Radiation Effects on Airless Bodies in Space: Radiolysis, Sputtering, and Sintering in Regoliths

Solar systems are interspersed with solids ranging in size from microscopic dust grains to moons. For Earth-like systems, rocks and minerals dominate the composition of inner solar system bodies, while surfaces in the outer solar system are predominantly ice covered. Small bodies, here meaning non-planetary bodies lacking appreciable atmospheres, are directly exposed to ambient radiation from numerous sources. Radiation of the solid state by ions, electrons and UV photons drives physical and chemical changes in the constituent materials, and understanding these effects is necessary to explain observations, guide spacecraft instrument design, and improve our understanding of the history of our solar system.

This thesis presents three case studies in radiation effects on airless bodies in space. First, laboratory experiments are used to estimate the formation rate of hydroxyl due to solar wind interactions with silicates on the surface of the Moon. Then, solar wind ion radiation and the subsequent ejection (sputtering) of surface species was studied by combining experimental measurements and Monte Carlo simulations of ion impact on lunar soil and meteorite compositions. Secondary ion fluxes produced by solar wind sputtering of small bodies are estimated, and it is shown that analysis of ejected ions can be used to determine composition and to classify small bodies according to the various known meteorite classes.

Incident plasma radiation can also modify the microstructure of a surface, and high energy ($>\:MeV$) electron radiation-induced sintering of ice grains has been implicated in the formation of thermal anomalies on several Saturnian moons. Here, the basic theory for radiation-induced diffusion and grain sintering are presented, and the formation timescales for anomalous features are estimated and compared with resurfacing rates due to grain impacts. Molecular dynamics (MD) simulations are used to obtain direct estimates for the number and average diffusion length of molecules mobilized by the incident electrons to further constrain the sintering model. Although the specific features considered are on the surfaces of the icy Saturnian moons, such processes occur throughout the outer Solar System and this work provides a template for quantifying radiation effects on other bodies within reach of human exploration.