Hydrothermal processing of microorganisms: Mass spectral signals of degraded biosignatures for life detection on icy moons

Life detection missions to the outer solar system are concentrating on the icy moons of Jupiter and Saturn and their inferred sub-surface oceans. Access to evidence of habitability, and possibly even life, is facilitated by the ejection of subsurface material in plumes and outgassing fissures. Orbiting spacecraft can intersect the plume material or detect past sputtered remnants of outgassed products and analyse the contents using instruments such as mass spectrometers. Hydrothermalism has been proposed for the subsurface environments of icy moons and the organic remains of any associated life would be expected to suffer some degradation through hydrothermalism, radiolysis, or spacecraft flyby impact fragmentation. Hydrothermalism is treated here for the first time in the context of the Europa Clipper mission. To assess the influence of hydrothermalism on the ability of orbiting mass spectrometers to detect degrading signals of life, we have subjected Earth microorganisms to laboratory hydrothermal processing. The processed microorganism samples were then analysed using gas chromatography-mass spectrometry (GC-MS) and mass spectra were generated. Certain compound classes, such as carbohydrates and proteins are significantly altered by hydrothermal processing, resulting in small one-ring and two-ring aromatic compounds such as indoles and phenols. However, lipid fragments, such as fatty acids, retain their fidelity and their provenance is easily recognised as biological in origin. Our data indicate that mass spectrometry measurements in the plumes of icy moons, using instruments such as the MAss Spectrometer for Planetary Exploration (MASPEX) onboard the upcoming Europa Clipper mission, can reveal the presence of life even after significant degradation by hydrothermal processing has taken place