Abstract EANA2024-103

The cryovolcanically active Saturnian moon Enceladus emits material in the form of gas and ice grains from its subsurface ocean into space through vents in the icy crust near its south pole. This ejected material was sampled and analysed by mass spectrometers onboard the Cassini spacecraft - the Cosmic Dust Analyzer (CDA) and the Ion and Neutral Mass Spectrometer (INMS). The detection of salt-rich grains, nanophase silica particles, molecular hydrogen, and a variety of organic compounds in the ejected material strongly indicated the presence of ongoing water-rock interactions and hydrothermal activity at the sea floor [1,2,3,4,5]. Previous analyses of organic material in ice grains from Saturn’s E ring led to the discovery of (i) complex macromolecular compounds with masses > 200 u and (ii) volatile compounds with masses < 100 u, with both categories containing various N-, O- and aryl-bearing groups [4,5]. The recent detection of phosphorus [6] and HCN [7] from the ocean of Enceladus has further enhanced its astrobiological potential.

Previously, organic-rich ice grains were only investigated by CDA in Saturn’s E ring but here, for the first time, we analyse organic material in freshly ejected ice grains in the vicinity of Enceladus. CDA time-of-flight mass spectra of freshly ejected ice grains were obtained directly from the plume during Cassini’s E5 flyby of Enceladus, with a closest approach of 21 km at ~ 17.7 km/s [8]. When compared to slower impact speed sampling of ice grains in the E ring, this high flyby speed offers previously unexplored fragmentation pathways of organics, providing new diagnostic possibilities for the identification of new organic components in plume ice grains. To verify these new diagnostics, we compare electron ionisation mass spectra extracted from open-source databases (NIST & MassBank; [9]) with our impact ionisation CDA spectra in a complementary fashion. Such a comparison offers important insights for mass spectra obtained at high impact speeds.

Our results confirm that organics containing aryl and oxygen moieties are present in fresh ice grains ejected in the plume, in line with previous observations in the E ring, providing new insights into the stability of these compounds at Enceladean hydrothermal sites. In addition, for the first time, we find ether/ethyl and ester/alkene (and mixed) moieties in these plume ice grains. These discoveries offer alterative pathways for organic synthesis in the hydrothermal systems on Enceladus, which carries significant implications for the habitability of the bulk Enceladus ocean.

References:

[1] Postberg et al. (2009), Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature, 459 (7250), pp. 1098-1101.

[2] Hsu et al. (2015), Ongoing hydrothermal activities within Enceladus. Nature, 519, pp. 207–210.

[3] Waite et al. (2017), Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes. Science, 356 (6334), pp. 155–159.

[4] Khawaja et al. (2019), Low-mass nitrogen-, oxygen bearing, and aromatic compounds in Enceladean ice grains. MNRAS, 489, pp. 5231–5243.

[5] Postberg & Khawaja et al. (2018), Macromolecular organic compounds from the depths of Enceladus. Nature, 558 (7711), pp. 564-568.

[6] Postberg et al. (2023), Detection of phosphates originating from Enceladus’ ocean. Nature, 618, 489-493.

[7] Peter, J.S., Nordheim, T.A. & Hand, K.P. (2024), Detection of HCN and diverse redox chemistry in the plume of Enceladus. Nat Astron 8, 164–173.

[8] Postberg et al. (2011), A salt-water reservoir as the source of a compositionally stratified plume on Enceladus. Nature, 474, pp. 620–622.

[9] Khawaja et al. (2022), Complementary mass spectral analysis of isomeric O-bearing organic compounds and fragmentation differences through analogue techniques for spaceborne mass spectrometers, Planet. Sci. J. 3 254.