Probing the inner boundaries of Saturn’s A ring with the Iapetus −1:0 nodal bending wave

The Iapetus −1:0 nodal bending wave, the first spiral wave ever described in Saturn’s rings, has been seen again for the first time in 29 years. We demonstrate that it is in fact the nodal bending wave, not the 1:0 apsidal density wave as previously reported. We use wavelet analysis to determine the wavelength profile, thus deriving the surface density at every point in the region covered by the bending wave. This profile is consistent with surface densities measured from more localized spiral density waves in the outer Cassini Division and the inner and mid-A Ring, varying smoothly from the low values of the former to the higher values of the latter. emarkably, our analysis indicates that there is no significant change in surface density across the boundary between the outer Cassini Division and the inner-A ring, despite the very abrupt increase in optical depth and reflected brightness at this location. We consider anew the nature of the classically identified “inner edge of the A ring,” given that it does not appear to be correlated with any abrupt increase in surface density. There is an abrupt increase in surface density at the Pandora 5:4 density wave, ∼300 km outward of the A ring’s inner edge. Further study is needed to robustly interpret our findings in terms of particle properties and abundances, much less to explain the origins of the implied structure.