Detecting hydrothermally processed peptides in the mass spectra of ice grains emitted by Enceladus and Europa

Results from the Cassini mission at Enceladus revealed the presence of a diverse organic inventory [1,2] in its subsurface ocean as well as ongoing hydrothermal activity at the water-rock interface [3,4]. Ice grains ejected into the plume are thought to contain compounds originating from the depths of the ocean, which are likely linked to hydrothermal (bio)geochemistry, as are other constituents in the vapour phase [1,2,5]. Understanding the nature and effects of hydrothermal activity on Enceladus and other icy ocean worlds (e.g. Europa) is crucial for evaluating the prospects of habitability as well as understanding how potential biosignatures may manifest in spacecraft data. Biosignatures that are present on the ocean floor may be chemically altered by the elevated temperature and pressure conditions in such environments, as well as by salts, mineral catalysts, or other solutes in hydrothermal fluids.

Analogue mass spectra, generated in the laboratory using the laser-induced liquid beam ion desorption (LILBID) technique [6], are a useful tool for interpreting impact ionisation mass spectra, as produced by Cassini’s mass spectrometer – the Cosmic Dust Analyzer (CDA). Thus, impact ionisation mass spectra of both hydrothermally processed and unprocessed material in ice grains obtained via spaceborne instruments can be recreated. Given the ubiquity of peptides utilised by all known life on Earth, Khawaja et al. [7] recently investigated the hydrothermal evolution of the simplest tripeptide – triglycine (GGG) - as both a potential biosignature and marker for hydrothermal activity. This work verified significant differences between the spectra of processed and unprocessed GGG, outlining an approach to verify a unique spectral fingerprint as evidence for the former. Here, we briefly discuss the results from the hydrothermally processed GGG, with similarities between the spectra before and after processing compared. We demonstrate that the presence of processed GGG can be elucidated with a two-step process: a) GGG is, in general, identified by the appearance of glycine, diglycine, and GGG peaks, along with common fragments indicative of peptides and N-bearing compounds, and b) processed GGG is identified by the additional presence of unique peaks related to products formed exclusively by the hydrothermal processes. In addition, certain peaks related to compounds not present in the initial solution appear in both spectra – e.g. a peak at m/z 115 in the cation spectrum assigned to diketopiperazine. The potential implications of such results are discussed briefly here and will form the basis of more detailed future investigations. We will apply our methodology used to deduce the presence of hydrothermally processed GGG to peptides of longer chain length and containing different amino acid constituents.

We also examine the influence of salts (NaCl) on the hydrothermal processing of GGG at pH 8.5-10, conditions typical for the ocean-core interface of Enceladus, and the peptide’s appearance in impact ionisation mass spectra. Previous experiments [8-10] have demonstrated that salts can suppress and alter expected organic features in LILBID and, by extension, impact ionisation mass spectrometry. Salinity and acidity are also thought to influence reaction pathways in hydrothermal chemistry [11-13]. This multi-faceted influence of salts presents significant implications for the detection of potential molecular biosignatures from icy ocean worlds with saline oceans (i.e. Enceladus and Europa). The solubility of GGG generally increases with greater departure from a neutral pH (i.e. more acidic or alkaline) and with salinity [14]. This is an important factor that not only influences the detectability of triglycine but also its behaviour as a biomolecule. The role of potential catalytic minerals relevant for the expected seafloor composition of Enceladus is also discussed in this work.

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