Friday, June 12, 2009
Shadows along the Keeler Gap
This Cassini image is looking towards the outer edge of Saturn's main A ring. The dark band near the ring's outer edge is the Keeler gap, which is maintained by the small 8km satellite Daphnis, which is the white speck there that casts a shadow across the ring plane. Daphnis' orbit is also inclined slightly relative to the ring plane, which carries it above/below the ring plane with each orbit about Saturn. Due to this up/down motion, Daphnis' gravity on the ring also pulls the nearby ring material at the gap's edge up/down by about 1 km. And because Saturn is near its equinox, the Sun's illumination here is almost horizontal across the ring plane, causing these km-high piles of ring particles to appear brightly lit on their sunward side, which also casts shadows across the ring plane. Check the Cassini/CICLOPS website for more details, or the recent paper in AJ by Weiss et al (subscription required).