Polarimetric Modeling and Observations for the Jovian satellites

At phase angles less than approximately 20 degrees, polarimetric phase curves of airless Solar System objects predominantly display a negative degree of linear polarization. Incorporating coherent backscattering (CB) into radiative transfer (RT) models provides a comprehensive modeling solution.

In our research, we model polarimetric phase curves for two of Jupiter’s satellites. We employ radiative-transfer coherent-backscattering (RT-CB, [1]) modeling with an ensemble-averaged scattering matrix. With this approach, parameterized phase matrix elements are utilized to replicate the observed low-phase-angle polarimetric phase curves for Io and the icy moon Ganymede [3] as was previously done for Europa [4].

We find a distinctive difference in the polarimetric phase curves of the RT-CB models of Jupiter’s icy satellites. Europa and Ganymede exhibit similar linear behavior, while Io’s results are much different. Despite Io and Europa sharing similar geometric albedos (A_g) of 0.63 and 0.67, respectively, their negative polarization branch (NPB) shapes differ. The NPB of Ganymede (Ag = 0.43) resembles that of Europa, albeit described by different parameters for the single-scattering properties. This discrepancy likely stems from the compositions of their surfaces. Polarimetric observations indicate only slight or no dependence on wavelength, suggesting wide particle size distributions with different real parts of the refractive index Re(m). For Europa and Ganymede, Re(m) was approximately 1.3, while for Io, it exceeded 1.4. Numerical computations using the RT-CB method successfully demonstrate a match to the polarimetric observations and to the geometric albedos.

The decomposition of ensemble-averaged scattering matrices into pure Mueller matrices [2] enables RT-CB computations for discrete random media of nonspherical particles. This decomposition will enable making conclusions about the structure and nature of regolith by comparing the RT-CB model results with observations. The RT-CB model can be effectively utilized to model airless objects based on ground-based observations. This is particularly useful as it enables modeling without expensive in situ measurements as Solar System geometry often limits ground-based observations to small phase angles.

[1] K. Muinonen et. al., ApJ 760, 118 (2012)

[2]K. Muinonen and A. Penttilä, JQSRT 324, 109058 (2024)

[3] N. Kiselev et al., Planet. Sci. J. 5, 10 (2024)

[4] N. Kiselev et al., Planet. Sci. J. 3, 134 (2022)