Sputtering, Radiolysis, and Photolysis: Physical Mechanisms and Astrophysical Applications

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AST 0507030JohnsonEnergetic charged particles, UV photons and x-rays alter materials in a number of astrophysical environments. Such radiations can change the composition of ices and other low temperature molecular solids and cause the ejection of atoms and molecules. Therefore, the chemistry induced in materials by photons, called photolysis, and that produced by charged particles, called radiolysis, are related to the production of gas-phase species, typically called desorption or sputtering. UV-photons, x-rays and cosmic-ray (CR) ions maintain the gas-phase species observed in regions of the interstellar medium (ISM), in discs around young stellar objects (YSOs) and in the recently discovered oxygen atmosphere over Saturn's rings. A giant toroidal atmosphere is produced in Saturn's inner magnetosphere by sputtering of grains. Europa and Ganymede have chemically altered surfaces and gravitationally bound oxygen atmospheres produced by energetic ions and electrons. The possible presence of a sub-surface ocean on Europa has lent importance to obtaining a description of the radiolytic production of oxidants in its relatively young surface. In order to understand these exciting observations, it is necessary to develop models of sputtering, radiolysis, and photolysis that can be used by astronomers. Because the surfaces and ambient gases are coupled by radiation processing, practical models will be constructed using the latest computational tools in materials science as guides. With such models, gas or plasma observations in an astrophysical environment can be used to determine the composition of embedded surface materials. Conversely, observations of the composition of the surface materials can be used to predict the ambient neutral and plasma environments. Although the commonality of the physical and chemical processes occurring in a number of space environments is the focus of this research, the models will be applied to the surface-plasma coupling in Saturn's magnetosphere, to the irradiation of the surface of Europa, and to x-ray-induced production of gas from the discs of YSOs. The fundamental physics and chemistry of radiation processing of molecular solids will be described. Because a massive amount of data is needed for a broad range of materials and radiation types (x-rays, CR ions, solar and magnetopheric plasmas), a research program is needed that summarizes the available data, then uses the modern computational tools in materials science to develop applicable models, and finally applies the new models to a number of topical problems.This work integrates problems in astronomy with those in material science. Because of the emphasis on underlying physics and chemistry, the models that are developed are of interest to a broad scientific community. In fact, this research, which is driven by the need to understand astrophysical observations, has already had an impact on the physics and chemistry of radiation modification of molecular materials and has altered the teaching of introductory materials courses. Because of its interdisciplinary nature, the research will be carried out by students in Engineering Physics at the University of Virginia. This is a model cross-disciplinary graduate program that has strong interactions with neighboring undergraduate institutions and high schools that serve primarily minority populations. ***