DOI: 10.17169/refubium-37960
title: Discriminating Aromatic Parent Compounds and their Derivative Isomers in Ice Grains from Enceladus and Europa using a Laboratory Analogue for Spaceborne Mass Spectrometers

description: The Cassini-Huygens space mission discovered a plume at Enceladus that ejects gases and frozen ice grains originating from an ocean of liquid water below its icy shell. In these ice grains, a range of interesting organic molecules were discovered by Cassini’s Cosmic Dust Analyzer (CDA) mass spectrometer. Organic molecules are important in the search for life beyond Earth as they form the basis of all known Earth life, and active biology elsewhere would likely have a discernible effect on the local inventory of organic species. One class of organic, with a ring structure of carbon atoms, called aromatics, were discovered in the plume. We investigate the spectral appearance of one example of aromatic compound, benzoic acid, as well as two similar compounds with additional chemical groups attached to the aromatic ring. The two similar compounds have the same mass and general structure, but slightly different arrangements of the additional groups, known as isomers. We find that it is simple to distinguish mass spectral features between benzoic acid and its related compounds, but more difficult to explain the differences between the isomers. This work will assist the analysis of mass spectrometry data from future habitability-investigating space missions to ocean-bearing icy moons.
description: Results from the Cassini-Huygens space mission at Enceladus revealed a substantial inventory of organic species embedded in plume and E ring ice grains originating from a global subsurface and putative habitable ocean. Compositional analysis by the Cosmic Dust Analyzer (CDA) indicated the presence of aromatic species and constrained some structural features, although their exact nature remains unclear. As indicated by many studies, among other organic species, low-mass aromatics likely played a role in the emergence of life on Earth and may be linked to potential prebiotic or biogenic chemistry on icy moons. Here, we study the behavior of single-ringed aromatic compounds – benzoic acid and two isomeric derivatives, 2,3- and 2,5-dihydroxybenzoic acid (DHBA) – using Laser-induced liquid beam ion desorption (LILBID), an analogue setup to simulate the impact ionization mass spectra of ice grains in space. These compounds share common structural features but also exhibit differences in functional groups and substituent positions. We investigate the fragmentation behavior and spectral appearance of each molecule over three simulated impact velocities, in both positive and negative ion mode. Parent compounds can be distinguished easily from their derivatives due to various spectral differences, including the (de)protonated molecular ion peaks appearing at different m/z values. We conclude that distinction between structural isomers in LILBID is more challenging, but some insights can be revealed by considering intermolecular bonding regimes. This work will guide future investigations into elucidating the composition of isomeric biosignatures in ice grains, relevant for future space missions to Enceladus and Europa.