Radiolysis & Photolysis

Production of Oxygen in Ice

Introduction

Although the radiolysis and photolysic of ice is an old subject, in 1982 we first suggested that it was important in of production of oxygen atmospheres on the icy Galilean satellites. Such atmospheres were confirmed to be present in 1998 and oxygen and ozone were observed to be present as trapped species in ice around the same time.

In 2003 we developed the first chemical model that is consistent with laboratory data for production of O2 in ice due to incident UV photons, low energy electrons and energetic ions. Since the production rates depend on sample formation, radiation history, sample temperature, and radiation type and energy, listing all of the data would be inappropriate. Therefore, below we described an approximate quantitative model that roughly fits the data. We also give parameters for a number of radiation types and the physical basis for these parameters.

Model

At low radiation doses, the production and trapping of O appears to be the precursor step needed for forming molecular oxygen in ice. O2 is formed by the production of a subsequent O (or OH) that interacts with the trapped O. The initially trapped O is called the precursor. Its formation depends on the defect density and density of competing reaction sites (e.g. trapped sulfur, carbon or nitrogen species). Since trapped O can be converted to peroxide and peroxide converted to trapped O, after high dose, O2 can also be formed efficiently from excitation of peroxide dimers in peroxide inclusions.

The yield, Y, is defined as the number of O2 molecules per incident particle (ion, electron or photon). Since O2 production from ice requires the formation of a precursor, the yield at low radiation doses can be written roughly as