dc.description.abstract
In the Solar System, the saturnian moon Enceladus and the jovian moon Europa are potentially habitable and the most promising candidates for space missions searching for extraterrestrial life. The subsurface oceans of these moons constitute the long-lived presence of large amounts of liquid water, a common component of icy moons throughout the outer Solar System. In addition, the rocky seafloors of both moons allow for rich geochemistry in their subsurface oceans, potentially including the formation of complex organic material at hydrothermal vent systems, which could also provide an energy source for another emergence of life. The exploration of icy ocean moons can be performed using impact ionization mass spectrometry, a powerful technique to thoroughly analyse the composition of ice grains ejected from both the surface ice shell and subsurface liquid reservoirs.
The Cosmic Dust Analyzer (CDA) onboard the Cassini mission detected a variety of salts, complex and reactive organic compounds, as well as bioessential elements in ice grains ejected by the plume of Enceladus into space. CDA mass spectra provided strong evidence for habitable conditions at the seafloor of the moon. These discoveries greatly benefitted from laboratory analogue mass spectra complementing the CDA data, obtained from a laboratory setup using Laser Induced Liquid Beam Ion Desorption (LILBID) as an ionisation source coupled to time-of-flight mass spectrometry. CDA paved the way for the SUrface Dust Analyzer (SUDA), its improved successor instrument onboard the upcoming Europa Clipper mission, which will provide a deeper understanding of Europa’s habitability. SUDA aims to characterize the composition of the subsurface ocean or liquid inclusions in the icy crust by analysing young endogenic surface material or possible cryovolcanic plume material.
Preparation for SUDA’s mission and related data analysis requires considerable laboratory work. This thesis describes LILBID experiments simulating the mass spectral signatures of both salt- and organic-rich ices, as expected to be encountered by SUDA or similar instruments on a future mission to Enceladus.
After a description of the current state of research into the habitability and astrobiological potential of both Europa and Enceladus in Part I, Part II details how SUDA-type instruments could detect organic molecules embedded in salt-rich ice grains. LILBID mass spectra were recorded for several organic species with various properties, derived from a range of functional groups, together with either NaCl, MgSO4 or H2SO4 at concentrations relevant for the surfaces of icy moons. Mass spectrometric signatures of the organic species can be detected via their molecular ions and a range of cluster species with Na+, Mg2+, Cl−, OH−, HSO4− ions, and NaCl, MgSO4 and H2SO4 molecules. The presence and intensity of these characteristic organic-rich peaks depends on the inorganic matrix and its concentration. The intensities of analyte signatures decrease with increasing salt concentrations due to suppression effects. In contrast, they are increased by the presence of sulfuric acid matrices in cation mode. Moreover, the sensitivity to different organic species strongly depends on the instrument polarity (cation or anion mode of the spectrometer) and on molecular properties, especially the functional groups present.
The recorded spectra are an important foundation to characterise both the organic and salt composition in ice grains from Europa and Enceladus and, by extension, potential constituents of the subsurface oceans.
Part III reveals a previously unknown capability of SUDA-type instruments: the ability to determine the oxidation state of iron in iron-bearing salts (or other minerals) embedded in ice grains. Such analytical capability may, in the near future, allow the elucidation of key parameters related to the geochemistry of subsurface oceans on icy moons, notably their redox chemistry and pH. These factors may have implications for hydrothermal mineralogy as well as possible metabolic activity of putative microbial life.
Further ongoing projects that have been conducted in the framework of this PhD project are briefly described in Part IV. They cover the irradiation of (i) icy samples containing molecular biosignatures to evaluate their degradation under the harsh radiation environment of Europa’s surface, and (ii) simple compounds (CH3OH:NH3:H2O ice) leading to the synthesis of a variety of complex organic structures. Moreover, an outlook is given on a future expedition to acquire samples in Antarctica - the best icy moon analogue on Earth - that has been planned during this PhD project. Finally, this work concludes with a summary of the most relevant results of this thesis (Part V).
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