UV Reflectance of Spacecraft Materials and Analog Soils: Implications for Bioburden Reductions on the Undersides of Mars Rovers

The Mars Sample Return mission architecture will utilize three spacecraft to collect, cache, recover, launch, and return to Earth a diversity of regolith and rock samples. However, no comprehensive Mars Microbial Survival (MMS) model currently exists. As an initial effort in building an MMS model, we examined the UV reflectance of 15 spacecraft materials and seven Mars analog soils within the context of the Perseverance mission. Data were used to predict the times required to achieve one lethal dose (syn., Sterility Assurance Level [SAL]; def. as a bioburden reduction of -12 logs). Results suggest that a single SAL dosage of UVC was achieved on exposed surfaces on the upper deck of Perseverance within a few hours to a few sols post-landing at Jezero Crater. The overall average for UVC reflectance from spacecraft materials was approximately 10%. The overall UVC reflectance from Mars analog soils was measured at 1.3%. The Adaptive Caching Assembly (ACA) on Perseverance is located on the forward edge of the underbelly of the spacecraft. Modeling of the accumulated UVC dosage for the ACA yielded a prediction of reaching one SAL for downward facing surfaces at 93 sols that receive "single-bounce" UVC photons from the local terrain. The SAL increases to 930 sols if an additional "bounce" of the solar UV irradiation is required to reach a partially protected site in the ACA hardware. The current study is the first to report the UVC reflectance from a diversity of spacecraft materials. A combination of three spacecraft will be used to collect, cache, launch, and return to Earth a diversity of rock and soil samples from Jezero Crater on Mars. The Perseverance rover is currently collecting, characterizing, and caching these samples. We sought to characterize the lethality of the ultraviolet (UV) environment at Jezero Crater to predict if the biocidal UVC (190-280 nm) flux on Mars could sterilize the upper and lower surfaces of the rover. The results suggest that the upper deck of the Perseverance rover exposed to direct solar UVC irradiation was sterilized within a few hours to a few sols on the Martian surface. Reflected solar UVC irradiation from spacecraft materials and soils (i.e., average UV reflectance of spacecraft materials and analog Mars soils were measured at 10% and 1.3%, respectively) is likely to kill the microorganisms attached to shielded or shaded surfaces. Shaded surfaces on the upper deck of Perseverance were likely sterilized within 8.3 sols (post-landing), while the sampling equipment on the underside of Perseverance likely required at least 93 sols to achieve one lethal dose of UVC irradiation. Reflected UVC can sterilize the undersides of landed roversSterilization can reach -12 logs within a few tens of sols on upper surfacesSterilization can reach -12 logs for the undersides of spacecraft within a few hundred sols