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

Abstract Saturn's rings are composed primarily of water ice with a small fraction of non-icy constituents that are likely both intrinsic and extrinsic in origin. The intrinsic material is thought to be characteristic of the ring progenitor, while the extrinsic material is derived from the continual stream of hypervelocity impacting micrometeoroids that pollute the rings over time. Thus constraining these relative volume fractions and their radial distribution provides a powerful tool by which one can reveal clues about the rings’ ultimate origin and age. In this companion to our first paper for the C ring ( Zhang et al., 2017 ), we present new measurements of the non-icy material fraction in Saturn's B ring, Cassini Division and A ring as determined from microwave radiometry observations acquired by the Cassini spacecraft. 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 (Grun 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.