The little moon Daphnis and its entourage of waves on the edge of the Keeler gap in Saturn's rings cast long shadows in this movie created from images taken as the planet approaches its mid-August 2009 equinox.

This movie is a sequence of 10 images, each taken about one minute 30 seconds apart. The small moon Daphnis (8 kilometers or 5 miles across) occupies an inclined orbit within the 42-kilometer (26-mile) wide Keeler Gap in Saturn's outer A ring. Recent analyses by imaging scientists published in the Astronomical Journal illustrate how the moon's gravitational pull perturbs the orbits of the particles forming the gap's edge and sculpts the edge into waves having both horizontal (radial) and out-of-plane components.

Measurements of the shadows in this and other images indicate that the vertical structures range between one-half to 1.5 kilometers tall (about one-third to one mile), making them as much as 150 times as high as the ring is thick. The main A, B and C rings are only about 10 meters (about 30 feet) thick. Daphnis itself can be seen casting a shadow onto the nearby ring.

These images of shadows cast onto the rings and others like it (see PIA11656 and PIA11653) 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 ringplane and causes out-of-plane structures to cast long shadows across the rings.

Bright specks in some movie frames are stars in the background.

This view looks toward the sunlit side of the rings from about 25 degrees below the ringplane. The images were taken in visible light with the Cassini spacecraft narrow-angle camera on May 25, 2009. The view was obtained at a distance of approximately 768,000 kilometers (477,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 52 degrees. Image scale is 4 kilometers (3 miles) per pixel.

The Cassini Equinox Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.

I never get tired of watching this clip. The increadible surprises of nature are truely stranger than fiction. Daphnis is one of my favorites.

CheshireCat (Jun 12, 2009 at 9:33 AM):

Les:
Yep, we actually started the research to study the relationship between the mass of the moonlet and the amplitude of the edge waves. Since we were doing computer simulations, when we found out that Daphnis has an inclined orbit, it was pretty easy to study that aspect, too. It was pretty cool to see how the edges of the rings would get vertical ripples from the interaction with the moon. (So, yes, it was a prediction we had made.) Serendipitously, these images came down just as the paper was about to be published and the journal was kind enough to allow us to add one of these to the paper.

Which lack of symmetry do you mean? On the one hand, on a given edge (inner or outer) of the gap, you only see ripples on one side of the moon. This is simply because the particles on the other side have not yet encountered the moon. (They're "upstream" if you will.) However, there is also an asymmetry between the two edges as well. This, we think, is due to Daphnis' orbital eccentricity (which means the two edges get forced differently at different times in Daphnis' orbit) and the fact that one edge is in a resonance with the moon Prometheus. Some members of the Imaging Team are studying this even now; watch for more about this in the future as the unravel the story further!

-John Weiss, CICLOPS

Les Porter (Jun 12, 2009 at 8:29 AM):

The "sequence of images" strung together shows the purely Newtonian [non-relativistic] behavior of particles in the rings-small-satellite-interaction when the satellite is small and embedded -- and it is fascinating to observe the "dampening" [if that is what happened] of the ripple behind the satellite as it is equally fascinating to see the building of the waves in the rings 'prior' to the arrival of the satellite -- since ring-particles and the tiny satellite are orbiting in slightly different period orbits. The crest to crest character, the "image definition," is better seen "ahead" of the satellite and seems to have more pronounced visual identity that vanishes more quickly after the passage through the responsive part of the system.

Has someone quantified the particle/satellite behavior, and been able to model the interaction observed? Star Trek could have used an accurate ring ripple effect composed by computer graphic simulation (somewhere, worked it into the action) but did anyone predict this? -- and has the quantification of the mass of the satellite been further enhanced by modeling of the particulate behavior?

Fascinating! The lack of pure symmetry in the ahead and behind visages of the ripple must tell us something subtle, don't you think? If worked out, the gap size becomes a function of the mass. . . the waves and their shapes implicit from the inclination. . .

I'll try to read the paper to see if those images are explained with respect to my curiosity, especially the apparent dampening. . .