Which moon is the inner shepherd of Saturn's F ring?

Prometheus is the inner shepherd moon of Saturn's narrow F ring. Its gravity continually tugs on nearby ring particles, helping shape and confine the ring's inner edge.

Prometheus is a small, irregularly shaped moon measuring approximately 137 by 81 by 56 kilometres. It was discovered in images taken by Voyager 1 on 24 October 1980 and was later named for the Titan Prometheus of Greek mythology. Its low density suggests that it is a porous body made largely of ice.

As Prometheus moves near the F ring, its gravitational pull creates kinks, strands and knots in the ring material. These structures can change over time because Prometheus and the nearby moon Pandora interact gravitationally. Pandora lies outside the F ring and was traditionally described as its outer shepherd, although later studies have questioned how strongly it confines the ring.

The other options belong to different ring environments. Atlas is associated with the outer A Ring, Pan maintains the Encke Gap, and Pandora is the moon most often paired with Prometheus in descriptions of the F ring. The phrase inner shepherd specifically identifies Prometheus.