  The crystalline state of water ice in the Solar System depends on the
temperature history of the ice and the influence of energetic particles to
which it has been exposed. We measured the infrared absorption spectra of
amorphous and crystalline water ice in the 10-50 K and 10-140 K temperature
range, respectively, and conducted a systematic experimental study to
investigate the amorphization of crystalline water ice via ionizing radiation
irradiation at doses of up to 160 \pm 30 eV per molecule. We found that
crystalline water ice can be converted only partially to amorphous ice by
electron irradiation. The experiments showed that a fraction of the 1.65 \mum
band, which is characteristic for crystalline water ice, survived the
irradiation, to a degree that strongly depends on the temperature. Quantitative
kinetic fits of the temporal evolution of the 1.65 \mum band clearly
demonstrate that there is a balance between thermal recrystallization and
irradiation-induced amorphization, with thermal recrystallizaton dominant at
higher temperatures. Our experiments show the amorphization at 40K was
incomplete, in contradiction to Mastrapa and Brown's conclusion (Icarus 2006,
183, 207.). At 50 K, the recrystallization due to thermal effects is strong,
and most of the crystalline ice survived. Temperatures of most icy objects in
the Solar System, including Jovian satellites, Saturnian satellites (including
Titan), and Kuiper Belt Objects, are equal to or above 50 K; this explains why
water ice detected on those objects is mostly crystalline.
