Analogue Experiments for the Mass Spectral Analysis of Organic Compounds form the Salt-rich Surface of Europa

Jupiter’s moon Europa is a prime target in the exploration of potentially habitable extraterrestrial ocean worlds. Its global subsurface ocean is likely in contact with a rocky silicate seafloor [1] and may have provided a long-term, stable environment with essential elements and energy sources for an independent origin of life [2]. Biogenic, prebiotic, or abiotic organic material from the ocean could be incorporated in ice grains ejected from the surface due to micrometeorite impacts [3] or in potential cryovolcanic plumes [4]. These ice grains could be detected during spacecraft flybys by the SUrface Dust Analyzer (SUDA [5]), an impact ionization mass spectrometer on board NASA’s upcoming Europa Clipper mission [6]. SUDA-type instruments are powerful tools for the identification of organic molecules, as demonstrated by the detection of complex organic macromolecules [7] and smaller reactive nitrogen- and oxygen-bearing compounds [8] in ice grains formed from Enceladus’s subsurface ocean by SUDA’s predecessor instrument, the Cosmic Dust Analyzer (CDA) [9] on board the Cassini spacecraft. These findings required laboratory analogue experiments using the Laser Induced Liquid Beam Ion Desorption (LILBID) approach, a proven technique for the investigation of mass spectral characteristics of organic molecules as identifiable by SUDA-type instruments. LILBID experiments have shown that bioessential molecules such as amino acids, fatty acids and peptides, as well microbial biosignatures, could be detected, and that abiotic and biotic mass spectral fingerprints could be distinguished by spaceborne impact ionization mass spectrometers down to the ppm or ppb level [10, 11, 12].

In impact ionization mass spectrometry, the matrix from which a sample is analyzed can influence ion formation and the resulting spectra [13]. Simulating the mass spectra of organic compounds in ice grains ejected from Europa therefore requires consideration of matrix compounds, particularly inorganic salts which seem to be a common ingredient of Europa’s surface ices [14, 15]. Here we investigate the detectability of several organic molecules (5-amino-1-pentanol, acetic acid, benzoic acid, butylamine, glucose, methanol, pyridine) containing a wide range of functional groups, namely hydroxyl, azine, (aromatic) carboxylic acid, (aromatic) amine, and alkanolamine, in salt-rich ice grains ejected from Europa. We focus on the effects of sodium chloride (NaCl), an abundant inorganic salt (0.1-1.2 mol/kg H₂O [16]), on the mass spectral signatures of these organics in both cation and anion mode.

Our results show that the salt-rich matrices modify the mass spectral characteristics of the organics by forming salt adducts, effectively suppressing characteristic organic peaks and complicating the spectral interpretation with interferences. We found that the spectral appearance of the organics fundamentally changes when using high (1M) NaCl concentrations as compared to low (0.01M) concentrations. At high concentrations in cation mode, the investigated organics typically form sodiated molecular ions [M+Na]⁺, clusters with NaCl and NaOH ([M+Na+(NaCl)]⁺, [M+Na+(NaOH)]⁺), as well as disodiated and trisodiated adduct cations ([M-H+ 2Na]⁺ and [M-2H+3Na]⁺). At the lowest salt concentration (0.01M NaCl), all the investigated organics are detected as protonated molecular peaks [M+H]⁺, whereas these peaks are mostly suppressed at higher NaCl concentrations. In anion mode, organics are typically detected as deprotonated molecules [M-H]^-, chlorinated adducts [M+Cl]^-, NaCl and NaOH clusters ([M+Cl+NaCl]^-, [M+Cl+NaOH]^-, [M-H+NaCl]^-) at concentrations of 0.01M and 0.1M NaCl. Suppression effects of organic-related peaks are stronger in anion mode than in cation mode, and prevent the detection of most organics at the highest NaCl concentration investigated (1M), although those organics appear as salt adducts in cation spectra. By contrast, the two organic acids investigated (acetic and benzoic acid) are detected with a high sensitivity in anion mode, even at the highest salt concentration. Deprotonated molecular ions of acetic acid were identified in a 1M NaCl matrix at a concentration of 0.1vol%, whereas most other organics need to be present at least at 10 times higher concentrations to be detectable at these high salt concentrations. In both ion modes, fewer characteristic fragment ions appear in the spectra with increasing salt concentrations, possibly due to either neutralization of charged fragments or suppression of the fragmentation process of the organics.  

Our results confirm the necessity for SUDA to record both cation and anion mass spectra to allow the identification in salt-rich ice grains of a wide range of organic compounds with different functional groups and pH and pKa values. Ongoing complementary investigations focus on other inorganic salts or acids expected to be found in the water ice matrix of Europa, including magnesium sulfate (MgSO₄) and sulfuric acid (H₂SO₄), which are expected to similarly induce complex matrix effects.

 

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