Polarization of Jupiter's large moons

Polarimetry is a powerful technique to study the surfaces of Solar System objects, especially at small phase angles where the so-called the negative polarization branch (NPB) is observed. The NPB can have different shape and polarization values for different composition of the surfaces. For rocky surfaces, it has a parabolic shape with a minimum around 10° and inversion point (the angle where the polarization changes from negative to positive) around 20°. However, for icy surfaces (satellites of giant planets, cometary nuclei, and Trans-Neptunian objects), the NPB becomes very asymmetric, and its minimum shifts to smaller phase angles. The cause of this has been attributed to Coherent Backscattering Mechanism (CBM), which is known to be very sensitive to the size of particles and porosity of the medium.

 

Recently, we have been able to accurately determine the NPB of jovian satellites Europa, Ganymede, and Io. The NPBs for Europa and Ganymede are very similar (despite the difference in their geometric albedo, 0.67 for Europa and 0.44 for Ganymede). Their NPBs appeared to be strongly shifted to small phase angles with a single narrow minimum (unlike a bimodal curve with a deep and narrow and broad and shallow overlapping NPBs as was suggested in Rosenbush et al. 2015). Io, unlike Ganymede and Europa, has a more extended, although still asymmetric, NPB, despite a similarity of Io’s albedo (0.63) to Europa’s albedo. As an example, we show the observed polarization phase curve for Europa (Fig. 1). Its NPB is evidently formed by CBM. To find the properties of the regolith particles, we accomplished computer modeling of the polarimetric curves using the radiative-transfer coherent-backscattering (RT-CB) method (e.g., Muinonen et al. 2015). The best fit for Europa, shown in Fig. 1, was achieved for a regolith layer of single-scattering albedo ~0.985 and extinction mean-free-path-length ~16 microns. We will also present the results for Io and Ganymede.

Figure 1. Phase-angle polarization dependence for Europa, observations in the R filter (open circles) and the best fit result for the RT-CB modeling (solid line).

References

Rosenbush, V., Kiselev, N., Afanasiev, V. Icy moons of the outer planets. In: Polarimetry of Stars and Planetary Systems. (Eds. L. Kolokolova, J. Hough, A-Ch. Levasseur-Regourd), Cambridge University Press, Cambridge, 340-359. 2015.

Muinonen, K., Penttilä, A., and Videen. Multiple scattering of light in particulate planetary media. In: Polarimetry of Stars and Planetary Systems. (Eds. L. Kolokolova, J. Hough, A-Ch. Levasseur-Regourd), Cambridge University Press, Cambridge, 117-129. 2015.