Geant4-IcyMoons: Simulating Electron Interaction Physics in Irradiated Astrophysical Ices

Abstract

Energetic particles continuously process water ice across astrophysical and planetary environments, from interstellar clouds and comets to icy planetary surfaces. Interpreting the resulting observables requires a physically grounded description of the underlying interactions, both to identify radiation-driven signatures and to distinguish them from superimposed chemical, thermal, and microphysical effects. We present Geant4-IcyMoons, an extension of Geant4-DNA developed for irradiated water ice and, ultimately, for materials embedded within it. In this study, we model the elastic and inelastic interactions of electrons with amorphous and hexagonal ice. For the first time, this enables a transport-ready Monte Carlo simulation of electron irradiation in water ice, linking incident-particle environments to the evolution of icy surfaces. We apply this framework to Jupiter's moon Europa as a representative case of electron bombardment of an icy surface. We show that, on the trailing hemisphere, the stronger low-energy electron bombardment confines much of the deposited energy to the upper $\lesssim 0.1$ cm, whereas on the leading hemisphere, the more energetic incident population drives deposition patterns to depths of tens of centimeters. This may contribute to the observed lens-like enrichment of radiolysis products centered on the equator of the trailing hemisphere. This work lays the foundation for treatments of ion irradiation and radiation chemistry in water ice and embedded materials.

Figures (12)

• Figure 1: Total cross-sections for elastic scattering. The light gray curve shows the 1--100 eV measurements of michaud2003cross. The slate gray curve shows the cross-sections of the ELSEPA model for liquid water. The dashed black curve shows the blended elastic model adopted in Geant4-IcyMoons.
• Figure 2: Total cross-sections of vibrational excitation channels, as reported in michaud2003cross for (a) intermolecular- and (b) intramolecular- modes.
• Figure 3: Experimentally-derived total dissociative electron attachment cross sections for amorphous water ice michaud2003cross.
• Figure 4: Amorphous (left) and hexagonal (right) dielectric properties. Upper panels: Best-fit Drude representations of the dielectric properties of (a) amorphous and (b) hexagonal ice at optical frequencies, based on emfietzoglou2007consistent, compared with experimental results daniels1971bestimmungkobayashi1983optical. Bottom panels: Residuals between the best-fit Drude model and optical experimental data.
• Figure 5: Total per-channel cross-sections for five ionization (red curves; O K-shell in purple) and five excitation (blue curves) channels in (a) amorphous and (b) hexagonal ice after relativistic correction. Note: The plotted cross-sections are molecular cross-sections computed for ice density of 1 g cm$^{-3}$. In Geant4-IcyMoons, bulk density effects are accounted for by using molecular number density to scale interaction rates at runtime.