Abstract DGP2026-85 |
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On the Origin and Evolution of Aromatic Compounds in Ice Grains Emitted from Enceladus
Saturn’s moon Enceladus lies amongst the major targets of space agencies searching for habitable planetary environments in the Solar System. Plume material, in the form of gas and ice particles, erupts from the moon’s fractured icy crust near its south pole and provides a unique opportunity to sample subsurface material originating from the core, ocean, and icy crust. Mass spectrometers onboard Cassini's spacecraft — the Cosmic Dust Analyzer (CDA, Srama et al. 2004) and the Ion and Neutral Mass Spectrometer (INMS, Waite et al. 2004) — found a variety of organic and inorganic compounds in the ice and gas phases of the plume material (Khawaja et al. 2025; Postberg et al. 2023, 2009; Khawaja et al. 2019; Postberg & Khawaja et al. 2018; Hsu et al. 2015, Waite et al. 2009).
Most ejected ice grains fall back to Enceladus’ surface, but a small fraction settle in Saturn’s E ring, where CDA performed most of its sampling during the mission’s lifetime from 2004 to 2017. To date, a large inventory of both simple and complex organic compounds has been found in ice grains detected by CDA. The mass spectral features revealed aromatic structures as a common constituent of the detected high mass (≥ 200 u) and low mass (≤ 100) organic material, in addition to N- and O-bearing moieties (Khawaja et al. 2025, 2019; Postberg & Khawaja et al. 2018). Aromatic compounds are amongst the most abundant and stable organic compounds in the universe, and thought to be involved in subsurface reaction chemistry on Enceladus. In the polycyclic aromatic hydrocarbon (PAH) world hypothesis, aromatic groups may have acted as stabilising compounds within early pre-cellular container elements before the evolution of modern lipid membranes (Groen et al. 2012). Therefore, it is essential to decipher the origin of aromatic species detected in Saturn’s E ring; whether they were delivered from primordial material during accretion, have evolved because of radiation in Saturn’s system, or emerged from subsurface regions of Enceladus. Enceladus’ seafloor is thought to be a location of active and complex chemistry, evidenced by the diverse array of organic components detected in ice grains from Enceladus (Khawaja et al. 2025, 2019; Postberg & Khawaja et al. 2018).
In recent results, Khawaja et al. (2025) detected spectral features related to aromatic species also in freshly ejected ice grains from Enceladus. The aromatic species in these freshly ejected young ice grains clearly indicates a subsurface origin. In this work, we discuss the latest understanding of Enceladus’ aromatic inventory, and the possibility that these compounds formed at the ice-ocean or water-rock interfaces. Within the new ERC Analogue Icy Moon Simulations (AIMS) project, we simulate hydrothermal processes under conditions similar to Enceladus’ hydrothermal seafloor that can potentially support high-pressure and high-temperature transformations of organic compounds, which we discuss here (Khawaja, Hortal- Sánchez, O'Sullivan et al. 2024). Deeper into the core, where pressures and temperatures rise, the synthesis, transformation, and possible degradation of aromatics compounds may take on different character that will be the part our future work.
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