Towards Low Earth Orbit Exposure Experiments on the ISS -Designing a Simulation Setup for Mars Like Conditions

AbstractIn the search for life, Mars is considered to be a major target due to its similarity and relativeproximity to Earth, which makes it accessible for scientific investigation. Considering the pastchemical, geological and physical environment on Mars, the planets surface might have beenhabitable to life during the so called Noachian. The quest to identify complex organic molecules onthe surface of Mars is an ongoing effort using instruments like SHERLOC onboard the NASAPerseverance Rover or the SAM and CheMin instruments onboard the NASA Curiosity Rover.Other investigations of possible biosignatures on Mars are focusing on the search for chemicalprocesses exclusive to life. Potential indicators are specific atmospheric gases like Methane ormineralogical signatures in composition or morphology that indicate past or present presence life.Recent measurements indicate not only the presence of frozen but also subsurface liquid water onMars. Such water could only be stable on the planet in the form of highly concentrated brines.Halophilic organisms, that are known to survive environments with high salinity, have thereforebecome a focus for Astrobiology in the context of Mars.The atmosphere of Mars mostly consists of CO2 (95%), but oxygen (0.174%) and water vapor (0.03%,variable) are also present at an atmospheric pressure of around 6 mbar. The high abundance of CO2blocks UV radiation below around 200 nm while any UV light at higher wavelengths reaches thesurface of Mars. This is in clear contrast to solar radiation on the surface of Earth where UV lightbelow around 300 nm is blocked from reaching ground level due to the higher concentration ofoxygen and ozone. Additionally, the absence of magnetic shielding around Mars means thatenergetic particle radiation can reach the surface of Mars. The average surface temperature of Marsis considered to be around 63 , reaching up to 20 in the equatorial regions and go as low as153 at the poles, with daily variations often exceeding 80 . The surface of Mars is covered in afine, unconsolidated regolith mostly originating from eroded volcanic rocks exhibiting a distinct redcolor caused by high abundances of iron oxides. Varying amounts of phyllosilicates have been foundindicating the past presence of water. To investigate the photochemistry of possible biosignatures ina laboratory or space born context it is necessary to reproduce these extreme conditions asaccurately as possible.A number of radiation exposure experiments under Mars-like conditions in Low Earth Orbit (LEO)involving organic molecules and other astrobiological samples have been performed or are currentlyunder development. Considering high costs and limited availability of space born experiments wehave developed a laboratory based Mars simulation setup. Our setup partly reuses concepts of LEOexperiments while adding simulation parameters that are not yet possible to recreate in LEO due totheir technical complexity. Specialized reaction cells have been developed for the NASA O/OREOScube satellite experiments. They hold samples, applied as thin films, in a sealed gas volume whilebeing transparent to irradiation and spectroscopy measurements. These reaction cells are alsoplanned to be used in the upcoming LEO experiments ExoCube Chem and OREOCube outside theInternational Space Station (ISS). The reaction cells consist of a central stainless steel ring, sealedusing indium rings with a sample window on either side The window materials are chosen to allowboth irradiation and transmission spectroscopy from the UV up to the IR range. Using FTIRspectroscopy, we can show that the reaction cells lose less than 60% of CO2 gas content over a spanof 18 months. The Radiation Background on Mars is complex and can not fully be recreatedaccurately. We therefore focus on simulating electromagnetic radiation as found on the surface ofMars. To do so we use a Xenon Arc lamp that produces a wide spectrum of light similar to solarradiation. It also produces significant amounts of UV radiation below 300 nm so that it can be usedas an adequate radiation source for Mars Simulation. The setup has space for up to 10 reaction cellsplaced in a ring for the most uniform irradiation. The irradiance was checked at each sample spotwith a relative variation in irradiance of less than 5%. The custom made sample holder can be cooledusing liquid nitrogen (LN2). An off the shelf solenoid valve is used to control the flow of LN2 whilethe temperature is controlled using a PT1000 temperature probe in the same form factor as thereaction cells. In practice this system can be used to cool samples to temperatures between roomtemperatures and about 150 . The custom PID control is not limited to a fixed temperature butalso allows to perform temperature protocols (e.g. diurnal cycles). The variance in temperature fromthe setpoint using this temperature control is typically below 1 .FTIR spectroscopy is performed using the ARCoptix OEM FT-IR module which is also planned to beused in the ExoCube Chem LEO experiment. For UV-VIS measurements we use an Ocean InsightFlame-S UV-VIS spectrometer, which is planned to be used in the OREOCube LEO experiment. Bothspectroscopy setups are placed on a xy-stage, measuring individual samples in transmission duringthe irradiation. The spectroscopic measurements are fully automated, such that only the exchange ofliquid nitrogen has to be performed manually. The setup will be used in the context of the ExoCubeHalo project to investigate photochemical processes involving halophilic organisms exposed toextreme Mars like conditions.AcknowledgementsThis work was supported by the Deutsche Forschungsgemeinschaft (DFG, grant number 490702919)and the Volkswagen Foundation and its Freigeist Program.References1 https://doi.org/10.1089/ast.2013.11062 https://doi.org/10.1007/s11214-021-00812-z3 https://doi.org/10.1016/j.pss.2016.06.007.4 https://doi.org/10.1016/j.chemer.2020.125605.5 https://doi.org/10.1038/s41550-020-1080-96 https://doi.org/10.1016/j.pss.2017.01.014.7 https://doi.org/10.1029/1999JE0010958 https://doi.org/10.1016/j.icarus.2018.08.0199 https://doi.org/10.1016/j.actaastro.2012.09.009.10 https://ui.adsabs.harvard.edu/abs/2022cosp44.2758W