Irradiation of Low Temperature Solids: Physical Mechanisms and Astrophysical Implications

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AST 0098523JohnsonDr. Robert Johnson, at the University of Virginia, will continue a research program to model the radiolysis and photolysis of low temperature solids in the outer Solar System and the interstellar medium (ISM). Irradiation by charged-particles or UV-photons of ice, molecules trapped in an ice matrix or hydrated minerals can determine the ambient neutral envelopes and the spectral properties of outer solar system satellites and of grains in the ISM. An understanding of the interaction of energetic ions, electrons and UV -photons with surfaces is needed to interpret many recent observations .A large body of laboratory data now exists on the sputtering and decomposition of low temperature condensed-gas solids. This data set has some inconsistencies, as well as large gaps in the materials studied and in the incident particle type and energies studied. It has only recently been shown that the long-ignored energetic electrons are important when describing the alteration of icy satellite surfaces. Therefore, Dr. Johnson will carry out a materials-based calculation program to model the available data and then use the results to expand the applicability of these data. His recent molecular dynamics calculations of energy transport during the sputtering of ices will be used to describe the sputtering of volatile and refractory species trapped in an ice matrix. In addition, he will incorporate and test those solid-state chemistry models that have been proposed for the production and desorption of new molecules due to irradiation of ice. The goal is to be able to calculate results for all charged particle types and energies, including cluster impact, for application to sputtering of a grain in the ISM, a ring particle, or a satellite regolith with a grain size and porosity much different than that used in laboratory studies. Such work is possible due to the rapid improvements in the computational tools available in materials science and because there exists an extensive set of data that can be used to test and calibrate these methods for the materials of interest in the outer Solar System and the ISM . Monte Carlo (MC) particle transport codes, which determine the distribution of energy deposition in a material, will be used along with molecular dynamics (MD) models and a continuum model to describe the energy transport and sputtering. The suggested chemical pathways for radiolysis and photolysis will be integrated in the MD model in order to describe aspects of the radiation-induced solid-state chemistry occurring in ice. Finally, Dr. Johnson will continue to assemble laboratory data from the large number of groups studying ion, electron and photon-induced sputtering and implantation and then incorporate these data into the proposed calculations .***