2009 northern vernal equinox.

In these two images, Cassini's narrow angle camera captured a 20,000-kilometer-long (12,000-mile-long) section arcing along the outer edge of the B ring in Saturn's rings. This particular section of the ring is known to be the site of vertical structures as tall as 3.5 kilometers (2.2 miles). See PIA11668 to learn more.

The B ring is shown at the top of both frames. The Cassini Division, a low-density ring region that separates the B ring from the A ring, runs from the bottom right to the upper left of the images. The A ring is at the bottom.

Cassini imaging scientists have concluded that this region and another in this vicinity are the sites of massive bodies, or moonlets, embedded near the ring's edge but independently orbiting Saturn. In the particular region shown in this image, the moonlets are likely big enough to cause ring material streaming past them to be excessively compressed and thrown vertically as a result.

This interpretation is supported by Cassini's previous discovery of a moonlet embedded in this region of the B ring (see PIA11665). The imaged moonlet, whose size is estimated at 300 meters (1,000 feet) across, was found only because it was betrayed by the shadow it cast during Saturn's August 2009 equinox period.

This image and others like it are only possible around the time of Saturn's equinox which occurs every half-Saturn-year (equivalent to about 15 Earth years). The illumination geometry that accompanies equinox lowers the Sun's angle to the ring plane and causes structures jutting out of the plane to cast long shadows across the rings. Cassini's cameras have spotted not only the predictable shadows of some of Saturn's moons (see PIA11657), but also the shadows of newly revealed vertical structures in the rings themselves (see PIA11654).

Both panels were re-projected to the same viewing geometry. The right-hand panel is a mosaic of two images. The images of the right-hand panel were taken in visible light with the Cassini spacecraft narrow-angle camera on August 19, 2009. Two stars are visible.

The left-hand panel was taken in visible light with the Cassini spacecraft narrow-angle camera on August 16, 2009. One star is visible.

This view looks toward the northern, sunlit side of the rings from about 11 degrees above the ring plane at a distance of approximately 2.3 million kilometers (1.4 million miles) from Saturn. Image scale is 13 kilometers (8 miles) per pixel.

Photo credit: NASA/JPL/Space Science Institute

Atlas and F-Ring Shadows

Delicate shadows are cast outward from Saturn's thin F ring in the lower left of this image taken as the planet approached its August 2009 equinox.

The moon Atlas (30 kilometers, or 19 miles across) is seen just above the center of the image between the A ring and thin F ring. Several background stars are also visible.

The novel illumination geometry created around the time of Saturn's August 2009 equinox allows out-of-plane structures and moons orbiting in or near the plane of Saturn's equatorial rings to cast shadows onto the rings. These scenes are possible only during the few months before and after Saturn's equinox which occurs only once in about 15 Earth years. To learn more about this special time and to see movies of moons' shadows moving across the rings, see PIA11651 and PIA11660.

This view looks toward the northern, unilluminated side of the rings from about 8 degrees above the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 11, 2009. The view was obtained at a distance of approximately 1.098 million kilometers (682,000 miles) from Atlas. Image scale is 7 kilometers (4 miles) per pixel.

Photo credit: NASA/JPL/Space Science Institute

A-Ring Propeller

A propeller-shaped structure, created by an unseen moon, can be seen in Saturn's A ring.

The propeller, which looks like a small, dark line interrupting the bright surrounding ring material, is in the upper left of this image near the edge of the Keeler Gap. See PIA12790 to learn more about propellers.

This view looks toward the southern, unilluminated side of the rings from about 16 degrees below the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on June 3, 2010. The view was acquired at a distance of approximately 279,000 kilometers (173,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 16 degrees. Image scale is 1 kilometer (3,300 feet) per pixel.

Photo credit: NASA/JPL/Space Science Institute

Promethean Shadow Before Equinox

Saturn's moon Prometheus casts a long shadow across the A ring in the middle-right side of this Cassini spacecraft image taken shortly before the planet's August 2009 equinox.

Prometheus (86 kilometers, or 53 miles across) orbits in the Roche Division between the thin F ring and the A ring. The novel illumination geometry created around the time of Saturn's August 2009 equinox allows out-of-plane structures and moons orbiting in or near the plane of Saturn's equatorial rings to cast shadows onto the rings. These scenes are possible only during the few months before and after Saturn's equinox, which occurs only once in about 15 Earth years. To learn more about this special time and to see movies of moons' shadows moving across the rings, see PIA11651 and PIA11660.

This view looks toward the northern, unilluminated side of the rings from about 31 degrees above the ringplane. Many stars are visible. The rings and stars have been brightened by a factor six relative to Prometheus to enhance visibility.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 29, 2009. The view was acquired at a distance of approximately 1.4 million kilometers (870,000 miles) from Prometheus and at a Sun-Prometheus-spacecraft, or phase, angle of 93 degrees. Image scale is 9 kilometers (6 miles) per pixel.

Photo credit: NASA/JPL/Space Science Institute

Note: The photo also shows several streamer channels and gravitational perturbations in the F-Ring. For more on this topic, see Fleeing the Scene and Fan Structures in Saturn's F-Ring.

Closest View of Daphnis

The Cassini spacecraft captures here one of its closest views of Saturn's ring-embedded moon Daphnis.

This image was taken July 5, 2010, at a distance of only about 75,000 kilometers (47,000 miles) from Daphnis. Seen at the upper left of this image, Daphnis (8 kilometers, or 5 miles across) appears in the Keeler Gap near the edge waves it has created in the A ring. The moon's orbit is inclined relative to the plane of Saturn's rings. Daphnis' gravitational pull perturbs the orbits of the particles of the A ring that form the Keeler Gap's edge, and sculpts the edge into waves having both horizontal (radial) and out-of-plane components. Material on the inner edge of the gap orbits faster than the moon so that the waves there lead the moon in its orbit. Material on the outer edge moves slower than the moon, so waves there trail the moon. See PIA11656 to learn more about this process.

Daphnis can also be seen casting a short shadow on the A ring.

This view looks toward the northern, sunlit side of the rings from about 14 degrees above the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera at a Sun-Daphnis-spacecraft, or phase, angle of 58 degrees. Image scale is 452 meters (1,483 feet) per pixel.

Photo credit: NASA/JPL/Space Science Institute

Fan Structures in Saturn's F-Ring

Bright clumps of ring material and a fan-like structure appear near the core of Saturn's tenuous F ring in this mosaic of images from NASA's Cassini spacecraft. Such features suggest the existence of additional objects in the F ring.

These discontinuous clumps near the core of the F ring may be created by the passage of the ring's shepherding moon Prometheus, and they can be seen casting narrow shadows that extend toward the bottom of the mosaic. The shadows are marked with arrows in the annotated version. On the right of the mosaic, a "fan" can be seen dissipating above the bright ring core. The fan (marked "F" in the annotated version) is a series of channels within the F ring's particles that appear to have a common origin but that spread outward radially in different directions. Gravitational perturbations on the ring material by a moonlet or clump of material can create these fans. The moonlet or clump orbits more or less elliptically compared to the rest of the F ring can create these fans. It is probably embedded in the ring and is causing the base of the fan channels to meet. See PIA1285 and PIA12786 for similar observations of such fans.

The diagonal streamer-channels are periodically created by the gravity of the potato-shaped moon Prometheus which is 148 kilometers (92 miles) on its longest side but is on average 86 kilometers (53 miles) across. To learn more and to watch a movie of this streamer-channel phenomenon, see PIA08397.

The images have been re-projected in this mosaic so that the F ring appears straightened rather than curved and compressed azimuthally (along the ring). This change represents a scale compression in the horizontal direction of about 33 to one which is why Prometheus looks like a bright line. Prometheus is marked "Pr" in the annotated version.

This sequence of 42 images was taken over a span of one hour, seven minutes. The earliest image is on the right, and time progresses moving left in the mosaic. Each image was cropped, re-projected and placed side by side in this montage. Scale in the original images was about 3 kilometers (2 miles) per pixel. The images were contrast enhanced and re-projected to a scale of 33 kilometers (21 miles) per pixel in the mosaic's horizontal direction and one kilometer (0.6 miles) per pixel in the mosaic's vertical direction. The single, cropped inset of the clumps included here was then magnified by a factor of two.

The view in the original images looked toward the northern, unilluminated side of the rings from about 70 degrees above the ring plane.

The images were taken in visible light with the Cassini spacecraft narrow-angle camera on December 8, 2008. The view was obtained at a range of distances from approximately 597,000 kilometers (371,000 miles) to 615,000 kilometers (382,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 77 degrees.

Photo credit: NASA/JPL/Space Science Institute

Notes: For other pictures and information in this series, see the following: PIA12784: Multiple F-Ring "Fans", PIA12786: "Fan" in the F Ring, and PIA12787: The Effect of Prometheus on the F Ring, the last of which includes a very interesting (but high memory-consuming) animation.