Photolysis and radiolysis of water ice on outer solar system bodies

In this paper the available laboratory data on photolysis and radiolysis of water ice are summarized for the purpose of interpreting remote sensing data for the icy satellites. The relevance of these data to the objects in the outer solar system is then discussed. It is pointed out that the dominant species seen in laboratory spectra can differ from that seen on icy objects in space because of the low ion and UV fluxes and the relatively long annealing times in space. In the absence of contaminants, O-2 is a relatively stable, photoresistant product which forms more efficiently with increasing temperature. O-2 can accumulate in voids in which O-3 is then formed by photolysis. In regions with temperatures always <100 K the absorption band seen in laboratory spectra at 0.28 mu m, associated with trapped OH, as well as bands (<0.25 mu m) associated with HO2 and H2O2 can produce a general reddening below similar to 0.4 mu m. This will contribute to the icy satellite reflectance spectra, as well as the reflectance spectra of more distant solar system objects. The stability of these species (O-2, H2O2, HO2, OH) is enhanced by the preferential loss of H-2, which also affects the D/H ratio in the surface. It is also shown that the luminescence produced by plasma bombardment might be observable by spacecraft on the nightside of certain satellites.