New Polarimetric Data for the Galilean Satellites: Europa Observations and Modeling

Abstract

This paper is dedicated to a long-standing problem of the shape of the negative branch of polarization (NBP) for Jupiter's moon Europa, determination of which is crucial for the characterization of the icy regolith on this satellite and similar objects, as well as for further progress in understanding light scattering by particulate surfaces. To establish the shape of Europa's NBP, in 2018–2021 we accomplished high-precision disk-integrated polarimetry of Europa in the UBVR I bands using the identical two-channel photoelectric polarimeters mounted on the 2.6 m Shajn reflector of the Crimean Astrophysical Observatory and the 2 m telescope of the Peak Terskol Observatory. We found that the polarization dependence on the phase angle in each filter is an asymmetric curve with a sharp polarization minimum P min ≈ − 0.3 % at phase angle α min ≤ 0 .° 4 , after which the polarization degree gradually increases to positive values, passing the inversion angle at α inv ≈ 6° − 7°. Within the error limits, the parameters P min, α min , and α inv of the NPB are independent of the wavelength in the visible spectrum. The polarization curve clearly demonstrates the so-called polarization opposition effect (POE). Our analysis of the previous and new polarimetric observations of Europa allows us to conclude that the POE is caused by coherent backscattering of sunlight on microscopic icy grains covering Europa’s surface. Computer modeling with the numerical radiative transfer coherent backscattering method demonstrates that the best fit to the polarimetric observations and geometric albedo of Europa is provided by a regolith layer of elementary single-scattering albedo ∼0.985 and extinction mean free path length 2π l/λ eff ≈ 150, λ eff representing the effective wavelength in the UBVR I spectral bands.

New Polarimetric Data for the Galilean Satellites: Io and Ganymede Observations and Modeling

“…A detailed description of the instrumentation, the method of polarization measurements of satellites and sky background, and the calculation of polarization parameters can be found in the paper by Kiselev et al (2022). The results of our current measurements of the polarization of Io and Ganymede in the UBVRI bands are summarized in the Appendix (Tables A1 and A2).…”

“…For this, we used the identical two-channel photoelectric polarimeters mounted on the 2.6 m Shajn reflector of the Crimean Astrophysical Observatory and the 2 m telescope of the Peak Terskol Observatory. In the study by Kiselev et al (2022), it was found that the NPB for Europa is a highly asymmetric curve with a sharp polarization minimum P min ≈ −0.3% at phase angle α min 0°.4, after which the polarization degree gradually increases to positive values, passing the inversion angle at α inv ≈ 6°-7°.…”

“…In the paper by Kiselev et al (2022), the authors specified the shape of the phase-angle dependence of polarization for Europa that allowed them to characterize the composition, size of particles, and surface structure (porosity) of the satellite regolith. In this work, we continue our study of the Galilean satellites of Jupiter, focusing on the polarimetry of Io and Ganymede.…”

“…The RT-CB method was extended to dense discrete random media of scatterers by Muinonen et al (2018) and Markkanen et al (2018). For more details on opposition phenomena, the mechanisms of their formation, and the modeling of these physical phenomena, see Kiselev et al (2022).…”

“…This means that a narrow polarization minimum located near the backscattering direction is superimposed on a broad parabolic curve, with the second minimum extending to the inversion angle. Detailed reviews of previous polarimetric observations of the Galilean satellites are given in Veverka (1977), Rosenbush et al (2015), and Kiselev et al (2022).…”