Spiral structure as an explanation for the asymmetric brightness of Saturn's A ring

Abstract

REITSEMA et al.¹ and Lumme and Irvine² have recently confirmed earlier observations which suggested that the A ring of Saturn is fainter in the quadrants following conjunctions of the particles with the Earth–Saturn line and brighter in the quadrants preceding conjunctions. Surprisingly, no intrinsic azimuthal brightness variation is found in the B ring. The brightness variation has been linked to the presence of synchronously rotating particles in the A ring, the effect caused by either a systematic variation in albedo over their surfaces or variations in their geometric projections. Of these two possibilities, the latter seems more promising since the asymmetry of the brightness variation with respect to the Earth–Saturn line is difficult to account for by means of an albedo variation. Furthermore, an albedo variation would yield the same brightness pattern for the B ring if it contained synchronously rotating particles. In this paper, however, we propose yet another mechanism to explain the phenomenon—spiral wakes in the A ring.