Tiscareno, M.S., Hedman, M.M., Burns, J.A,, Weiss, J.W., Porco, C.C. (2009). "Saturn's A Ring Has No Inner Edge" American Astronomical Society, DPS meeting #41, #25.04.
The Iapetus -1:0 nodal bending wave, the first spiral wave ever reported in Saturn's rings (Cuzzi et al. 1981, Nature), has been seen for the first time in 29 years in Cassini images taken during April and June 2009. As when Voyager 1 discovered this wave, Saturn is now close to equinox, so that incident sunlight approaches the rings nearly edge-on and thus highlights any vertical structure. The extensive long-wavelength Iapetus -1:0 bending wave begins in the outer Cassini Division and continues several thousand km into the inner A Ring.|
We use wavelet analysis (Tiscareno et al. 2007, Icarus) to determine the wavelength profile, thus deriving a surface-density profile for 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 (Tiscareno et al. 2006, ApJL; 2007, Icarus; Colwell et al. 2009, Icarus), varying smoothly from the low values of the former to the higher values of the latter.
Most remarkably, our derived density profile shows 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 brightness and optical depth at this location. What is the nature of the classically identified "inner edge of the A Ring" if it is not correlated with any significant change in surface density?
We will discuss the strengths and weaknesses of possible theories, including that the gradually increasing surface density brings about a sudden onset of self-gravity wakes. But in any case, previous work that has looked for a major mass-confining mechanism at the A Ring's inner edge must be revisited in view of these new data.