Mass Spectrometric Fingerprints of Organic Compounds in NaCl-Rich Ice Grains from Europa and Enceladus

Abstract

Europa and Enceladus, respective moons of Jupiter and Saturn, are prime targets in the exploration of potentially habitable extraterrestrial ocean worlds. Organic material could be incorporated from the ocean into ice grains ejected from the surface or in potential plumes and detected via spacecraft flybys with impact ionization mass spectrometers, such as the SUrface Dust Analyzer (SUDA) onboard Europa Clipper or the Cosmic Dust Analyzer (CDA) onboard the past Cassini mission. Ice grains ejected from both Europa and Enceladus are expected to contain sodium salts, specifically sodium chloride (NaCl), in varying concentrations. Consequently, it is important to understand its effects on the mass spectrometric signatures of organic material in salt-rich ice grains. Previous studies have only focused on the detection of biosignatures, such as amino acids, in salt-rich ice grains. We here perform analogue experiments using the Laser Induced Liquid Beam Ion Desorption (LILBID) technique to study how a wide variety of abiotic and potentially biotic organic molecules could be identified by SUDA-type instruments. We investigate their mass spectral characteristics and detectability at various typical NaCl concentrations expected for salt-rich ice grains and in both cation and anion modes. Results show that organics in salt-rich ice grains can still be detected because of the formation of molecular ions and sodiated and chlorinated species. However, high salt concentrations induce compound- and concentration-dependent suppression effects, depending on the chemical properties and functional groups of the analytes. Our results emphasize the need of both ion modes to detect a wide range of organics embedded in complex matrices and to discriminate between abiotic and potentially biotic species. This work complements a spectral reference library for Europa Clipper and other ocean world missions.