Analogue experiments for the detection of microorganisms on Enceladus and Europa

Abstract

<p>From its south pole, the Saturnian moon Enceladus ejects plumes of gas and water ice grains, formed from its subsurface ocean, into space. A similar phenomenon is suspected to occur on Jupiter’s moon Europa. The emitted ice grains can be analyzed by impact ionization mass spectrometers, such as the Cosmic Dust Analyzer (CDA) on-board the Cassini spacecraft, rendering the ocean accessible for analysis by spacecraft flybys [1]. The data gathered by CDA in the Saturnian system showed that Enceladus’ ocean is salt-rich and contains a variety of organic material. Some of the detected organics indicate the presence of complex macromolecules whereas others are typical of low mass amino acid precursors, capable of interacting within or near Enceladus’ hydrothermal vent system, or Enceladus’ water-percolated rocky core [2-6]. Although this increases Enceladus’ relevance as a potentially habitable environment, biosignatures have so far not been identified.</p><p>Interpreting the space-based measurements of icy grains requires terrestrial calibration. The Laser Induced Liquid Beam Ion Desorption (LILBID) technique is known to accurately reproduce the impact ionization mass spectra of ice grains recorded in space [7]. Previous LILBID experiments have shown that potential biosignatures, namely amino acids, fatty acids, and peptides can be detected in the ice grains, and that even abiotic and biotic chemistry can be distinguished from each other [8,9]. Here we report our next steps, to investigate whether deoxyribonucleic acid (DNA) – as an indicator for earthlike life, and able to be encased in emitted ice grains - can also be detected and characterized using impact ionization mass spectrometers such as the Surface Dust Analyzer (SUDA) on-board Europa Clipper [10,11] or one on a future Enceladus mission [12,13]. We therefore conducted high-sensitivity LILBID analogue experiments with genomic DNA isolated from <em>Escherichia coli</em> bacteria, as well as lysed and disrupted <em>E. coli</em> cell material, to predict their spectral appearance in cationic and anionic impact ionization mass spectra. To identify any potential effects of cold and alkaline environments on spectral appearance, treatments including variations in pH and temperature were also applied to the cell material.</p><p>We clearly identify all four nucleobases (adenine, thymine, guanine, and cytosine) and compounds deriving from the phosphate deoxyribose backbone in the <em>E. coli</em> DNA mass spectra. An effect of a high adjusted pH on the cell material can be observed and will be further investigated in the future. This is just the first step of a large experimental campaign to investigate and predict the potential spectrometric fingerprints of organisms in ice grains emitted from Enceladus or Europa. The spectra of <em>E. coli</em> DNA, cell material, as well as of other biosignatures will be incorporated into a comprehensive spectral reference library, to provide comparable analogue data of a wide range of compounds applicable to impact ionization mass spectrometers on-board Europa Clipper or other future ocean world missions.</p><p> </p><p><strong>References</strong></p><p>[1] F. Postberg et al. (2011) A salt-water reservoir as the source of a compositionally stratified plume on Enceladus. Nature 474:620–622.</p><p>[2] F. Postberg et al. (2008) The E-ring in the vicinity of Enceladus II. Probing the moon’s interior—the composition of E-ring particles. Icarus 193:438–454.</p><p>[3] F. Postberg et al. (2009) Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature 459:1098–1101.</p><p>[4] F. Postberg (2018) Macromolecular organic compounds from the depths of Enceladus. Nature 558:564–568</p><p>[5] H.-W. Hsu et al. 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