Abstract EANA2025-119

The subsurface oceans of icy moons, such as Jupiter’s moon Europa and Saturn’s moon Enceladus, are prime targets in the search for extraterrestrial life. Enceladus has even been selected for ESA’s next large-class mission (L4). At Enceladus’ south pole, ice grains of ocean water are ejected into space. The Cosmic Dust Analyser (CDA), an impact ionization mass spectrometer onboard the Cassini spacecraft, analyzed the composition of the ice grains and thereby the ocean, that is salty, alkaline, and contains organic compounds. Although less is known about the subsurface of Europa, this will soon change with the Europa Clipper and JUICE missions. Europa Clipper is equipped with a next-generation impact ionization mass spectrometer, the SUrface Dust Analyser (SUDA). Based on CDA and SUDA, the High Ice Flux Instrument (HIFI), has been developed for a future Enceladus mission. HIFI and SUDA will be able to analyze ice grains ejected from the icy moons with an improved mass resolution over CDA. Interpretation of CDA data was aided by a laboratory experiment that employs the Laser Induced Liquid Beam Ion Desorption (LILBID) technique to simulate impact ionization with laser desorption. LILBID can also predict the detectability of molecular biosignatures enclosed in ice grains for future mass spectrometers like SUDA or HIFI.

At the ocean floor of the icy moons, hydrothermally heated seawater mixes with the cold ocean, generating temperature gradients which could support microbial life. On Earth, spore-forming bacteria have been isolated from warm hydrothermal environments. Spores form under environmental stress, such as extreme temperatures, pH, radiation, and oligotrophic conditions, and can survive in these extreme conditions. If life existed in or near Europa’s or Enceladus’ hydrothermal systems, resilient endospores could be dispersed widely into cold waters and survive over long timescales. This renders them promising targets for the analysis of ejected ocean material by impact ionization mass spectrometers.

In this study, we measured B. subtilis spores (1.80 mg/mL) and vegetative cells (4.77 mg/mL) in a water matrix with LILBID to determine the detectability of their molecular biosignatures in water ice grains by impact ionization mass spectrometers, such as SUDA or HiFi.

We found that biosignatures of B. subtilis spores and vegetative cells could be detected in ice grains in positive and negative ion modes. Mass spectral features included mass peaks of various amino acids and small organic compounds that, in combination, can indicate the presence of life. For spores specifically, the amino acids arginine in cation spectra and glutamic acid in anion spectra were readily detectable. Dipicolinic acid (DPA) also yielded characteristic mass peaks. This would be a unique identifier for sporulation, since DPA is a characteristic component of various bacterial spores but absent in vegetative cells. The analogue spectra are collected in a database for interpretation of mass spectra obtained on future space missions.

We show that SUDA onboard Europa Clipper or a similar instrument on a future Enceladus mission, would be able to identify characteristic biomolecules derived from B. subtilis spores and vegetative cells, and also to distinguish their differing biosignatures.