Decoding Solar System Volatile Compositions: Laboratory Investigations of Ice Entrapment and Deuterium Fractionation

Abstract

Stars and planets form within dense molecular clouds composed of gas and microscopic dust grains. On the cold surfaces of these grains, volatile molecules such as H2O, CO2, CH4, and N2 condense to form icy mantles that serve as active chemical reactors. Within these ices, physical trapping, isotopic exchange, and radical chemistry govern how volatile elements are stored, transformed, and ultimately delivered to planets. Understanding these processes is key to tracing the molecular history that connects interstellar clouds to comets, protoplanetary disks, and potentially habitable worlds. In protoplanetary disks, and in planet formation more broadly, the distribution of volatiles between gas and ice is critical for setting the compositions of the solids that become incorporated into planets, moons, and comets. If ices were pure, this distribution could be described by a sequence of snowlines. In reality, astrophysical ices are complex mixtures, and their compositions at different disk temperatures depend on how different volatiles interact within the ice matrix. Physical trapping, or entrapment, of hyper-volatile molecules such as CO, CH4, N2, and Ar within less volatile ices like H2O or CO2 can delay sublimation to temperatures above their pure desorption points, altering the inventory of gases released into the disk and of the solids incorporated into planetary bodies. Laboratory desorption experiments conducted under astrophysically relevant conditions show that entrapment efficiency is primarily governed by the physical structure of the ice rather than by chemical interactions between species, indicating that entrapment is a mechanical trapping process. These results provide quantitative constraints on how hyper-volatiles are retained or released, influencing the volatile distribution inherited by disks and icy bodies. Icy grains are not simply passive reservoirs of interstellar species; photochemical processing actively introduces new chemical and isotopic complexity. In the second part of this thesis, ultraviolet irradiation of mixed H2O:CD4 and D2O:CH4 ices was used to investigate solid-state hydrogen–deuterium exchange. These experiments reveal that photodissociation of both water and methane produces reactive OH and OD radicals that drive abstraction and recombination reactions, leading to the formation of HDO and CD3H/CH3D. The efficiency and directionality of this exchange differ between the two isotopic systems, indicating that isotopic equilibration is incomplete. The results suggest that local irradiation conditions can partially reshape primordial D/H ratios, providing a mechanism to explain the diversity of isotopic signatures measured in cometary volatiles and interstellar ices. The final part of this work explores oxygen-atom insertion as a pathway to form methanol and its deuterated analogs under cold, barrierless conditions. Experiments using mixed CH4 and CD4 ices with 18O2 demonstrate that excited O(1D) atoms readily insert into C–H and C–D bonds to form CH3OH and CD3OD. A subtle isotopic bias toward the formation of deuterated methanol indicates that zero-point energy differences and bond energetics influence the insertion outcome. These findings identify oxygen insertion as a new solid-state pathway for deuterium fractionation in interstellar organics, offering an explanation for otherwise puzzling isotopic enrichments observed in methanol and related molecules. Together, these studies provide a unified view of how the physical structure and photochemistry of ices regulate volatile retention, isotopic fractionation, and the emergence of molecular complexity in star- and planet-forming regions. By combining systematic laboratory experiments on entrapment, H/D exchange, and oxygen insertion, this thesis connects the chemical evolution of interstellar ices to the processes shaping planetary compositions. The results reveal how cold solid-state chemistry governs the inheritance of volatiles from molecular clouds to planetary systems, bringing us closer to understanding the molecular origins of habitability.