Identifying Amino Acids Isomers with Dust Analyzers in Ice Grains from Enceladus and Europa

The icy ocean moons Enceladus and Europa offer potentially habitable environments below their icy crusts. Ice grains ejected from cryovolcanic plumes [1,2] and micrometeorite bombardment can be sampled by impact ionization mass spectrometers, as performed in the past by the Cosmic Dust Analyzer (CDA; [3]) onboard Cassini in the Saturnian system. Successor instruments to the CDA include the SUrface Dust Analyzer (SUDA; [4]) onboard NASA’s Europa Clipper mission and the HiFi instrument for a future Enceladus mission [5]. The strongly enhanced capabilities of SUDA and contemporary instruments, relative to CDA, allow the identification of molecular biosignatures. Among possible molecular biosignatures, amino acids are essential building blocks of proteins and play a crucial role in the formation of water-based life as we know it, thus their identification on extraterrestrial water worlds is key to the search for life beyond Earth.

Laboratory analogue experiments using laser-induced liquid beam ion desorption (LILBID [6]) have demonstrated that impact ionization mass spectrometers can detect amino acids [7] down to the ppm or ppb level, if they are entrapped in emitted ice grains, and can distinguish between abundance patterns of abiotic and biotic formation processes [8]. However, at any given molecular mass of an amino acid, several isomers (identical molecular formula but distinct arrangements of atoms in space) exist, which are indistinguishable by their molecular peaks in recorded mass spectra. Until now, it was unclear whether isomeric amino acids can be discriminated from each other, e.g., by fragmentation patterns in impact ionization mass spectra.

Here, using LILBID mass spectrometry, we conducted a cation mode analysis of eight isomeric amino acids with an identical molecular mass of 131.173 u and formula C₆H₁₃NO₂ [9]. The recorded mass spectra were investigated for spectral features that enable differentiation of the different isomeric amino acids, with the aid of quantum chemistry calculations.

We show that the amino acid isomers (including diastereoisomers) can be uniquely identified due to their distinct mass spectral features and fragmentation patterns. Several observed fragments and their intensities can be explained through intramolecular hydrogen bonding and other structural effects originating from the parent molecules. Importantly, α-amino acids can be clearly differentiated from non-α-amino acids, because they have lower proton affinities than non-α-amino acids, which result in lower ionization efficiencies for α-amino acids. Additionally, we further complement the LILBID database [10], which already contains a large variety of analogue mass spectra of both organic and inorganic compounds, for upcoming missions to icy ocean moons.

The ability to discriminate amino acid isomers in a robust and reliable manner highlights a novel ability of impact ionization mass spectrometers that has significant implications for the search of biosignatures in the solar system, in particular for SUDA on Europa Clipper and other future instruments onboard missions exploring ocean worlds.

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[8] F. Klenner et al., Astrobiology, 20, 1168-1184 (2020)

[9] J. Bönigk, et al. Astrobiology 15311074261443835 (2025)

[10] F. Klenner et al., Earth Space Sci., 9, e2022EA002313 (2022)