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Comparative Analysis of Photopolarimetric Characteristics and Surface Properties of the Galilean Satellites

Presentation #504.08 in the session Galilean Satellites: Scratching the Surface (Oral Presentation)

Published onOct 23, 2023
Comparative Analysis of Photopolarimetric Characteristics and Surface Properties of the Galilean Satellites

We present the results of a high-precision polarimetric study conducted on Jupiter’s moons Europa, Io, and Ganymede. The primary objective of this study was to elucidate the characteristics of their negative polarization branch (NPB) and utilize them to gain insights into the properties of their surfaces through computer modeling. The observations were performed using the “POLSHAKH” two-channel photoelectric polarimeters, which were mounted on the 2.6m Shajn reflector of the Crimean Astrophysical Observatory and the 2m Ritchey-Chretien-Coude telescope of the Peak Terskol Observatory from 2018 to 2023.

For Europa, we found that the NPB displayed an asymmetric curve with the polarization minimum Pmin ~ –0.3% at a phase angle αmin ≤ 0.4°; then the polarization gradually increased to positive values, crossing the inversion angle αinv ~ 6°–7°. Observations of Io revealed a polarization degree decrease, reaching Pmin ~ –0.25% as the phase angle changed from 0 to 2°, then it exhibited a slight linear increase remaining below approximately –0.15% until the phase angle 12°. We estimated αinv ~ 27°, representing the largest inversion angle among the satellites. Ganymede’s NPB exhibited a sharply asymmetric curve characterized by Pmin ~ –0.4% at αmin ~ 0.7°, and αinv ~ 11°. A comparison of the NPB shapes and parameters among all three satellites clearly demonstrated the presence of the polarization opposition effect, likely resulting from coherent backscattering of sunlight on microscopic grains covering their surfaces.

Interestingly, despite Io and Europa sharing similar geometric albedo values (Io: ρv = 0.63, Europa: ρv = 0.67), their NPBs exhibited significant differences. Conversely, Ganymede, characterized by ρv = 0.43, distinct from Europa, displayed similar Pmin, αmin, and αinv values.

We used the radiative transfer coherent backscattering (RT-CB) approach to simulate the observed polarimetric data and albedo of the satellite surfaces. Our analysis revealed at what parameters of the modeling (single scattering albedo ω and mean free path length l) we can achieve the best fit to the observations. Tentative parameters, which provided a close match to the data, were found to be kl = 150 (k is the wave number) for all satellites, and ω = 0.985 for Europa and Io, and ω = 0.942 for Ganymede, indicating compositional differences in the regolith of the satellites.

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