Water-Group Ion Irradiation Studies of Enceladus Surface Analogues

Introduction

Saturn’s magnetosphere contains trapped plasma and energetic charged particles which constantly irradiate the surface of Enceladus. The plasma consists of a variety of charged particles including water-group ions (O⁺, OH⁺, H₂O⁺, H₃O⁺) with a wide energy range, formed primarily from high energy electrons interacting with plume material [Howett et al., 2018; Johnson et al., 2008; Tokar et al., 2009, 2008]. Observations by Cassini’s CIRS and ISS show that on Saturn’s inner icy satellites, such as Mimas and Tethys, irradiation by the cold plasma darkens UV-IR reflectance spectra and produces bullseye-shaped features on the moons’ surfaces [Howett et al., 2018]. However, at Enceladus, the effects of plasma bombardment are unknown and difficult to determine because of the competing processes of E-ring grain bombardment and plume deposition [Paranicas et al., 2014]. Therefore, laboratory experiments are necessary to both isolate and understand aspects of the irradiation process that modify the composition of the ice.

 

Methodology

In this study, Enceledean surface ice analogues containing H₂O, CO₂, NH₃, and CH₄ were irradiated by water-group ions (including O⁺, O³⁺, OH⁺, and H₂O⁺ ions) with energies between 10-45 keV, the aim being to explore the physical chemistry of the ices and characterise the extent to which the surface material of Enceladus is weathered by ions in Saturn’s radiation environment. All experiments were carried out using the ATOMKI-Queen’s University Ice Laboratory for Astrochemistry (AQUILA) ice chamber at the HUN-REN Institute for Nuclear Research (HUN-REN ATOMKI), which is interfaced to the Atomki Electron Cyclotron Resonance Ion Source (ECRIS) [Biri et al. 2012, Biri et al. 2021, Rácz et al. 2024]. The ECRIS facility is unique in that it is capable of producing molecular beams of ions, which can be used to simulate the Saturnian plasma environment. The AQUILA chamber is capable of achieving temperatures and pressures suited to Enceladus’ surface (pressures of 10⁻⁷ mbar to 10⁻¹⁰ mbar [Waite et al., 2006] and temperatures between 33 K to 180 K [Spencer et al., 2006]).

Enceladus surface ice analogues were prepared at 20 K by co-depositing the gas and water vapour samples via their simultaneous introduction from the pre-mixing dosing lines into the main chamber. The thickness and composition of the ices were monitored throughout the deposition process using Fourier Transform Infrared (FTIR) transmission absorption spectroscopy across the 4000-650 cm^–1 wavenumber range at a nominal resolution of 2 cm^–1. Once at the required thickness (> 300 nm), the ices were warmed to 70 K, a temperature that is more representative of the Enceledean mean surface temperature [Spencer and Nimmo 2013].

In five separate experiments, ices were irradiated by 10 keV O⁺ ions, 45 keV O³⁺ ions, 10 and 15 keV OH⁺ ions, and 15 keV H₂O⁺ ions. Radiation-induced chemical changes in the ices were studied in situ using FTIR. Ices were irradiated until the main H₂O absorption peak appeared to be destroyed, with FTIR spectra acquired at pre-determined fluence intervals. At the end of each irradiation experiment, ices were thermally annealed at a rate of 2 K min^–1, with FTIR spectra acquired at 10 K intervals.

 

Results

Examples of the FTIR spectra are shown in Figure 1, for the experiments using 15 keV H₂O⁺ as the irradiating ion. Analysis of the spectra showed that every irradiation experiment resulted in the formation of CO, OCN^–, and NH₄⁺. Post-irradiative thermal annealing produced carbamic acid, ammonium carbamate, and an alcohol (likely methanol or ethanol) in most experiments. Other organic species, such as acetylene, acetaldehyde, formamide, and CH₂OH radicals, were also tentatively detected as radiolytic products. Although many of these products have not previously been detected on Enceladus’ surface, some have been detected in Enceladus’ plumes, which leads to questions about whether plume material is formed within the radiation-rich space environment or whether it originates in the subsurface ocean. Since our experiments have shown that the timescales over which these radiolytic products can form are on the same order of magnitude as the exposure timescales of ice material on the surface of Enceladus or within its plumes, caution is suggested if future studies attempt to use the plumes or near-plume surface ice composition as a proxy for the composition of the subsurface ocean.

Figure 1. FTIR spectra for the 15 keV H₂O⁺ experiment. The top spectrum shows the initial ice at 70 K compared to the spectrum taken after irradiation, with all parent ice components labelled. The middle two spectra show regions of interest of a difference spectrum produced by subtracting the spectrum acquired before irradiation from that acquired at the end of the irradiation experiment. The radiolytic products are labelled. The bottom panel shows a difference spectrum produced by subtracting the spectrum acquired at the end of irradiation at 70 K from that acquired during the post-irradiative thermal annealing experiment at 160 K. AC/CA refers to ammonium carbamate/carbamic acid.

 

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