October 13, 2009 | 2
At the Saturnian equinox, which takes place every 15 Earth years or so, the plane of Saturn's rings aligns with the sun, casting shadows that bring out subtle details in their structure. NASA's Cassini spacecraft was on hand in August to observe Saturn's latest equinox up close and captured this image showing how the planet's moons influence the rings.
The narrow outer band in this photograph, known as the F ring (Saturn's main rings are named A to G in the order they were discovered), features channels carved into the ring by Prometheus, the oblong moon visible just inside the F ring. These so-called streamers are formed when the elliptical orbit of Prometheus brings the moon into the F ring. The gravity of the moon pulls material out of the ring, carving out a new streamer on each 15-hour orbit.
A more subtle deformation is visible toward the outer portion of the A ring, the wider band in the center of the image. The A ring is nearly 15,000 kilometers across (for comparison, the F ring is just a few hundred kilometers broad at its widest point), but its thickness is measured in the tens of meters.
Within the A ring is a 40-kilometer span known as the Keeler Gap, in which the moon Daphnis orbits. This miniature moon has an inclined orbit, meaning its orbital path is tilted with respect to the rings. Due to Daphnis's inclination, its gravity pulls material out of the ring plane, creating waves on either side of the Keeler Gap (visible as a bump near the bottom of the photograph, toward the outer edge of the A ring). Due to Cassini's unique vantage point at Saturn's equinox, Daphnis's deformation of the A ring was set in stark relief by the low angle of the sun, casting shadows that revealed the waves to be approximately four kilometers high.
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