1,1,1- 1Freie Universität Berlin, Institute for Geological Sciences, Department of Planetary Science and Remote Sensing, Berlin, Germany (tr.osullivan@fu-berlin.de))
- 2NASA Ames Research Center, Moffett Field, Mountain View, California, USA
- 3Bay Area Environmental Research Institute, Moffett Field, Mountain View, California, USA
- 4Institute for Space Systems, University of Stuttgart, Stuttgart, Germany
Ice grains emitted by the geologically active Saturnian moon Enceladus were sampled by Cassini’s Cosmic Dust Analyzer (CDA) – an impact ionisation mass spectrometer [1, 2] – allowing their compositional analysis. CDA exploited the kinetic energy from hypervelocity impacts of ice grains onto its metal target to ionise and fragment molecular compounds embedded in a water ice environment. Cassini’s observations at Enceladus have revealed the presence of a diverse chemical inventory, with CDA detecting a wide range of organic and inorganic compounds incorporated into ice grains ejected through the south polar plume [3-8]. The observed species imply a rich (geo)chemistry in its subsurface liquid water ocean and ongoing hydrothermal activity at its rocky, warm seafloor [9, 10]. The combination of liquid water, rich and active chemistry, and a source of energy – which together hint at habitable conditions - have cemented Enceladus’ place as a prime target in the search for life beyond Earth.
Across the entire mass range of detected organic species, which include nitrogen- and oxygen-bearing compounds and span a wide range of chemical properties, aromatic compounds are common in ice grains sampled from both the fresh plume and Saturn's E ring [3, 6, 11]. The assignment of mass spectral features to specific aromatic parent molecules has hitherto been challenging, as single-ringed aromatics generally fragment via similar pathways during impact ionisation. Organic compounds in ice grains sampled at hypervelocity by CDA generally undergo a degree of fragmentation closely correlated with the impact speed and molecular structure [12]. Unlike other mass spectral techniques, little empirical data is available as a reference for spaceborne impact ionisation mass spectrometry, due to the technical difficulties of accelerating ice grains in the laboratory. Analogue techniques such as laser-induced liquid beam ion desorption (LILBID) mass spectrometry, can successfully recreate impact ionisation mass spectra and thus represent a critical mode of data analysis for past and future space missions [12, 13]. There remains, however, a need for a deeper theoretical understanding of the physics and chemistry behind impact ionisation mass spectrometry, enabling both the prediction of mass spectral appearances for a large variety of organic compounds and – vice versa – the reconstruction of parent molecules from a given mass spectrum.
As a first case study, we employ quantum chemical calculations using the ORCA theoretical chemistry package [14] to investigate the relative energies of various pathways for the dissociation of aromatic compounds in water matrices, representative of the (semi-)polar aromatics detected in ice grains by CDA. These dissociation channels are compared to LILBID mass spectra simulating those obtained from impacts of aromatic-containing ice grains onto a spacecraft detector. We discuss the general applicability of quantum chemistry to impact ionisation and its efficacy in explaining the observed fragment ions of LILBID. We investigate phenol in particular, which is a compound representative of the (semi-)polar aromatics detected by CDA in Enceladean ice grains. We also discuss the influence of the water ice matrix on fragmentation using an explicit solvation model. In general, we find that fragment ions in LILBID match those predicted by some low-energy dissociation channels, but find inconsistencies related to peak intensities. Our work here not only guides the interpretation of existing data from Cassini’s CDA, but will also assist in planning for the SUrface Dust Analyzer (SUDA) instrument, which is based on CDA heritage, onboard the recently launched Europa Clipper [15].
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How to cite: O'Sullivan, T. R., Bera, P. P., Khawaja, N., Napoleoni, M., and Postberg, F.: Towards understanding mass spectra from icy moons using quantum chemistry: A case study for aromatic compounds., EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-544, https://doi.org/10.5194/epsc-dps2025-544, 2025.
