VLA multi-wavelength microwave observations of Saturn's C and B rings

Abstract

We present multi-wavelength VLA observations in the microwave of Saturn's rings. The data were obtained over two observational windows and cover a range from the Q to S bands (0.7–13 cm). Key ring particle properties are determined in the C and B rings such as particle porosity and non-icy material fraction in order to compare them directly with previously derived results using the 2-cm Cassini RADAR radiometry data (Zhang et al., 2017a, b). We confirm that these new data are consistent with the particles in the C ring being quite porous with porosity values of roughly 75–90% depending on different scattering phase functions, as well as the presence of a “hot band” in the middle C ring which is associated with both an anomalously high non-icy material fraction and low opacity relative to the rest of the C ring. Furthermore, with our multi-wavelength study we find that the amount of intrinsic thermal emission is almost constant with wavelength, which is unexpected since particles tend to become more absorbing at higher frequencies. If we assume that the non-icy material in the rings is intramixed within the ring particles, this result suggests a corresponding decrease in the imaginary part of the non-icy material dielectric constant at higher frequencies, which is most noticeable in the middle C ring. We do not see evidence for such decrease in the B ring particles. If true, this supports the idea that the non-icy material in the middle C ring has a different origin, and that the larger particles in the middle C ring may be composed of a rocky core covered by a porous, icy mantle. If the non-icy materials in the middle C ring are embedded as large chunks, a core-mantle model for the ring particles there naturally explains this almost constant intrinsic thermal emission. Furthermore, because the VLA has more complete azimuthal angle coverage than the Cassini radiometry observations, we are able to investigate the ring brightness in more detail. In particular, we notice a flatter scattering profile than that predicted by using a pure Mie phase function at higher frequencies (Q and K bands; 0.7–1.3 cm) for azimuthal angles between |40|o and |60|o. We can match the observed scattering profile better by introducing a semi-empirical phase function for large particles to account for nonsphericity effects. Finally, the non-icy material fraction in the B ring is less than 1%, which is in agreement with that derived from Cassini observations. The frequency-dependence of the B ring thermal emission sets the lower limit of B ring particle porosity. We confirm that the B ring particles are likely over 80% porous, which at the same time explains the high opacity in the B ring as measured from density waves. Because we are limited by the signal-to-noise ratio, the optical depth of the middle B ring could be higher than the currently accepted values. However, we found that even if the middle B ring is six times more optically thick than the value measured from UVIS occultations, the surface mass density is still likely to be less than 300 g/cm2, while the exposure time to non-icy pollutants due to micrometeoroid bombardment is less than 200 Myr.