Saturn's Moon Pandora Stripped of 'Shepherd' Status

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A decade-long study tracking the intricate motions of Saturn’s ring moons shows that Pandora, located just outside the F Ring, actually is not gravitationally herding the narrow ring’s outer edge.

It, along with partner moon Prometheus, located just the inside the F-ring, actually stir up particles in the rings, but in the midst of the chaos, a stable core exists.

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“The long-term stability of the narrow F Ring core has been hard to understand. Instead of acting as ‘shepherds,’ Prometheus and Pandora together stir the vast preponderance of the region into a chaotic state,” planetary scientist Jeffrey Cuzzi, with NASA’s Ames Research Center in Moffett Field, Calif., writes in a paper published in the April edition of Icarus.

Extrapolating from nearly 10 years worth of data collected by the Saturn-orbiting Cassini spacecraft, Cuzzi and colleagues ran computer simulations showing how complicated orbital dynamics can lead to what they call “calm in the midst of chaos.”

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The explanation is quite nuanced, but basically has to do with the difference in timing between the ring particles’ motions and the moons’ orbits. The study showed a neat connection between Prometheus’ orbit and ring particles darting in and out which ends up keeping the ring in balance.

“Essentially, we find that the F Ring core is not confined by a combination of Prometheus and Pandora, but a combination of Prometheus and precession,” the authors write.

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Precession is the change in orientation of a spinning body due to gravitational influences.

Kudos to The Planetary Society’s Emily Lakdawalla for scoping out the paper.