Water-group ion radiation chemistry of Enceladean surface ice analogues: Quantifying cross-sections and G-values

Introduction

Future missions to Enceladus will aim to characterise the chemical composition of its subsurface ocean by sampling material ejected through the south polar plumes, as well as surface ice near the plume vents. A primary scientific objective of such missions is the detection of prebiotic organic molecules as potential biosignatures of extant or extinct life. However, correctly distinguishing true biosignatures from organics produced via abiotic processes remains a critical interpretive challenge.

At the surface of Enceladus, radiation from Saturn's magnetosphere is a driver of molecular destruction and synthesis within the ice phase. Ions and electrons from magnetospheric plasmas are well established as agents of chemical change on the icy surfaces of Solar System moons, including Europa, Ganymede, and Enceladus itself. Water-group ions dominate the Saturnian magnetospheric ion population at the orbit of Enceladus, with H₂O⁺ ions alone accounting for over 60% of the ion flux (Johnson et al. 2008; Tokar et al. 2008, 2009).

Our previous work (Richards et al. 2025) demonstrated that such radiation can produce simple inorganic species (e.g., CO, NH₄⁺, OCN⁻) as well as prebiotically relevant organics such as formamide (HCONH₂) and acetaldehyde (CH₃CHO). The radiolytic products were formed within Encealdus timescales of a few tens of hours within a plume, or a few years on the icy surface; short enough to make discrimination between subsurface-sourced and abiotically formed molecules extremely difficult.

Previous laboratory studies have quantified the radiolytic destruction of Enceladean ice analogues and the formation of radiolytic products using various forms of ionising radiation, including keV electrons, high-energy oxygen ions, X-rays, and Lyman-α photons (Bergantini et al. 2014, Bründl et al. 2026, Pilling et al. 2019, Rachid et al. 2020). Key parameters extracted from these studies include the effective destruction cross-section, σ (cm²), a probabilistic measure of molecular destruction by radiolysis or sputtering, and analogous formation cross-sections for radiolytic products. They also include the G-value, which describes the number of molecules formed or destroyed per 100 eV of energy deposited into the target. This study aims to quantify these parameters under irradiation by water-group ions in the energy range of tens of keV.

Methodology

Experiments were performed using the AQUILA chamber at the HUN-REN Institute for Nuclear Research, described in detail by Rácz et al. (2024). The AQUILA consists of an ultrahigh-vacuum chamber operating at a base pressure of a few 10⁻⁹ mbar, equipped with a cryogenically cooled sample holder hosting a ZnSe deposition substrate. Astrophysical ice analogues are deposited via background condensation of gas mixtures dosed into the chamber. The facility also hosts a dedicated electron cyclotron resonance ion source (ECRIS; Biri et al. 2021), which delivers keV ion beams to the sample at 45° to the surface normal. Chemical and structural changes to the ice are monitored in situ using mid-infrared transmission absorption spectroscopy (Bruker V70v; 4000–650 cm⁻¹; 1 cm⁻¹ resolution).

Ice analogues composed of H₂O, CO₂, CH₄, and NH₃ were deposited to thicknesses exceeding 300 nm onto the pre-cooled (20 K) substrate, then warmed to 70 K to better represent Enceladean surface conditions (Spencer & Nimmo 2013). In separate experiments, ices were exposed to five water-group ion beams: 10 keV O⁺, 45 keV O³⁺, 10 keV OH⁺, 15 keV OH⁺, and 15 keV H₂O⁺. Infrared spectra were acquired at regular fluence intervals throughout each irradiation run.

From the resulting spectra, we derive: (i) effective destruction cross-sections for the parent ice constituents; (ii) formation cross-sections for selected radiolytic products; and (iii) G-values (molecules destroyed or formed per 100 eV deposited). Together, these parameters provide a quantitative description of radiation chemistry relevant to the surface environment of Enceladus, and constitute input for models seeking to distinguish abiotic chemistry from genuine biosignatures in future Enceladus mission data. The results of this analysis will be presented during the conference.

Acknowledgements

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 871149.

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