Daphnis Making Waves

The Cassini spacecraft catches Saturn's moon Daphnis making waves and casting shadows from the narrow Keeler Gap of the planet's A ring in this view taken around the time of Saturn's August 2009 equinox.

Daphnis (8 kilometers, or 5 miles across) is almost invisible in this view, but the shadows cast on the wide A ring can be seen below the center of the image. The Encke Gap of the A ring, which is wider than the Keeler Gap, is on the right. Saturn's thin F ring is on the left of the view. See PIA11629 for a similar, closer view.

More than a dozen background stars are visible in this image.

Daphnis has an inclined orbit and its gravitational pull perturbs the orbits of the particles of the A ring forming 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.

The novel illumination geometry that accompanies equinox lowers the sun's angle to the ringplane, significantly darkens the rings, and causes out-of-plane structures to look anomalously bright and cast shadows across 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. Before and after equinox, 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 PIA11665).

This view looks toward the northern, sunlit side of the rings from about 13 degrees above the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on August 16, 2009. The view was acquired at a distance of approximately 2.1 million kilometers (1.3 million miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 108 degrees. Image scale is 12 kilometers (8 miles) per pixel.

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

Pan Amid the Rings

NASA's Cassini spacecraft has recently resumed the kind of orbits that allow for spectacular views of Saturn's rings. This view, from Cassini's imaging camera, shows the outer A ring and the F ring. The wide gap in the image is the Encke Gap, where you see not only the embedded moon Pan but also several kinky, dusty ringlets. A wavy pattern on the inner edge of the Encke gap downstream from Pan and aspiral pattern moving inwards from that edge show Pan's gravitational influence. The narrow gap close to the outer edge is the Keeler gap.

Photo credit: NASA/JPL-Caltech/SSI

Daphnis, Pan and the Rings

Saturn's moons Daphnis and Pan demonstrate their effects on the planet's rings in this view from the Cassini spacecraft.

Daphnis (5 miles, or 8 kilometers across), on the left of the image, orbits in the Keeler Gap of the A ring. The moon's gravitational pull perturbs the orbits of the particles of the A ring forming the gap's edge and sculpts the edge into waves that move both in the ring's plane (radially) and out of the ring's plane. See PIA11655 and PIA12698 to learn more.

Pan (17 miles, or 28 kilometers across), in the top right of the image, orbits in the Encke Gap of the A ring. The effects of that moon's gravity can be seen as dark wakes on the parts of the rings below Pan in the image, propagating towards the middle of the image. See PIA07528 and PIA10529 to learn more.

This view looks toward the southern, unilluminated side of the rings from about 6 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 obtained at a distance of approximately 329,000 miles (529,000 kilometers) from Saturn. Image scale is 2 miles (3 kilometers) per pixel.

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

Galactic Behavior for the Outer B Ring

Keeping a close watch on the outer portion of Saturn's B ring, NASA's Cassini spacecraft records the complex inward and outward movement of the edge of the ring. This ring movement resembles the suspected behavior of spiral disk galaxies.

The position of the outer edge of the B ring, shown here crossing the middle of the frame, varies with time in this concatenation of 301 images taken an average of 1 minute, 50 seconds apart, over the span of about nine hours. The total variation of the edge, from the innermost to outermost locations, is 200 kilometers (120 miles). The eccentric Huygens Ringlet, another very narrow ringlet discovered by Cassini, and the innermost of the bands of ring material in the Cassini Division, a low-density region once thought to be empty, all appear in the top of the frame.

Cassini scientists have determined that the complicated radial variations in the B ring edge are caused by the presence of four scalloped patterns, all independently moving around the ring. One pattern, with two lobes, is present because of the gravitational perturbations from the moon Mimas, which alter the ring particle orbits because of a repetitive configuration of particle and satellite orbital positions known as a Lindblad resonance; this pattern always stays fixed with respect to Mimas.

The other patterns with one, two, and three lobes respectively, travel around the ring with differing speeds and are believed to be natural modes of oscillation of the ring in this vicinity, excited by a process known as "viscous overstability." In this process, the small, random motions of the ring particles feed energy into a wave that propagates outward across the ring from an inner boundary, reflects off the outer edge of the B ring (which becomes distorted as a result), and then travels inward until it reflects off the inner boundary. This continuous back-and-forth reflection is necessary for these wave patterns to grow and become visible as distortions in the outer edge of the B ring.

In supporting these so-called "self-excited" modes, the outer edge of the B ring is behaving the way astronomers believe spiral galaxies behave. However, such modes are not directly observable in galaxies. Cassini's observations of the outer B ring edge constitute the first time such large-scale modes in a broad disk of material have been observed in nature.

The movie repeats twice. The second time the movie runs, the location of the Mimas resonance (marked with a green line), the locations of the inner boundaries for the one-lobed (blue), two-lobed (yellow), and three-lobed (red) modes, and the location of the mean radius of the outer edge of the B ring (white) are all indicated.

The images were re-projected into the same viewing geometry and magnified by a factor of two to increase visibility of features. Image scale was about 2 kilometers (about 1 mile) per pixel in the original images. These images have not been cleaned of cosmic rays that struck the camera's sensor during exposure. These cosmic ray hits appear as small white streaks on the images.

The view looks toward the southern, sunlit side of the rings from about 44 degrees below the ring plane.

The images were taken in visible light with the Cassini spacecraft narrow-angle camera on January 28, 2008. The view was acquired at a distance of approximately 424,000 kilometers (264,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 52 degrees.

Video credit: NASA/JPL/Space Science Institute

Note: For a similar article and video, see PIA12795: Oscillations at B Ring Edge.

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

Daphnis and the Rings

Rather than being an unchanging disk of peaceful particles, the material that makes up Saturn's rings is constantly pushed and pulled into spectacular shapes.

On the left of the image, the moon Daphnis (8 kilometers,or 5 miles across) affects material as it orbits in the A ring's Keeler Gap. The moon's orbit is inclined relative to the plane of Saturn's rings. Daphnis' gravitational pull perturbs the orbits of the particles forming the Keeler Gap's edge. This 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.

On the right, the material at the edge of the Encke Gap shows waves caused by Pan (28 kilometers, or 17 miles across). See PIA09881 for a similar view.

This view looks toward the southern, unilluminated side of the rings from about 6 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 531,000 kilometers (330,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 33 degrees. Image scale is 3 kilometers (2 miles) per pixel.

Photo credit: NASA/JPL/Space Science Institute

Earhart Propeller in the A Ring

A propeller-shaped structure created by an unseen moon is brightly illuminated on the sunlit side of Saturn's rings in this image obtained by NASA's Cassini spacecraft.

The moon, which is too small to be seen, is at the center of the propeller structure visible in the upper left of the image, near the Encke Gap of the A ring. The A ring is the outermost of Saturn's main rings.

The moon is likely about a kilometer (half a mile) across. Disturbed ring material to the upper left and lower right of the moon reflects sunlight brightly and appears like a white airplane propeller. Several density waves are also visible in the ring. A spiral density wave is a spiral-shaped accumulation of particles that tightly winds many times around the planet. It is the result of gravitational tugs by individual moons whose orbits are in resonance with the particles' orbits at a specific distance from Saturn.

A propeller's appearance changes with viewing geometry, and this image shows the way a propeller looks when viewed from the sunlit side of the rings. Contrasts can reverse when the structure is observed on the dark side of the rings: for example, the bright structure of this propeller corresponds to the dark portion at the center of the propeller seen in PIA12791 which was imaged from the unilluminated side of the rings.

This image is part of a growing catalog of "propeller" moons that, despite being too small to be seen, enhance their visibility by creating larger disturbances in the surrounding fabric of Saturn's rings. Cassini scientists now have tracked several of these individual propeller moons embedded in Saturn's disk over several years.

These images are important because they represent the first time scientists have been able to track the orbits of objects in space that are embedded in a disk of material. Continued monitoring of these objects may lead to direct observations of the interaction between a disk of material and embedded moons. Such interactions help scientists understand fundamental principles of how solar systems formed from disks of matter. Indeed, Cassini scientists have seen changes in the orbits of these moons, although they don't yet know exactly what causes these changes.

Imaging scientists nicknamed the propeller shown here "Earhart" after the early American aviatrix Amelia Earhart. The propeller structure is 5 kilometers (3 miles) in the radial dimension (the dimension moving outward from Saturn which is far out of frame to the lower right of this image). It is 60 kilometers (35 miles) in the azimuthal (longitudinal) dimension.

This same propeller can be seen casting a shadow around the time of the planet's equinox in PIA11672. See PIA07791 and PIA07792 to learn more about propeller shapes and to see smaller propellers.

Scale in the original image was 2 kilometers (1 mile) per pixel. The image has been rotated and contrast-enhanced to aid visibility.

This view looks toward the southern, sunlit side of the rings from about 81 degrees below the ring plane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 11, 2008. The view was obtained at a distance of approximately 364,000 kilometers (226,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 82 degrees.

Photo credit: NASA/JPL/Space Science Institute

Note: For other images in this series, see PIA12789: Tracking a Propeller, PIA12791: Propeller from Unlit Side (mentioned above), and PIA12792: Propeller Churns the A Ring, which features a short movie of the Propeller "Bleriot," named after the French aviator Louis Blériot.