Biocidal Conditions in Low-Mars-Orbit can Inactivate Bioburdens on External Spacecraft Surfaces and Dust Particles within a few Sols

Mars Sample Return Program planning includes a series of spacecraft staged both on the Martian surface and in low-Mars-orbit (LMO). During the transfer of samples into orbit, external spacecraft surfaces might be exposed to Mars dust carried on the sample container exterior and possibly extant microbiota (if present). This study was designed to characterize the synergistic effects of LMO ultraviolet irradiation, vacuum, and solar heating on the survival of four UV-resistant and heat-tolerant bacteria, yeast, and fungus. The species tested were Aspergillus fumigatus ISSFT-021 conidia, Naganishia onofrii DBVPG 5303, Bacillus pumilus SFR-032 spores, and Geobacillus stearothermophilus ATCC 12980 spores. A. fumigatus and B. pumilus were also exposed to LMO conditions with and without Mojave Mars Simulant (MMS) dust. The time required to reach the desired Sterility Assurance Level (SAL; 10-6 from a dose predicted to yield 12-log reduction) was 2.0 hours for A. fumigatus and 76.6 minutes for B. pumilus if exposed directly to the solar UV beam in LMO. With MMS, predicted SAL 10-6 values were extended to 22 hours and 1.72 hours, respectively. Analysis of UV transmittance through cell stacks of up to 12 µm thick were performed for A. fumigatus. Results indicated that only ~4-5% of UVC photons can penetrate through 12 µm stacked aggregates. Divergent results among experiments were likely caused by presence of stacked aggregates of spores among the various coupons tested.