The moons ' gravity maintain the rings current shape with a well defined edge . Any material that drifts closer to a moon 's orbit , it can either move back into the body of the ring , deflect out into space , or is added onto the moon 's surface . The gravity of shepherd moons serves to maintain a sharply defined edge to the ring ; material that drifts closer to the shepherd moon 's orbit is either deflected back into the body of the ring , ejected from the system , or accreted onto the moon itself .