Thursday, May 7, 2009

Shadows at the edge of Saturn's B ring

This fascinating image is from the April 15 Astronomy Picture of the Day. The image was acquired by the Cassini spacecraft, which is in orbit about Saturn. The brighter part of this image is a close-up of the outer edge of Saturn's B ring, while the lower darker part shows the fainter ring material that orbits in the Cassini Division. Saturn is approaching equinox, which means that the Sun is near the ring plane, which also allows the satellite Mimas to cast its shadow on the ring plane (dark vertical streak).

Note also the dark `cookie bites' missing from the outer edge of the B ring. These seem to be shadows cast by something that lies right at the ring edge, possibly very large ring particles orbiting there. But note the bright ringlet that also appears at the B ring's outer edge; if that ringlet is puffy, or otherwise kinky in the vertical direction, then that ringlet might be casting these shadows. Also keep in mind that Mimas has a 2:1 resonance at the B ring's edge, which is where a ring particle orbits twice for every orbit of Mimas. So it is conceivable that resonance might be 'snowplowing' the B ring edge, with ring material also piling up in a vertically above and below the ring-plane as Mimas also shoves it radially inwards. If so, then this snowpiling might instead be responsible for these shadows.

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