Thermal and Radiation Processing of Porous Extraterrestrial Water Ice

Mitchell, Emily, Engineering Physics - School of Engineering and Applied Science, University of Virginia
Reinke, Petra, Department of Materials Science and Engineering, University of Virginia
Johnson, Robert, Department of Materials Science and Engineering, University of Virginia

This dissertation is an experimental study of the factors responsible for the state of water ice in extraterrestrial environments, and the changes induced in these ices due to pervasive space phenomena such as thermal processing and UV and energetic particle irradiation. The properties of frozen water are strongly dependent on the astronomical environment that harbors the ice. Ice morphology, phase, and chemical composition continually evolve due to processes such as thermal cycling, exposure to exospheric gases, and interaction with cosmic rays and stellar UV and particle fluxes. In this project, analytical techniques such as quartz crystal microgravimetry, infrared spectroscopy, ultraviolet-visible spectroscopy, and mass spectrometry were used in tandem to probe physical and chemical changes induced in laboratory-scale analogs of astronomical ices

First, we characterized the transformation of amorphous ice to the cubic crystalline phase, extrapolating our results to estimate the crystallization time of ices on Outer Solar System satellites, where a puzzling distribution of amorphous and crystalline ice has been detected. We vapor-deposited amorphous ice at 10 K and examined the effects of ice porosity on crystallization kinetics at 130 to 141 K, finding that ices of high porosity crystallize significantly faster than less porous ices. The experiments reveal a surface-driven crystallization process that can explain observations of crystalline ice on Jovian moons.

We have also simulated monolayer coverage of water on the lunar surface and performed 193-nm photon and 4-keV He+ irradiations to investigate processes responsible for the observed diurnal and latitudinal variation in water abundance. We found that photodesorption by solar photons is four orders of magnitude more efficient than the solar wind in removing water molecules from the lunar regolith, with a lifetime of ~12 hours for average solar activity at the sub-solar point.

We then studied synergistic effects induced in ices exposed to ambient methane and ion irradiation, to mimic surficial gas trapping observed on trans-Neptunian objects by space missions such as NASA’s New Horizons. We found that adsorption of methane by the ice increased during 100-keV proton irradiation at 40 to 50 K, due to competition between enhanced uptake in radiation-modified ice pores and pore collapse. Additionally, we observed complex radiation chemistry between the ice and adsorbed methane, detecting more than a dozen new molecular species synthesized from dissociated radicals in the radiolyzed film.

Finally, we simulated irradiation effects of ice-covered dust grains immersed in gaseous hydrogen in the interstellar medium. In contrast to the irradiation of ices exposed to ambient methane, we determined that 100-keV proton irradiation led to net loss of adsorbed H2 from porous ice films at 7 K, with a desorption cross-section independent of film thickness, H2 flux, and ion flux. We found residual amounts of hydrogen trapped within the radiolyzed ice at high ion doses, and estimate as high as 8% concentration of trapped gas at interstellar H2 accretion rates due to the cosmic ray flux. We also found that the ambient H2 caused suppression in radiolytic peroxide production due to H-enrichment and the decrease in OH concentration in the film, compared to the irradiation of H2O ices without deliberate exposure to H2.

These experiments show that the ice pores strongly affect the changes induced in extraterrestrial ices by thermal and radiation processing. The pores facilitate crystallization, trap atmospheric gases, harbor radiolytic molecules, and govern physical and chemical transformations of icy surfaces. The results obtained in this dissertation can be compared with results from astronomical observations and used to interpret the history and predict the evolution of icy surfaces in space.

PHD (Doctor of Philosophy)
ice, laboratory simulations, astronomical surfaces
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