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

Abstract
In the search for life, Mars is considered to be a major target due to its similarity and relative
proximity to Earth, which makes it accessible for scientific investigation. Considering the past
chemical, geological and physical environment on Mars, the planets surface might have been
habitable to life during the so called Noachian¹. The quest to identify complex organic molecules on
the surface of Mars is an ongoing effort using instruments like SHERLOC onboard the NASA
Perseverance Rover² or the SAM³ and CheMin⁴ instruments onboard the NASA Curiosity Rover.
Other investigations of possible biosignatures on Mars are focusing on the search for chemical
processes exclusive to life. Potential indicators are specific atmospheric gases like Methane or
mineralogical 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 on
Mars. 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 therefore
become a focus for Astrobiology in the context of Mars⁵.
The atmosphere of Mars mostly consists of CO2 (95%), but oxygen (0.174%) and water vapor (0.03%,
variable) are also present⁶ at an atmospheric pressure of around 6 mbar. The high abundance of CO2
blocks UV radiation below around 200 nm while any UV light at higher wavelengths reaches the
surface of Mars. This is in clear contrast to solar radiation on the surface of Earth where UV light
below around 300 nm is blocked from reaching ground level due to the higher concentration of
oxygen and ozone. Additionally, the absence of magnetic shielding around Mars means that
energetic particle radiation can reach the surface of Mars. The average surface temperature of Mars
is considered to be around −63 ℃, reaching up to 20 ℃ in the equatorial regions and go as low as
−153 ℃ at the poles, with daily variations often exceeding 80 ℃⁷. The surface of Mars is covered in a
fine, unconsolidated regolith mostly originating from eroded volcanic rocks exhibiting a distinct red
color caused by high abundances of iron oxides. Varying amounts of phyllosilicates have been found
indicating the past presence of water⁸. To investigate the photochemistry of possible biosignatures in
a laboratory or space born context it is necessary to reproduce these extreme conditions as
accurately 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 currently
under development. Considering high costs and limited availability of space born experiments we
have developed a laboratory based Mars simulation setup. Our setup partly reuses concepts of LEO
experiments while adding simulation parameters that are not yet possible to recreate in LEO due to
their technical complexity. Specialized reaction cells have been developed for the NASA O/OREOS
cube satellite experiments⁹. They hold samples, applied as thin films, in a sealed gas volume while
being transparent to irradiation and spectroscopy measurements. These reaction cells are also
planned to be used in the upcoming LEO experiments ExoCube Chem and OREOCube¹⁰ outside the
International Space Station (ISS). The reaction cells consist of a central stainless steel ring, sealed
using indium rings with a sample window on either side The window materials are chosen to allow
both irradiation and transmission spectroscopy from the UV up to the IR range. Using FTIR
spectroscopy, we can show that the reaction cells lose less than 60% of CO2 gas content over a span
of 18 months. The Radiation Background on Mars is complex and can not fully be recreated
accurately. We therefore focus on simulating electromagnetic radiation as found on the surface of
Mars. To do so we use a Xenon Arc lamp that produces a wide spectrum of light similar to solar
radiation. It also produces significant amounts of UV radiation below 300 nm so that it can be used
as an adequate radiation source for Mars Simulation. The setup has space for up to 10 reaction cells
placed in a ring for the most uniform irradiation. The irradiance was checked at each sample spot
with a relative variation in irradiance of less than 5%. The custom made sample holder can be cooled
using liquid nitrogen (LN2). An off the shelf solenoid valve is used to control the flow of LN2 while
the temperature is controlled using a PT1000 temperature probe in the same form factor as the
reaction cells. In practice this system can be used to cool samples to temperatures between room
temperatures and about −150 ℃. The custom PID control is not limited to a fixed temperature but
also allows to perform temperature protocols (e.g. diurnal cycles). The variance in temperature from
the 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 be
used in the ExoCube Chem LEO experiment. For UV-VIS measurements we use an Ocean Insight
Flame-S UV-VIS spectrometer, which is planned to be used in the OREOCube LEO experiment. Both
spectroscopy setups are placed on a xy-stage, measuring individual samples in transmission during
the irradiation. The spectroscopic measurements are fully automated, such that only the exchange of
liquid nitrogen has to be performed manually. The setup will be used in the context of the ExoCube
Halo project to investigate photochemical processes involving halophilic organisms exposed to
extreme Mars like conditions.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft (DFG, grant number 490702919)
and the Volkswagen Foundation and its Freigeist Program.
References
1 https://doi.org/10.1089/ast.2013.1106
2 https://doi.org/10.1007/s11214-021-00812-z
3 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/s41550-020-1080-9
6 https://doi.org/10.1016/j.pss.2017.01.014.
7 https://doi.org/10.1029/1999JE001095
8 https://doi.org/10.1016/j.icarus.2018.08.019
9 https://doi.org/10.1016/j.actaastro.2012.09.009.
10 https://ui.adsabs.harvard.edu/abs/2022cosp…44.2758W