A reappraisal of the amorphization kinetics of water ice: Combined experimental and numerical investigations and amorphization cross-sections in the 90–120 K range

Airless icy objects in the outer Solar System are continually exposed to energetic ion irradiation from the solar wind, solar energetic particles, galactic cosmic rays or magnetospheric particles in the case of the rings or satellites of the giant planets. This irradiation can induce the amorphization of crystalline water ice, a phenomenon observed on several icy objects. However, the kinetics of water ice amorphization remains poorly constrained at temperatures above 90 K, typical of Galilean satellites. To address this gap, new irradiation experiments were conducted using Mg, O and S ions with energies ranging from 36 to 126 keV at temperatures from 20 to 120 K. We demonstrate that the amorphization cross-section σ a m correlates with the electronic stopping power S e , invalidating the assumption commonly made in previous studies that total stopping power (i.e. total dose) controls the water ice amorphization process. Our data also show that the amorphization cross-section decreases as the temperature increases from 20 to 120 K, and it depends on the ion flux. Numerical simulations using the inelastic thermal spike model fail to reproduce the evolution of the amorphization cross-section with temperature. The amorphization mechanism remains largely unknown. The inelastic interactions possibly involve the production of isolated defects, which eventually coalesce to form continuous amorphous domains. An irradiation-induced recrystallization process may explain the observed temperature dependency. Though the amorphization cross-sections do not correlate with the nuclear stopping power, the contribution of elastic interactions cannot be fully ruled out. However, the interplay between the two types of interactions, inelastic and elastic, remains elusive. • Only few studies have been devoted to the determination of the amorphization cross-sections of water ice above 90 K, although they are of major interest for investigating the evolution of the surfaces of icy Jovian satellites (90–160 K). • Ion irradiation experiments were conducted with O, Mg and S ions with energies ranging from 36 to 126 keV at temperatures from 20 to 120 K, and interpreted with the numerical inelastic Thermal Spike model (i-TS). • Our data show that the amorphization cross-section is linearly correlated with the electronic stopping power. It decreases as the temperature increases, and its dependence on ion flux increases with the temperature. • The i-TS model fails in reproducing experimental results. The mechanism of amorphization by electronic interactions possibly results from the accumulation of defects. Alternative models are discussed, addressing the respective contributions of elastic and inelastic interactions.