Volatile organic products resulting from the electron irradiation of methanol ice: Implications for Europa’s surface organics

- Context. The constant flux of energetic particles reaching the surface of the Jovian Moons, in particular Europa[1], can process and destroy the potential organic species that could be found on their surface. Endogenic organics could be a window into the composition of the subsurface ocean, therefore it is critical to understand the result of their alteration to interpret the future measurements of the Europa Clipper [2] and JUICE [3] missions.

- Goals. This study was performed to determine the diversity of volatile organic products that can be obtained by irradiating methanol in conditions relevant to Europa’s surface Methanol is the simplest of alcohols, widely present in early solar system materials, and tentatively detected in another ocean world, Enceladus [4]. Its radiation chemistry is well studied but primarily in colder conditions, more relevant to small bodies of the early solar system (e.g., [5]).

 

- Experimental methods.

We grew pure CH₃OH ices, ~5 µm thick on a copper sample holder connected to a closed cycle Helium cryostat inside a vacuum chamber. Their growth and evolution was monitored using a FTIR (Fourier-Transform Infrared) Spectrometer in the Mid-Infrared range. We then irradiated them with 10 keV electrons.

The experiments were performed at three different temperatures relevant to Europa’s surface (50 K, 80 K and 130 K), and at three different fluences: 2.12·10¹⁵ e⁻/cm², 6.36·10¹⁵ e⁻/cm² and 1.27·10¹⁶ e⁻/cm^2. This last value corresponds to an exposition lasting from ~100 days to ~400 years on Europa, depending on the area of the surface [6]. After the irradiation was completed, the sample was brought back to room temperature and the resulting volatiles were transferred into a GCMS (Gas Chromatographer−Mass Spectrometer)[7], allowing for separation and unambiguous identification of volatile organic compounds that could otherwise not be detected with FTIR spectroscopy.

 

- Results. Post-irradiation FTIR spectra allows the identification of several common products of methanol radiation chemistry: CO₂, CO, CH₄, ethylene glycol and formaldehyde [7]. GCMS analysis of the volatile products shows great chemical diversity (22 species identified). These compounds include aldehydes, ketones, ethers, esters, alcohols, alkenes and some heterocycles, in different abundances depending on dose and temperature. The quantity and diversity of products differ from previous results obtained with UV irradiation[5], suggesting different branching ratios of radicals resulting from electron irradiation such as the predominance of •OCH₃. The products of this experiments show that radiation processing of even simple organics could complicate the assessment of the interior conditions of Europa. As an example, the propylene/propanol ratio we obtain could, in a proposed framework based on geochemical modelling of hydrothermal fluids [8], wrongly be interpreted as evidence for high temperature hydrothermalism.

 

- Acknowledgements. This work was supported by CNES, focused on the JUICE mission. This work was also supported by the Programme National de Planétologie (PNP) of CNRS-INSU cofunded by CNES. We acknowledge support from CNRS Ingéniérie as part of the DERCI Programme (European Research and International Cooperation Directorate). We acknowledge support from the French government under the France 2030 investment plan, as part of the Initiative d'Excellence d'Aix-Marseille Université—A*MIDEX AMX-21-PEP-032. This research is part of the project ROC-ICE and has benefited from funding provided by l'Agence Nationale de la Recherche (ANR) under the Generic Call for Proposals 2024

 

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