Europa’s Icy Regolith characterized by modeling of radar and UV-Vis-NIR Scattering

Europa’s photometric and polarimetric behaviour in UV-Vis-NIR light provides constraints on the microphysical properties of its uppermost regolith. At small phase angles, approximately below 20 degrees, atmosphereless bodies exhibit non-linear brightening near the backscattering direction as well as negative linear polarization, in which the scattered light is polarized parallel to the Sun–object–observer scattering plane. These phenomena are sensitive to particle size, shape, and refractive index, as  well as to regolith packing density, as they arise from electromagnetic wave scattering in discrete media composed of small particles, including interference between reciprocal light paths traversed in opposite directions. Photometric and polarimetric observations can therefore be used to probe the structure and composition of the near-surface scattering medium.

 

In earlier work, we modeled the small-phase-angle polarimetric phase curves of the Galilean satellites [1 , 2] using radiative-transfer coherent-backscattering (RT-CB) calculations [3 –5] with ensemble-averaged scattering matrices. For Europa, this approach reproduced key characteristics of the observed negative polarization branch using disk-integrated polarimetric observations. A recent study [6] further demonstrated the diagnostic potential of Mueller matrix elements beyond those describing linear polarization alone.

 

In the present work, we extend this modeling framework by replacing generic single-scattering inputs with scattering matrices derived from physically constrained particle models for Europa’s icy surface. The particle ensembles considered here include randomly shaped and randomly oriented particles with effective sizes in the approximate range 0.1–2.0 μm. Their single-scattering properties are computed using numerical electromagnetic methods, including the Discrete Dipole Approximation (ADDA) [7] (0.1–0.5 μm) and the surface-integral-equation method [8] (0.1–2.0 μm). These calculations are complemented by the scattering properties of larger Gaussian particles generated with the SIRIS-4 code [9 , 10], resulting in ensemble-averaged scattering matrices representative of a physically motivated near-surface composition. The resulting ensemble-averaged scattering matrices are used as input for the RT-CB modeling of Europa’s photometric and polarimetric phase curves. Furthermore, decomposition of the ensemble-averaged scattering matrix into polarization-conserving Mueller matrices [4] enables RT-CB computations for discrete random media composed of nonspherical particles [5]. This decomposition facilitates the interpretation of near-surface structure and composition by comparing RT-CB model results with observations [11].

 

We apply extended RT-CB computations to a surface model consisting of a layer of wavelength-scale amorphous ice particles overlying crystalline ice particles that are large compared with the wavelength; see Muinonen et al., present conference. At radar wavelengths, we model absorbing particles embedded in pure crystalline water ice. Using radar observations of same-circular (SC) and opposite-circular (OC) polarization [12, 13], we aim to estimate the relative abundance of pure water ice and more absorbing impurities. Previous studies have addressed this using coherent-backscatter and multiple-scattering models (e.g., [14]). Here, we apply the extended RT-CB model to investigate whether these polarization measurements can provide additional constraints on the relative abundance of pure water ice and more absorbing impurities.

 

By simulating light scattering from near-surface composition models with specified physical properties and comparing the results with ensemble-averaged scattering matrices and observations, we obtain constraints on the compositional and structural characteristics of Europa’s surface, including particle size distribution, packing density, and impurity content. The RT-CB model, combined with photometric and polarimetric measurements, provides a valuable tool for characterizing Europa, other icy satellites, and airless bodies using both ground-based observations and spacecraft measurements. Regolith parameters retrieved through RT-CB modeling can serve as boundary conditions for simulations of particle release into the surface-bounded exospheres of Jovian moons, thereby supporting the science goals of the JUICE and Europa Clipper missions.

references

[1] N. Kiselev et al., ”New Polarimetric Data for the Galilean Satellites : Europa Observations and Modeling” Planet. Sci. J. 3, 134 (2022)

[2] N. Kiselev et al., ”New Polarimetric Data for the Galilean Satellites: Io and Ganymede Observations and Modeling,” Planet. Sci. J. 5, 10 (2024)

[3] K. Muinonen et al., ”Coherent Backscattering Verified Numerically for a Finite Volume of Spherical Particles,”ApJ 760, 118 (2012)

[4] K. Muinonen, A. Penttilä, ”Scattering matrices of particle ensembles analytically decomposed into pure Mueller matrices,” JQSRT 324, (2024)

[5] K. Muinonen et al., ”Coherent backscattering in discrete random media of particle ensembles,” JQSRT 330,(2025)

[6] A. Leppälä, K.Muinonen, A. Penttilä, et al., JQSRT, submitted (2026)

[7] M. A. Yurkin, A. G. Hoekstra, ”The discrete-dipole-approximation code ADDA: Capabilities and known limitations,” JQSRT 112, 13 (2011)

[8] J. Markkanen, ”Surface-integral-equation solution for solid particles with wavelength-scale surface roughness,” JQSRT 341, (2025)

[9] K. Muinonen et al., ”Light scattering by Gaussian particles with internal inclusions and roughened surfaces using ray optics,” JQSRT 110, (2012)