Thermal and radiolytic processing of astrophysical ice analogs

Tribbett, Patrick David (2023) Thermal and radiolytic processing of astrophysical ice analogs. Doctoral thesis, Northern Arizona University.

Text Tribbett_2023_thermal_radiolytic_processing_astrophysical_ice_analogs.pdf - Published Version Download (88MB)

Abstract

Volatile molecules (water (H2O), carbon dioxide (CO2), methane (CH4), hydrogen sulfide (H2S), etc.) present in astrophysical environments condense as ices onto surfaces as large as planetary bodies, and as small as interstellar dust grains. Once condensed, these ices are exposed to both ionizing radiation and thermal processing, which alters their chemical history. To understand the distribution of molecular ices in the solar system and the interstellar medium (ISM), determine reaction pathways to complex molecules in the condensed phase, and interpret spectral data from observational, mission, and modeling studies, it is fundamental to describe the processes that alter molecular ices at low temperatures and pressures. Here I present experimental studies characterizing these astrophysically-relevant ices and several of the radiolytic and thermal processes that alter these ices. Experiments are performed in a new ultra-high vacuum chamber in the Processes Environments and Astrochemistry on Extraterrestrial Surfaces (PEAXS) Laboratory at Northern Arizona University (NAU), which was constructed in partial fulfillment of this dissertation. Charged particle bombardment of H2O ice surfaces simultaneously produces radiolytic products and sputters material, providing source material for the tenuous atmospheres around icy satellites, including Europa. We measure the total sputtering yield, or number of ejected molecules per incident ion, for low energy argon ions, analogous to the cold, heavy ion population within the magnetosphere of Jupiter, at temperatures relevant to Europa. We find that current theoretical sputtering models over predict our empirical radiolytic oxygen sputtering yields, and consequently the contribution of sputtered molecular oxygen (O2) due to the cold, heavy magnetospheric ions that irradiate Europa. The observational absence of the stable low temperature phase of H2O ice, microporous amorphous solid water (ASW), in the outer solar system and ISM is surprising given that temperatures should favor the formation of microporous ASW during ambient condensation. However, we demonstrate that energetic electrons efficiently compact amorphous ice, destroying porosity and internal surface area. We find that microporous ASW should only be expected in the youngest ices in interstellar molecular clouds. To enable future detection of microporous ASW using the James Webb Space Telescope (JWST), we identify several new near-infrared (NIR) features indicative of microporous ASW and demonstrate how they may also probe, the currently elusive homonuclear molecules. Thermally-driven chemical reactions within astrophysical ices receive significantly less attention than radiation-driven chemistry due to the low temperatures and minimal diffusion associated with astrophysical ices. However, we find several instances of thermally-driven oxidation reactions that occur at temperatures as low as 70 K. We demonstrate that a reaction occurs in ice mixtures of H2S, ozone (O3), and H2O producing sulfur dioxide (SO2), sulfur anions, and O2 at low temperatures, and hydrated sulfuric acid at high temperatures. This reaction is consistent with the observed hemispheric separation of sulfur species and radiolytically-produced oxidants on the Jovian satellites. We also demonstrate that a reaction occurs within ammonia (NH3), O3, and H2O ice mixtures that produces the nitrate ion, which is thermally stable as ammonium nitrate salt at high temperatures. Ammonium nitrate exhibits several NIR spectral features that may be consistent with the observed 2 μm features on several icy solar system worlds, including Charon, Miranda, and Enceladus.

Item Type:	Thesis (Doctoral)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	Astrophysical ices; Ionizing radiation; Thermal procession; Atmospheres; Europa; Jovian moons
Subjects:	Q Science > QB Astronomy
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > Physics and Astronomy
Date Deposited:	20 Oct 2025 21:31
Last Modified:	20 Oct 2025 21:31
URI:	https://openknowledge.nau.edu/id/eprint/6224

Actions (login required)

IR Staff Record View

Downloads