Ringing in a new moon

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.

MOON GAP. Tiny moon discovered in the Keeler gap, a clearing in Saturn’s A ring. Space Science Institute, JPL/NASA

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.

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