Photochemical Stability of Organic Compounds in Mineral Matrices and Metal Oxide Surfaces in Space: In-situ Spectroscopy on the International Space Station and Laboratory Simulation Experiments
Authors: Wipf Severin , Nitsche Ruben , Sultana Zeba , Burr David , Drauschke Janina , Hofmann Florence , Elsaesser Andreas 
Published: July 2024

In the search for organic compounds associated with the origin and self-preservation of lifeforms in our solar system, inorganic materials are widely recognized as an important factor influencing the chemical stability of such organic compounds, so called biosignatures, under hostile space conditions like extreme temperature, pressure and radiation[1]. Here, spaceborne experiments and laboratory simulations play a crucial role to investigate the underlying interaction mechanisms of organic compounds and specific inorganic materials [2-4] in order to support the efforts of biosignature search by space missions [5, 6]. As next-generation spaceborne experiments, OREOcube (Organics Exposure Orbit cube) and ExocubeChem (Exposure of organics/organisms cube Chemistry) will be mounted on the outside of the International Space Station in 2026, enabling the exposition of biosignatures in and inorganic materials to elevated levels of electromagnetic radiation in the low Earth orbit for approxiimately 6 months. Both experiments will measure the photostability of organic molecules spectroscopically with robust, miniaturized and space-qualified spectrometers; while OREOcube will make use of an ultraviolet-visible (UV-VIS) spectrometer using the sun as light source, ExocubeChem will utilize a Fourier-transform infrared (FTIR) spectrometer with an integrated Globar to track structural changes of organic compounds and provide detailed information about their kinetics and photochemical evolution. The UV-Vis spectrometer of OREOcube covers a wavelength range of 200-1100nm while the IR spectrometer of ExocubeChem detects molecular vibrations in the 2.5-12&#181;m range. To enable such in-situ measurements in space, the samples will be prepared as thin films of several nanometer up to thicknesses of some micrometer on Magnesium Fluoride (MgF2) substrates with high transparency for electromagnetic radiation from the UV- up to the mid-infrared wavelength range (approximately 120 to 8500 nm). To obtain uniform sample films, the organic and inorganic molecules will be deposited by controlled thermal evaporation or pipetting onto the MgF2 substrates which then can be hermetically sealed in specially designed gas tight cells, allowing control of different gas atmospheres and pressures. In this way, the organic/inorganic thin films can be exposed to specific gas mixtures giving the possibility to mimic different planetary atmospheres and the development of gaseous photolytic products of the sample species can be detected. The sample cell design and parts of the experimental setups are a heritage of the O/OREOS (Organism/Organic Exposure to Orbital Stresses) nanosatellite[7], optimized for spectroscopic measurement in space. Supportively, samples from OREOcube and ExocubeChem will be returned to the Earth, opening the possibility for further advanced post-flight anylsis of the samples. Additional microscopy and in-depth chemical analyses will provide a deeper understanding of the astrochemical processes undergone by the organic compounds and the impact of inorganic surfaces on their photochemical stability.The ongoing preparation process of OREOcube and ExocubeChem includes the final selection of the sample species, considered to be most interesting combination of biosignatures and inorganic material in terms of relevance to the origin of lifeforms, photo-stabilization and preservation effects. In this context, we constructed a Mars simulation chamber at Freie Universitaet Berlin, which is designed to be compatible with the experimental design of OREOcube and ExocubeChem. The sample species, contained in the gas tight cells described above, can be exposed to the Martian surface temperature and atmospheric conditions (with the possibility of simulating atmospheric humidity) and subjected to simulated solar light irradiation with UV-light intensities which are modeled on those on the Martian surface environment. A fully automated measurement system using prototypes of the UV-Vis and FTIR-spectrometer of OREOcube and ExocubeChem can provide detailed information about the photochemical evolution and kinetics at 5-hour intervals without interfering with the experimental process.Several experiments have been carried out to examine which organic/inorganic compounds are appropriate for the OREOcube and the ExocubeChem experiments, including the suitability of their chemical composition to the different spectroscopic methods. In a first set of experiments, we investigated the photochemical interactions of the proteinogenic amino acid alanine and the Martian soil analogue montmorillonite clay; we intercalated the amino acid into the interlayers of the montmorillonite, forming a so called organoclay. The results, in short, indicate that the alanine is photochemically unstable under Mars-like UV-radiation and would require millimeter-thick clay layers as sunshield to persist for extended periods of time. But, interestingly, the clay matrix is capable to confine gaseous carbon dioxide (a photolytic byproduct of the alanine decay) in its interlayers, indicating the past existence of an organic compound within the clay. This confinement process, however, has to be further analyzed with addressing the desorption rate of the carbon dioxide from the clay. In another, recent series of experiments, the photostability of biosignatures such as pigments (beta carotene and quercetin) or metal-porphyrins on iron oxide (FexOx) surfaces was investigated. Pigments play a key role in photoautotrophic metabolism and are potent scavangers of (UV-induced) free radicals [8], porphyrins are photochemically robust and an unambiguous indicator of biological activity [9] and iron oxides have found to be abundant on the Martian surface [10]. Preliminary results suggest, that the oxidation state of iron impacts the photostability of the organic compounds.Further experiments with other classes of biosignatures such as polycyclic hydrocarbons and quinones in combination with inorganic materials found on the Martian surface such as titanium oxides (TiO2), manganese oxides (MnOx) and mineral clays are scheduled for the second half of 2024.&#160; &#160;&#160;&#160;References 1) Kopacz, N., et al., Icarus, 2023. 394.2) Elsaesser, A., et al., Acta Astronautica, 2020. 170: p. 275-288.3) Baratta, G.A., et al., Astrobiology, 2019. 19(8): p. 1018-1036.4) Stalport, F., et al., Astrobiology, 2019. 19(8): p. 1037-1052.5) Enya, K., et al., Life Sci Space Res (Amst), 2022. 34: p. 53-67.6) Parker, E.T., et al., Geochimica Et Cosmochimica Acta, 2023. 347: p. 42-57.7) Ehrenfreund, P., et al., Acta Astronautica, 2014. 93: p. 501-508.8) Baque, M., et al., Science Advances, 2022. 8.9) Suo, Z., et al., Astrobiology, 2007. 7(4): p. 605-15.10) Morris, R.V., et al., Journal of Geophysical Research: Planets, 2006. 111(E12).&#160;&#160;&#160;Acknowledgements:Ministry of Economics and Energy(DLR, grants 50WB1623 and50WB2023). Volkswagen Foundation and Freigeist Program.&#160;