UV photochemistry of ices - The role of photons in the processing of ices

The potential impact of UV photoprocessing on the composition and the properties of solar system ices is discussed. The various energetic processes to which solar system ices are exposed (photon irradiation and charged particle bombardement) are taken into account as well as the variation of these processes with the type of object considered (planet, satellite, comet, rings...) and its environment. An attempt is made in each case to assess the relative contribution/importance of photoinduced chemistry compared to the chemistry induced by charged particle bombardement. Thus, it is found that UV photons should dominate the processing of ices in the outer layers of the surface. This, of course, needs to be scaled to the flux of solar UV photons available in each case. The information available on the composition of solar system molecular ices is reviewed and the known/potential chemical reactions induced by photon irradiation are examined. The laboratory techniques most commonly used to simulate the photoprocessing of ices are described. The information derived from the laboratory studies of the photochemistry of planetary, cometary and interstellar ice analogs is reviewed through representative examples. It is found that the laboratory effort devoted to the study of the UV photochemistry of planetary ice analogs has been virtually nonexistent until very recently. This is in sharp contrast with the case of interstellar ices where a fair amount of information has been derived from laboratory simulations over the last two decades. This is also in sharp contrast with the effort devoted to the study of the chemistry induced in ices by ion bombardment. A similar effort (in size, scope, and continuity) is called for to study the photon induced chemistry in solar-system ices. Moreover, it is fundamentally important to study the combined effects of UV photon irradiation and ion bombardment for the processing of ices to realistically simulate the irradiation effects on solar system ices.