Space Topics: Cassini-Huygens
The Year in Pictures: 2009
Side-lit Structures in Saturn's Rings
Daphnis shadow and wake
Cassini captured this image of Daphnis, the moon that orbits within the Keeler gap, on June 26, 2009. The "wake" structures orbiting in front of it clearly cast shadows onto the A ring.
Credit: NASA / JPL / SSI
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August 11, 2009 marked the equinox on Saturn, a once-in-15-years event when
the Sun passes through Saturn’s ring plane. With the Sun striking the vast, flat
ring system from the side, tiny vertical perturbations in the rings’ flatness were
brought into relief as they cast long shadows. Here, the tiny moon Daphnis, about
8 kilometers (5 miles) in diameter, orbits within the Keeler gap in the A ring,
casting a needle-like shadow onto the A ring. Also casting shadows are sawtooth
structures on the edges of the Keeler gap. These structures are vertical
perturbations in the ring particles caused by up-and-down gravitational tugs
from Daphnis, which has a slightly inclined orbit. Particles closer to Saturn than
Daphnis, on the right side of the Keeler gap, orbit Saturn faster than Daphnis
does, so the sawtooth pattern spreads out ahead of Daphnis in its orbit. Particles
farther from Saturn than Daphnis, on the left side of the Keeler gap, orbit Saturn
more slowly, so that half of the sawtooth pattern spreads out behind Daphnis
in its orbit. Cassini’s scientists employed the extremely low angle lighting
geometry around the equinox as a probe to observe previously undiscovered
vertical structures in Saturn’s rings.
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