Effects of Hydrothermal Conditions on a Model Simple Aromatic Compound using an Enceladus Ocean Laboratory Analogue

Ice grains ejected by the ocean world Enceladus were measured in situ by multiple instruments onboard the Cassini spacecraft during its exploration of the Saturnian system. Analysis of material emitted within a plume at the moon's south pole revealed the presence of a global subsurface salty liquid water ocean containing organic compounds and evidence of hydrothermal activity [1-5]. Ice grains sampled by Cassini’s impact ionization mass spectrometer, the Cosmic Dust (CDA), have revealed a diverse suite of organic species, including high-mass organic compounds, intermediate mass species, and small aromatic moieties [6-8]. The coexistence of liquid water, organic material, and hydrothermal chemistry satisfies key criteria for habitability, making interpretation of Enceladus' ocean chemistry particularly important in the search for life in the solar system. This is highlighted by the recently announced ESA L4 mission to investigate the habitability of Enceladus and search for biosignatures.

To investigate the possible origin of organic compounds on Enceladus, it is necessary to understand their evolution under different physio-chemical conditions from the core to the plume. We performed laboratory experiments simulating Enceladean hydrothermal conditions to understand the chemical evolution of organics identified in past and future observations. Low-mass organic compounds with functional groups including aromatic rings and carbonyl groups have been identified in Enceladean ice grains [7]. Benzoic acid, consisting of phenol and carbonyl groups, thus serves as a model compound for which to test the effects of Enceladus-like hydrothermal conditions on small aromatic compounds. The ERC-CoG Analogue Icy Moon Simulations (AIMS) project’s hydrothermal laboratory simulates the processing of ocean material within the temperature range 80–150°C and the pressure range 80–130 bar, representing conditions suggested for the water–rock interface on Enceladus [9]. In this study, we prepared solutions of benzoic acid (0.008 M) in an analog “Enceladus Ocean” consisting of NaCl (10^-6 M), and a sodium carbonate-bicarbonate buffer system (0.025 M) adjusted to pH 10. The solution was then processed in a hydrothermal reactor at a pressure of 80 bar and temperature of 80 °C for 16 days. Mass spectra produced in the laboratory by the laser-induced liquid beam ion desorption (LILBID) technique serve as an analog [10] for those obtained by CDA or future Enceladus dust analyzers like the High Ice Flux Instrument (HiFi) [11]. The LILBID spectra were evaluated and the contrast between hydrothermally processed and unprocessed benzoic acid is discussed.

The outcome of this work will improve our understanding of the hydrothermal chemistry of small aromatic compounds occurring in the core of Enceladus. These results can guide the interpretation of existing CDA data and future measurements obtained by Europa Clipper's SUrface Dust Analyzer (SUDA) [12], while supporting science planning for ESA'S L4 mission. Future work will extend these experiments to additional aromatic compounds to further constrain the evolution and detectability of hydrothermally processed organics in Enceladus-like environments.

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