Ringing in a new moon
By Ron Cowen
Last July, the Cassini spacecraft passed through a clearing in Saturn’s outermost major ring and recorded a set of wavy features at the gap’s edges. To explain the ripples, scientists predicted the existence of a new Saturnian moon. But, they couldn’t tell for sure, because until this spring, Cassini orbited Saturn in the same plane as the rings. So, it could view them only as a pencil-thin line. In April, Cassini rose out of the plane, enabling the craft to peer down on the rings.
![](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2005/05/5028.jpg?resize=150%2C128&ssl=1)
On May 1, the spacecraft proved the moon prediction correct when it spotted the second known moon lying entirely within the planet’s system of icy rings. Although the moon is only 7 kilometers across, it has enough gravity to scallop the edges of what’s known as the Keeler gap in the A ring, the outermost of the planet’s bright main rings.
The other ring-bound Saturnian moon, 16-km-wide Pan, orbits along a closer-in clearing called the Encke gap and creates similar wavelike features. The moons Prometheus and Pandora, which lie on either side of Saturn’s narrow F ring, also produce the scalloping effect.
Cassini will fly above the rings until December, and researchers expect that it will uncover many more hidden moons.