Abstract DGP2026-40

The joint ESA/NASA Cassini–Huygens mission sampled various populations of ice grains in the plume of Enceladus and Saturn’s E Ring, where plume material is deposited, revealing a substantial fraction of organic-rich ice grains. These organic compounds are largely derived from the subsurface ocean and can therefore act as tracers of complex chemical networks within the colder bulk ocean, at the hydrothermally active seafloor, and inside the porous, water-filled rocky core. Cassini’s Cosmic Dust Analyser (CDA) sampled ice grains at hypervelocities (≳2 km s⁻¹), ionising embedded organic molecules and fragmenting them into constituent ions and neutrals. The resulting ions were collected by the instrument’s mass analyser and the electronic signal amplified to produce time-of-flight mass spectra. Whilst this mode of sampling has enabled the detection of a wide range of organic and inorganic material, confidently assigning mass spectra to specific chemical species has remained a major challenge in the analysis of CDA data. Until recently, the acceleration of ice grains to hypervelocity under laboratory conditions to generate reference spectra has been extremely difficult, and laser-based analogue methods have therefore been employed. In particular, laser-induced liquid beam ion desorption (LILBID) has been instrumental in the interpretation of existing data and in science planning for ongoing and future missions, although deeper theoretical insights into the underlying chemical fragmentation processes are often absent.

Here, we perform ab initio calculations using density functional theory (DFT) to investigate the energetic preferences governing the dissociation of phenol, used as a model aromatic compound which are ubiquitous in CDA datasets of organic-rich ice grains. The computed fragmentation channels are compared directly with experimental mass spectra obtained using LILBID. We find that protonation is the dominant ionisation mechanism for aromatics in water, and that the observed fragmentation patterns cannot be reproduced by dissociation of neutral phenol or its radical cation. Notably, the most stable protonated isomer of phenol is not the direct source of dissociation; instead, proton transfer must occur prior to fragmentation. This implies that multiple, more labile protonated structures must be considered to explain the dominant fragments observed in the LILBID mass spectrum of phenol. These include the losses of CO and H₂O, for which we also determine reaction mechanisms. Both dissociation pathways are found to be thermodynamically and kinetically accessible. We further examine the role of water–molecule interactions and microsolvation effects on the potential energy surface of protonated phenol, demonstrating that the presence of water significantly alters preferred protonation sites and favours alternative protonated structures relative to the gas phase.

The robust determination of organic-rich ice grain compositions is essential for assessing the chemistry and habitability of subsurface oceans. Quantum chemical calculations can unlock information that remains obscured within CDA datasets. This work also provides a framework for interpreting data from Europa Clipper’s SUrface Dust Analyzer (SUDA) and for science planning of ESA’s L4 mission to Enceladus currently under development.