Laboratory studies of astrophysical ices

Citation

Dissly, Richard William (1995) Laboratory studies of astrophysical ices. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/c0mj-np35. https://resolver.caltech.edu/CaltechETD:etd-10312007-083146

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. This thesis reports the results of three laboratory studies, each concerned with some aspect of ices in an astrophysical environment, presented as independent papers: 1. Molecular hydrogen is the most abundant molecule in interstellar space, and is therefore of central importance to the physics and chemistry of that environment. Experiments simulating the codeposition of molecular hydrogen and water ice on interstellar grains demonstrate that amorphous water ice at 12 K can incorporate a substantial amount of H2, up to a mole ratio of H2/H2O = 0.53. We find that the physical behavior of ~80% of the hydrogen can be explained satisfactorily in terms of an equilibrium population, thermodynamically governed by a wide distribution of binding site energies. Such a description predicts that gas phase accretion could lead to mole fractions of H2 in interstellar grain mantles as high as 0.3. Accretion of gas phase H2 onto grain mantles, rather than photochemical production of H2 within the ice, could be a general explanation for recent observations of frozen H2 in interstellar ices. The possibility of interstellar grains that are rich in H2 could strongly affect our understanding of grain surface chemistry and gas-grain interactions. 2. Photochemical models of Triton's atmosphere predict ethylene (C2H4) as a primary product of methane photodissociation, formed at a high enough level that it should be readily observable as a surface condensate in [...] years, yet it has not been observed. Ultraviolet photolysis experiments on C2H4 ice were done to simulate its irradiation on Triton's surface. Our results show that C2H4 ice is readily dissociated by radiation of wavelengths [...], with C2H4 ice as a primary product. Dilution in an inert N2 matrix does not affect the photochemical yield of C2H4, suggesting that the reaction C2H4 [...] C2H2 is unimolecular. Quantum yields for both the destruction of C2H4 and the formation of C2H2 are discussed, as functions of both irradiation wavelength and dilution in N2 ice. Applying these results to Triton, we find that the ambient UV flux reaching Triton's surface is more than adequate to prevent the build-up of an ethylene ice layer. 3. Thermal models of icy satellite surfaces that allow the scattering and absorption of incident sunlight at significant depths predict an enhancement of subsurface temperatures over the mean surface temperature known as the solid-state greenhouse effect. We verify that a solid-state greenhouse can readily be produced in a bed of evacuated glass beads, used as a crude analog for the surface of an icy body. Measurements of the thermal and radiative properties thought to govern the size of this temperature enhancement confirm that it can be reasonably predicted from these parameters.

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