Heavy ion irradiation of crystalline water ice

TL;DR

This study investigates how high-energy cosmic ray-like ions amorphize crystalline water ice and measure the sputtering yield, revealing that cosmic rays significantly contribute to ice evolution in space, especially for strongly bonded ices.

Contribution

It provides experimental data on high-energy ion-induced amorphisation and sputtering of crystalline water ice, extending previous work to more realistic cosmic ray energies.

Findings

Amorphisation cross-section varies with temperature and ion energy.

Sputtering yield depends quadratically on electronic stopping power.

Cosmic ray sputtering may dominate over UV photodesorption for certain ices.

Abstract

Under cosmic irradiation, the interstellar water ice mantles evolve towards a compact amorphous state. Crystalline ice amorphisation was previously monitored mainly in the keV to hundreds of keV ion energies. We experimentally investigate heavy ion irradiation amorphisation of crystalline ice, at high energies closer to true cosmic rays, and explore the water-ice sputtering yield. We irradiated thin crystalline ice films with MeV to GeV swift ion beams, produced at the GANIL accelerator. The ice infrared spectral evolution as a function of fluence is monitored with in-situ infrared spectroscopy (induced amorphisation of the initial crystalline state into a compact amorphous phase). The crystalline ice amorphisation cross-section is measured in the high electronic stopping-power range for different temperatures. At large fluence, the ice sputtering is measured on the infrared spectra, and the fitted sputtering-yield dependence, combined with previous measurements, is quadratic over three decades of electronic stopping power. The final state of cosmic ray irradiation for porous amorphous and crystalline ice, as monitored by infrared spectroscopy, is the same, but with a large difference in cross-section, hence in time scale in an astrophysical context. The cosmic ray water-ice sputtering rates compete with the UV photodesorption yields reported in the literature. The prevalence of direct cosmic ray sputtering over cosmic-ray induced photons photodesorption may be particularly true for ices strongly bonded to the ice mantles surfaces, such as hydrogen-bonded ice structures or more generally the so-called polar ices.