  Water (H2O) ice is ubiquitous component of the universe, having been detected
in a variety of interstellar and Solar System environments where radiation
plays an important role in its physico-chemical transformations. Although the
radiation chemistry of H2O astrophysical ice analogues has been well studied,
direct and systematic comparisons of different solid phases are scarce and are
typically limited to just two phases. In this article, we describe the results
of an in-depth study of the 2 keV electron irradiation of amorphous solid water
(ASW), restrained amorphous ice (RAI) and the cubic (Ic) and hexagonal (Ih)
crystalline phases at 20 K so as to further uncover any potential dependence of
the radiation physics and chemistry on the solid phase of the ice. Mid-infrared
spectroscopic analysis of the four investigated H2O ice phases revealed that
electron irradiation of the RAI, Ic, and Ih phases resulted in their
amorphization (with the latter undergoing the process more slowly) while ASW
underwent compaction. The abundance of hydrogen peroxide (H2O2) produced as a
result of the irradiation was also found to vary between phases, with yields
being highest in irradiated ASW. This observation is the cumulative result of
several factors including the increased porosity and quantity of lattice
defects in ASW, as well as its less extensive hydrogen-bonding network. Our
results have astrophysical implications, particularly with regards to H2O-rich
icy interstellar and Solar System bodies exposed to both radiation fields and
temperature gradients.
