Low-Energy Plasma–Surface Interactions at Airless Icy Bodies

Citation

Grayson, Robert Wall (2024) Low-Energy Plasma–Surface Interactions at Airless Icy Bodies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fn7c-k591. https://resolver.caltech.edu/CaltechTHESIS:06032024-222110855

Abstract

Low-energy plasma surface interactions occur in many solar system environments and are especially important in the magnetospheres of gas giants. Within these magnetospheres orbit a catalogue of icy moons, some of which famously host interior liquid-water oceans. They are continuously exposed to a cold, corotating plasma “wind,” resulting in bombardment by heavy reactive ions, with peak number fluxes in the hyperthermal energy regime (10s to 100s of eV). Despite their abundance, these low-energy ions have been mostly overlooked in planetary science because they are poor drivers of radiolysis. In this thesis we take a combined experimental-theoretical approach to understanding the interaction of hyperthermal water group molecules/ions with relevant surfaces, motivated by some specific solar system observations, mostly from the Saturn system.

We begin with experimental case studies of water-group ion scattering on carbonaceous (Chapter 2) and chloride-salt surfaces (Chapter 3), focusing on the emission of secondary negative ions. For carbonaceous surfaces, we detect surprisingly energetic carbon fragments, apparently emitted by near-threshold sputtering processes. The most abundant products (O⁻, C2⁻, C2H⁻) are consistent with mass range for negative PUIs of unknown origin observed near Dione and Rhea. The reported mass ranges, however, have been estimated for pick-up of initially stationary ions, which is a poor assumption for the products we observe. Our experiments with chloride salts (relevant to Jupiter’s moon Europa) are complicated by surface charging but provide kinematic evidence of reactive scattering and single knock-on sputter processes. Specifically, we observe abstraction of Cl from Pt to form chlorine monoxide anions. We then describe a modification of our scattering apparatus to enable exposure of ice targets, developing a one-of-a-kind experimental facility (Chapter 6). Some limited and preliminary results for Ar⁺ and O⁺ bombardment of amorphous water ice follow, which are more revealing of experimental challenges than of surface chemistry and dynamics.

Our theoretical efforts include Reactive Molecular Dynamics simulations of collision-induced chemistry in ices using the ReaxFF formalism. These reveal a novel non-radiolytic process (an Eley-Rideal reaction) for formation of molecular oxygen in low-energy (2−50 eV) water-group molecule bombardment of crystalline water ice, relevant to the maintenance of O₂ exospheres at Saturn’s moons Dione and Rhea (Chapter 4). With the addition of CH₄ to the ice (as a clathrate), bombardment results in formation of methanol and formaldehyde at yields as great as 10% and 5%, respectively (Chapter 5). Two mechanisms are observed for methanol synthesis: one a typical radiolysis process and the second a two-step non-radiolytic mechanism. We provide preliminary results for an HCN/CH₄/H₂O ice target in Chapter 8 to motivate further study of the role that hyperthermal reactive ions play in synthesis of prebiotic organics. Finally, in Chapter 7, we describe a Monte-Carlo model for the production and transport of H₂ in the Enceladus due to plasma-surface interactions. Radiolysis by suprathermal electrons is the primary contributor, but the calculated mixing ratio falls several orders of magnitude short of the reported ~1%, which lends credibility to the notion that H₂ is being emitted from Enceladus’ internal ocean.

Thesis Files