Prometheus, Pandora and Saturn's F Ring . ... Credit Copyright: NASA , JPL , Voyager Explanation: The third and fourth innermost moons of Saturn were unexpectedly discovered to be gravitational "shepards." The inner moon Prometheus and the outer moon Pandora use their gravitational attraction to define Saturn 's outermost ring. ... This complex interaction creates a ring structure with two narrow braids and several unusual knots. ... NASA Web Site Statements, Warnings, and Disclaimers . ...