A Model Study of the Thermal Evolution of Astrophysical Ices

We address the question of the evolution of ices that have been exposed to radiation from stellar sources and cosmic rays. We studied in the laboratory the thermal evolution of a model ice sample: a mixture of water, hydrogen peroxide, dioxygen, and ozone produced by irradiating solid H sub(2)O sub(2) with 50 keV H super(+) at 17 K. The changes in composition and release of volatiles during warming to 200 K were monitored by infrared spectroscopy, mass spectrometry, and microbalance techniques. We find evidence for voids in the water component from the infrared bands due to dangling H bonds. The absorption from these bands increases during heating and can be observed at temperatures as high as 6155 K. More O sub(2) is stored in the radiolyzed film than can be retained by codeposition of O sub(2) and H sub(2)O. This O sub(2) remains trapped until 6 155 K, where it desorbs in an outburst as water ice crystallizes. Warming of the ice also drastically decreases the intrinsic absorbance of O sub(2) by annealing defects in the ice. We also observe loss of O sub(3) in two stages during heating, which correlates with desorption and possibly chemical reactions with radicals stored in the ice, triggered by the temperature increase.