Analogue experiments for the identification of organics in ice grains from Europa using mass spectrometry

Jupiter’s moon Europa is predicted to harbor a global liquid water ocean beneath its icy crust [1,2].  Like Saturn’s moon Enceladus, Europa could be cryovolcanically active, with evidence for plumes of water recently reported (e.g. [3,4]). Such plumes could eject gas and water ice grains from the subsurface ocean into space. Sputtering and micrometeorite bombardment may also eject icy surface particles to high altitudes [5].

Ice grains ejected from icy moons can be analyzed during spacecraft flybys by impact ionization mass spectrometers [6], such as the Cosmic Dust Analyzer (CDA) onboard the Cassini spacecraft or the Surface Dust Analyzer (SUDA) that will be onboard the upcoming Europa Clipper mission [7]. These instruments can determine the composition of the ice grains and potentially indirectly sample subsurface oceans. In the Saturnian system, data collected by the CDA instrument showed that Enceladus’ interior ocean is salt-rich [8], sustains water-rock hydrothermal interactions [9], and contains a variety of organic material, including complex macromolecules [10] and low mass volatile compounds, potentially acting as amino acid precursors [11]. On Europa, the subsurface ocean is predicted to be in direct contact with silicates and possible seafloor magmatic activity, enhancing the potential habitability of the moon [12]. The moon’s surface is also exposed to the harsh radiation environment of Jupiter, which may induce oxidation and potentially other chemical reactions involving both ice and non-ice compounds [13].

Interpreting mass spectra acquired in space requires terrestrial calibration by analogue experiments. The Laser Induced Liquid Beam Ion Desorption (LILBID) technique reproduces the impact ionization mass spectra of ice grains recorded in space [14]. Previous LILBID experiments have shown that bioessential molecules, such as amino acids and fatty acids, can be detected in ice grains at concentrations as low as the µM or nM level [15], and that the abiotic and biotic formation processes of these molecules can be distinguished from each other based on spectral features [16]. Microbial biosignatures were also investigated recently, showing that nucleobases, fatty acids and other bacterial fragments can be clearly identified [17].

Here we investigate whether Europa-relevant organic compounds encased in ice grains can also be detected and characterized using impact ionization mass spectrometry in space. High-sensitivity LILBID experiments have been performed with formamide (CH₃NO), farnesol (C₁₅H₂₆O), cholesteryl linoleate (C₄₅H₇₆O₂) and N-dodecanoyl-L-homoserine lactone (C₁₆H₂₉NO₃) to predict their spectral appearance in both anion and cation mass spectra. Formamide is expected to form by radiolytic reactions on Europa’s surface [18], while farnesol, cholesteryl linoleate and N-dodecanoyl-L-homoserine lactone are lipids, representing potential biosignatures and selected for their high resistance to degradation. These compounds were investigated in water matrices with varying NaCl concentrations designed to mimic Europa’s predicted salty ocean and surface composition [19,20]. Results show that the identification of molecular peaks as well as characteristic fragments is possible for both formamide and lipids. Additionally, the spectra of formamide in salt-rich matrices show that formamide can be detected via sodiated molecular clusters ([CH₃NO+Na]⁺) and other Na-rich complexes.

The next steps will be to investigate these compounds in H₂O₂-rich matrices, designed to simulate the highly oxidizing surface environment of Europa [21]. Sulfate salts and sulfuric acid are also under consideration as important matrices relevant to the surface chemistry of Europa. Other potential biosignatures, as well as their irradiation products, will be investigated to study the likelihood of their survival and detection under Europa’s radiation environment. The recorded mass spectra will complement a comprehensive spectral reference library [22], which provides analogue data of a wide range of compounds applicable to impact ionization mass spectrometers onboard Europa Clipper or other future ocean world missions.

 

 

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[22] Klenner et al., in prep.