Exposure age of Saturn's A and B rings, and the Cassini Division as suggested by their non-icy material content

We investigate the near-zero azimuthal angle observations and demonstrate a radially varying scattering phase function for B ring particles which transitions from half-Mie-half-isotropic in the inner and outer B ring to purely isotropic in the middle B ring. This variation follows the trend of the optical depth in that as the optical depth increases, more particles scatter isotropically. In the A ring radially inward of the Encke gap, we find that the phase function can vary from purely isotropic for 55% porous particles to a combination of 30% Mie/70% isotropic for porosities of 90%, while outwards of the Encke gap particles are most likely 90% porous and scatter light isotropically. We derive the non-icy material fractions, assuming silicate as the non-icy material, and show that there is a significant dependence on the assumed porosity in the B ring, but the radial distribution follows the same trend as the optical depth. Owing to the B ring's high opacity (i.e. high optical depth but low surface density), the particles there are likely to have 85%-90% porosity, with corresponding non-icy volume fractions of ∼0.3%-0.5% in the inner and outer B ring, and ∼0.1%-0.2% in the middle regions. For the A ring interior to the Encke gap, the derived non-icy material volume fraction is ∼0.2%-0.3% everywhere for porosities ranging from 55%-90%. Finally, our results for the Cassini Division indicate a non-icy material fraction of ∼1%-2% similar to most regions in the C ring, except that the Cassini Division particles are more likely to have a porosity ≳ 90% due to the high opacity there. We find that the overall pollution exposure time for the A and B rings and the Cassini Division ranges from ∼30-150 Myr, which is in line with the ∼15-90 Myr we previously derived for most regions in the C ring. These exposure times assume an initially nearly pure-ice ring that has been continuously contaminated by in-falling micrometeoroids since its formation, using the currently accepted value of the micrometeoroid flux (Grün et al., 1985; Cuzzi and Estrada, 1998; Kempf et al., 2013; Altobelli et al., 2015). Our results here, taken together with our previous findings for the C ring, further support the idea that Saturn's rings may be ≲150 Myr old suggesting an origin scenario in which the rings are derived from the relatively recent breakup of an icy moon.