Cassini’s New Look at Organic Material in Enceladus’ Plume Ice Grains with CDA: Implication for the Habitability of Ocean Worlds

The search for habitable environments in the outer solar system is at the forefront of contemporary space exploration. The presence of subsurface liquid water, energy sources, and organic molecules make some icy moons with subsurface oceans potential sites to search for extraterrestrial life. Among these bodies are the Jovian moon Europa and the Saturnian moons Enceladus and Titan. The recent detection of phosphorus (Postberg et al. 2023) and HCN (Peter et al. 2024) in the ocean of Enceladus has further enhanced its astrobiological potential.

Enceladus ejects subsurface material into space in the form of ice grains and vapours from the moon’s south polar region. Most of the ice grains fall back onto the surface, with only a fraction of these grains escaping the moon’s Hill Sphere and forming part of Saturn’s E ring. Cassini’s on-board mass spectrometers — the Cosmic Dust Analyzer (CDA) and Ion and Neutral Mass Spectrometer (INMS) - sampled gas and ice grains both from the plume and in the E ring. INMS detected O- and N-bearing organics alongside hydrocarbon species in the gas phase directly in the plume (Waite et al. 2009). On the other hand, CDA was also able to detect different classes of organic species in the ice grains sampled in Saturn’s E ring (Khawaja et al. 2019). The detections of sodium salts, nanophase SiO2 particles, and molecular hydrogen confirm the presence of water-rock interactions and hydrothermal activity in and around the rocky core of Enceladus (Postberg et al. 2009, Hsu et al. 2015, Waite et al. 2017). CDA revealed a diverse inventory of organic compounds that led to the classification of organic species in E ring ice grains: (i) complex, hydrophobic, solid, macromolecular organic compounds with molecular masses > 200 u (Postberg et al. 2018) and (ii) the more abundant recondensed volatile organic compounds in ice grains, which produce spectral features due to low mass (< 100 u) nitrogen-, oxygen-, or single aromatic ring-bearing compounds (Khawaja et al. 2019). These organic compounds are evidence of organic chemistry in the subsurface ocean.

Thus far, organic material has only been subject to detailed investigation by CDA in E ring ice grains. Here, for the first time, we analyse organic material in freshly ejected Enceladus plume ice grains. For this purpose, Cassini’s flybys of the Enceladus plume provided a unique opportunity for CDA to collect freshly ejected subsurface oceanic material, particularly organic compounds, as opposed to settled E ring grains. Flyby data were hitherto only used to classify ice grains by general composition (i.e. organic-bearing) without an in-depth compositional analysis (Postberg et al. 2011). We analyse CDA time-of-flight mass spectral data of freshly ejected ice grains sampled at a velocity ~ 17.7 km/s during Cassini’s E5 traversal of Enceladus in 2008. These high speeds support previously unexplored fragmentation pathways of organics, opening up new diagnostic possibilities for identifying such organic components in plume ice grains. For this work, we used electron ionisation (EI) mass spectra extracted from open-source databases in a complementary fashion (NIST & MassBank; Khawaja et al. 2022) to compare the fragment ions of certain organic compounds with those obtained at such high impact velocities.

Our results confirm the presence of aryl and oxygen moieties in ice grains that were previously sampled in the E ring, providing fresh insights into the stability of these compounds at Enceladean hydrothermal sites. In addition, mass spectra of these freshly-ejected organic-bearing grains also exhibit certain spectral features which were not observed at lower impact speeds in the E ring. For the first time, we find ether/ethyl and ester/alkene group moieties in these plume ice grains that provide a basis for alterative pathways for organic synthesis in hydrothermal systems on Enceladus, which carries significant implications for the habitability of the Enceladus ocean.

References

Hsu et al. (2015), Ongoing hydrothermal acGviGes within Enceladus. Nature, 519, pp. 207–210.

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

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

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.

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

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

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

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.

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

Waite et al. (2009), Liquid water on Enceladus from observaGons of ammonia and 40Ar in the plume. Nature, 460, pp. 487–490.