Implementing bioburden reduction and control on the deliquescent hydrogel of the ExoMars HABIT instrument

The HABIT (HabitAbility, Brines, Irradiation and Temperature) instrument will be the first Swedish instrument that will land on the surface of Mars as a part of the ExoMars 2020 mission (ESA/IKI). It is also the first European ISRU (In-situ Resource Utilization) instrument capable of producing liquid water on Mars extracting atmospheric water vapor using salt deliquescence to form a stable liquid brine. HABIT also will study current habitability conditions on Mars investigating the air and surface thermal ranges and UV (Ultraviolet) irradiance. The BOTTLE (Brine Observation Transition To Liquid Experiment) is the container element of HABIT with four independent cells housing deliquescent salts, which have been found on Mars, exposing them to the Martian atmosphere. In order to prevent capillarity of deliquescent or hydrated salts, a mixture of deliquescent salts with Super Absorbent Polymer (SAP) based on polyacrylamide is utilized. This mixture has deliquescent and hydrogel properties and can be reused by applying a thermal cycle, complying thus with the purpose of the instrument. A High Efficiency Particulate Air (HEPA) grade filter made of polytetrafluroethylene (PTFE) porous membrane sandwiched between spunbounded non-woven fabric stands as a physical barrier allowing interaction between the gaseous molecules of the Martian atmosphere and the salt mixtures, and at the same time preventing the passage of any potential biological contamination from the cells to the outside or vice-versa. In addition to the physical barrier, a strict bioburden reduction and analysis procedure is applied to the hardware and the contained salt mixtures adhering to the European Cooperation for Space Standardization protocol of microbial examination of flight hardware (ECSS-Q-ST-70-55C). The deliquescent salts and the SAP products need to be properly treated independently to adhere to the planetary protection protocols. In this paper, we have described the bioburden reduction process utilized to sterilize the salt mixtures in BOTTLE and the assays adopted to validate the sterilization. The sterilization process involves Dry Heat Microbial Reduction (DHMR) of the deliquescent salts and the SAP mixtures. The performance of SAP after DHMR is validated to ensure its working efficiency after sterilization. A slightly modified version of the standard swab assay is used in the validation process and a comparison is made between samples exposed to a thermal shock treatment and those without thermal shock, to determine the best assay to be applied for future space hardware utilizing such salt mixtures for planetary investigation and ISRU. The demonstration of the compatibility of these products with the processes commonly required for space applications has implications for the future exploration of Mars.