Vertical Relaxation of a Moonlet Propeller in Saturn's A Ring

Abstract

Two images, taken by the Cassini spacecraft near Saturn's equinox in 2009 August, show the Earhart propeller casting a 350 km long shadow, offering the opportunity to watch how the ring height, excited by the propeller moonlet, relaxes to an equilibrium state. From the shape of the shadow cast and a model of the azimuthal propeller height relaxation, we determine the exponential cooling constant of this process to be λ = 0.07 ± 0.02 km^-1, and thereby determine the collision frequency of the ring particles in the vertically excited region of the propeller to be ω _c /Ω = 0.9 ± 0.2.