Heavy ion irradiation of crystalline water ice
  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.
