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.

Mass Spectrometric Fingerprints of Organic Compounds in Sulfate-Rich Ice Grains: Implications for Europa Clipper

“…63,68 This effect (in MgSO 4 matrices) is similar or stronger than that seen for the same organic compounds in NaCl matrices. 66 While detection limits can typically drop by orders of magnitude with high concentrations of salts, this effect is strongly compound-dependent and was mild for some organic species (e.g., for 5-amino-1-pentanol in 1 M MgSO 4 as seen in Figure S5).…”

“…Magnesium adducts including [M + Mg–H] + cations have previously been observed by Han et al with electrospray ionization (ESI) mass spectrometry. As the LILBID ionization method tends to produce singly charged ions and magnesium (Mg 2+ ) and sodium (Na + ) ions have different charges, the organic cations formed in MgSO 4 matrices are different from the species observed in NaCl matrices, where sodium adducts form with organic molecules via sodiation processes. ,, The replacement of structural elements by Na + cations (i.e., sodiation) seems to be more efficient than by Mg 2+ cations, as protonated organic species typically dominate the cation spectra of MgSO 4 matrices, whereas sodiated cations typically dominate in NaCl matrices. , In organic molecules, it is easier to replace H + with Na + than with Mg 2+ because Na + and H + have the same charge. Additionally, the larger size of Mg 2+ ions compared to that of Na + slows down the attachment to organic molecules by steric hindrance.…”

“…This can be achieved by combining variations in laser power density with delay time. 62 Recently, the LILBID technique was used to investigate the mass spectral fingerprints of a wide range of organic compounds in NaCl-rich water ice grains 66 as present at Enceladus 19 and probably Europa. 30 We showed that despite high salt concentrations, organic material�depending on the NaCl concentration�can be detected via molecular peaks and/or sodiated and chlorinated adducts.…”

“…30 We showed that despite high salt concentrations, organic material�depending on the NaCl concentration�can be detected via molecular peaks and/or sodiated and chlorinated adducts. 66 Matrix effects were shown to play an important role in the detectability of organics and were dependent on the structural and compositional properties of the organic compounds, including the presence of functional groups, and the salt concentration. Building on these experiments, 66 we here measured LILBID spectra of the same range of organic species (5-amino-1pentanol, acetic acid, benzoic acid, butylamine, glucose, methanol, pyridine) in both cation and anion modes to investigate their mass spectral signatures in 0.01, 0.1, and 1 M concentration MgSO 4 -and H 2 SO 4 -rich matrices, i.e., concentrations representative of those expected at the surface of Europa and similar to those used for NaCl matrices.…”

“…66 Matrix effects were shown to play an important role in the detectability of organics and were dependent on the structural and compositional properties of the organic compounds, including the presence of functional groups, and the salt concentration. Building on these experiments, 66 we here measured LILBID spectra of the same range of organic species (5-amino-1pentanol, acetic acid, benzoic acid, butylamine, glucose, methanol, pyridine) in both cation and anion modes to investigate their mass spectral signatures in 0.01, 0.1, and 1 M concentration MgSO 4 -and H 2 SO 4 -rich matrices, i.e., concentrations representative of those expected at the surface of Europa and similar to those used for NaCl matrices. 66 The goal of this work is to understand the interactions between salts or sulfuric acid and organics in simulated impact mass spectra of SUDA for organic content on the order of several % in concentration.…”