Organic biosignatures from alkaline hydrothermal environments at icy moons

The icy moons Europa and Enceladus are key targets for life detection space exploration missions, due to the evidence for subsurface liquid water oceans beneath their icy crusts. Hydrothermal vents at the interface of the rocky cores and water oceans of icy moons are considered a promising region for habitability, owing to the potential for organisms to harness energy from surrounding temperature and redox gradients [1]. For life in these environments to be detected, chemical biosignatures must be delivered to surface plumes for direct measurement by mass spectrometric instruments, such as MASPEX and SUDA onboard the Europa Clipper space probe [2]. Due to the high temperature conditions surrounding hydrothermal vents, this transportation process will involve significant thermal degradation of any organic materials. It has been previously shown, through laboratory simulation, that organic biosignatures survive hydrothermal processing [3][4]. However,  hydrothermal fluids at icy moons may be significantly alkaline and saline [5], and this has not been accounted for in laboratory simulations to date.

Here, we simulated hydrothermal processing of microbial biomass under alkaline and saline aqueous conditions. Roughly 10 mg biomass from multiple microbial strains, including those relevant to habitable icy moon environments, was individually loaded into glass tubes. 0.4 ml deionised water, or sodium hydroxide solution for the alkaline experiments, was added to the tubes. For saline experiments, 20 mg solid magnesium or sodium salts were added. The tubes were then flame-sealed under vacuum and heated at 150 °C and 250 °C for 3 days in a stainless-steel reactor vessel. The organic products were extracted, derivatised and then analysed using gas chromatography-mass spectrometry (GC-MS).

In comparison to control experiments performed in deionised water, saline conditions led to subtle changes in the organic products. Generally, condensation reactions that formed nitrogen heterocycles were favoured in the presence of dissolved salts, likely as a result of the reduction in water activity. Under alkaline conditions, more significant changes were observed, with the deamination of amino acids substantially promoted. This mechanism could hinder the detection of amino acids, which if unaccounted for, could lead to false-negative detections of proteinaceous organic molecules at icy moons. However, some detectable organic products from amino acid degradation persisted through hydrothermal processing, which could be targeted as a potential biosignature by future chemical analysis at icy moons.

References:

[1] Jebbar et al., 2020, Space Science Reviews; [2] Waite et al., 2024, Space Science Reviews; [3] Salter et al., 2022, ACS Earth and Space Chemistry; [4] Tan et al., 2023, Astrobiology; [5] Glein and Truong, 2025, Icarus