Ion irradiation of crystalline water ice: investigation of amorphization kinetics and application to Ganymede

Water ice in the Solar System is predominantly observed in two forms: amorphous or crystalline [1,2]. Spectroscopic measurements, particularly the absorption bands from about 1.5 to 3 µm, enable the determination of water ice structure. On Jovian satellites such as Europa and Ganymede, a majority of crystalline water ice with a superficial amorphous layer has been observed [3]. While an amorphous water ice layer is detected across the entire leading hemisphere of Europa [4], amorphous layers on Ganymede are primarily found on the polar caps [5,6]. Ganymede's magnetic field shields its equatorial regions from charged particle bombardment, resulting in higher irradiation at the poles [7,8]. Therefore, these amorphous water ice layers are likely produced by the amorphization of crystalline water ice due to charged particles coming from the Jovian magnetosphere. To better model the distribution between amorphous and crystalline water ice on Jovian satellite surfaces, a thorough understanding of the water ice amorphization process is essential. However, the kinetics of amorphization is poorly constrained in the literature, with variations observed between low- and high-energy ion irradiation (0,1-100 MeV) [9,10]. Most previous experiments were conducted at temperatures below 90 K with light ions, temperatures too low to be applied to the surfaces of Galilean satellites (see the review [9]).

In this study, we investigate the kinetics of water ice amorphization at temperatures between 90 and 120 K, typical of Jovian satellites. New irradiation experiments were conducted at GANIL (Grand Accélérateur National d’Ions Lourds, Caen), where water ice films were irradiated with Mg, O and S ions at energies around 100 keV. These ions have significant nuclear stopping powers, allowing us to explore a different energy loss regime compared to previous studies [11,12,13]. During irradiation, the temporal evolution of the amorphous fraction was monitored using infrared spectra. The amorphization fraction follows an exponential behavior either as a function of the dose (controlled by the K parameter, i.e. the dose for which it reaches 63.2 % of its maximal value during the experiment) or as a function of the fluence (controlled by the compact amorphization cross-section σ_am). We show that the K parameter depends not only on temperature but also on flux and ion energy, and it is not correlated with stopping powers. However, we found that the compact amorphization cross-section σ_am is very well correlated with the electronic stopping power S_e [Fig.1], but not with the nuclear stopping power S_n. Expressions linking σ_am and S_e were determined at 90, 100 and 110 K, enabling the estimation of the amorphous fraction based solely on the electronic stopping power of the ions. The correlation between σ_am and S_e is discussed using the thermal spike model [14], which estimates the temperature reached within the track radius induced by ion irradiation in materials. This model has been developed to simulate the effects of swift heavy ions on insulators and metals at room temperature. Here, it successfully reproduces the linear correlation between σ_am and S_e observed in the experimental data, thereby confirming that amorphization results from water ice melting and quenching along the ion track. However, the model does not explain the temperature dependence of the amorphization cross-section by varying the energy transfer from excited electrons to phonons, suggesting that it is most likely controlled by recrystallization or even thermal agitation.

In parallel, we develop a numerical model to determine the distribution between crystalline and amorphous water ice on Ganymede. As part of this work, doses received at Ganymede’s surface have been estimated [Fig.2]. Our experiments have shown that the electronic stopping power of the incident energetic particles controls the amorphization of water ice, rather than the dose. Our model will be refined with the new experimental data to provide more accurate numerical simulations of the evolution of water ice on Ganymede.

Fig. 1: Water ice amorphization cross-section σ_am as a function of electronic stopping power S_e, derived from fitting the amorphous fraction as a function of fluence. Power regressions with their associated uncertainties are included.

Fig. 2: Doses delivered by ions at Ganymede’s surface as a function of depth, computed for different regions. A dynamic surface was considered, taking sputtering into account.

 

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