Probing Enceladus’ sub-surface ocean by mass spectrometry: the quest for inorganic and organic biosignatures Bernd Abel 1 , Frank Postberg 2 1 University Leipzig, Germany, 2 Freie Universitaet Berlin, Germany Saturn’s moon Enceladus harbors a global water ocean, which lies under an ice crust and above a rocky core. Through warm cracks in the crust a cryo-volcanic plume ejects ice grains and vapor into space that contain materials originating from the ocean. The analysis of salt-rich ice grains probed by the CASSINI spacecraft identified a number of inorganic salts and an alkaline pH of the water below the ice crust. Hydrothermal activity is suspected to occur deep inside the porous core, powered by tidal dissipation. For a long time only simple organic compounds with molecular masses mostly below 50 atomic mass units have been observed in plume material. Recently we reported observations of emitted ice grains containing concentrated and complex macromolecular organic material with molecular masses above 200 atomic mass units. The data constrain the macromolecular structure of organics detected in the ice grains and suggest the presence of a thin organic-rich film on top of the oceanic water table, where organic nucleation cores generated by the bursting of bubbles allow the probing of Enceladus’ organic inventory in enhanced concentrations. Very recently, we discovered phosphate-bearing minerals in ice grains emitted by Enceladus. Phosphorus (P) is the least abundant of the bio-essential “CHNOPS" elements, with earlier work indicating an even more drastic depletion on Ocean Worlds without dry land, such as Enceladus. However, recent results provide firm evidence that phosphorus is readily available in Enceladus's ocean in the form of orthophosphates, with P concentrations at least 100 times higher than in Earth's oceans in the moon’s near surface waters. References 1. Postberg, F.; Khawaja, N.; Abel, B.; Choblet, G.; Glein, C. R.; Gudipati, M. S.; Henderson, B. L.; Hsu, H. W.;Kempf, S.; Klenner, F.; Moragas-Klostermeyer, G.; Magee, B.; Nolle, L.; Perry, M.; Reviol, R.; Schmidt, J.; Srama, R.; Stolz, F.; Tobie, G.; Trieloff, M.; Waite, J. H., Macromolecular organic compounds from the depths of Enceladus. Nature 2018, 558 (7711), 564-568. 2. Postberg, F.; Kempf, S.; Schmidt, J.; Brilliantov, N.; Beinsen, A.; Abel, B.; Buck, U.; Srama, R., Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature 2009, 459 (7250), 1098-1101. 3. Klenner, F.; Postberg, F.; Hillier, J.; Khawaja, N.; Reviol, R.; Stolz, F.; Cable, M. L.; Abel, B.; Noelle, L., Analog Experiments for the Identification of Trace Biosignatures in Ice Grains from Extraterrestrial Ocean Worlds. Astrobiology 2020, 20 (2), 179- 189. 4. Klenner, F.; Postberg, F.; Hillier, J.; Khawaja, N.; Reviol, R.; Srama, R.; Abel, B.; Stolz, F.; Kempf, S., Analogue spectra for impact ionization mass spectra of water ice grains obtained at different impact speeds in space. Rapid Commun Mass Spectr 2019, 33 (22), 1751-1760.
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