A Lunar Microbial Survival Model for Predicting the Forward Contamination of the Moon

Abstract

The surface conditions on the Moon are extremely harsh with high doses of ultraviolet (UV) irradiation (26.8 W $ m-2 UVC/UVB), wide temperature extremes (-171°C to 140°C), low pressure (10-10 Pa), and high levels of ionizing radiation. External spacecraft surfaces on the Moon are generally >100°C during daylight hours and can reach as high as 140°C at local noon. A Lunar Microbial Survival (LMS) model was developed that estimated (1) the total viable bioburden of all spacecraft landed on the Moon as *4.57 • 10 10 microbial cells/ spores at contact, (2) the inactivation kinetics of Bacillus subtilis spores to vacuum as approaching-2 logs per 2107 days, (3) the inactivation of spores on external surfaces due to concomitant low-pressure and hightemperature conditions as-6 logs per 8 h for local noon conditions, and (4) the ionizing radiation by solar wind particles as approaching-3 logs per lunation on external surfaces only. When the biocidal factors of solar UV, vacuum, high-temperature, and ionizing radiation were combined into an integrated LMS model, a theoretical-2479 log reduction in viable bioburden was predicted for external spacecraft surfaces per lunation at the equator. Results indicate that external surfaces of landed or crashed spacecraft are unlikely to harbor viable spores after only one lunation, that shallow internal surfaces will be sterilized due to the interactive effects of vacuum and thermal cycling from solar irradiation, and that deep internal surfaces would be affected only by vacuum with a degradation rate of-0.02 logs per lunation.

Correction to:Astrobiology 2019; 19(6):730–756

“…Changes were made only to the <LMS.m> MatLab program in the new Supplemental Table S5. All other data remain as published in the original LMS paper (Schuerger et al, 2019). The changed MatLab code in Supplemental Table S5 has been highlighted in yellow.…”

“…By increasing the biocidal nature of UV irradiation by 60x in the LMS model, solar UV becomes the dominant biocidal factor on external surfaces on the Moon. The term lethal dose (LD) was defined in the LMS model as a -10 log reduction in spacecraft bioburden (Schuerger et al, 2019). In practice the first -6 logs in bioburden reductions are empirically derived in the lab, and then the kill rates extrapolated an additional -4 logs.…”

“…A Lunar Microbial Survival (LMS) model (Schuerger et al, 2019) was developed to estimate the persistence of spacecraft bacteria over long periods of time on the Moon. Recently, we were extending the LMS model to include predictions for microbial survival on interplanetary spacecraft (Moores and Schuerger, 2020) and detected errors in Eqs.…”

Biological safety in the context of backward planetary protection and Mars Sample Return: conclusions from the Sterilization Working Group

“…For example, in the same study by Weber and Greenberg (1985), UV exposures under vacuum at 21°C were significantly more lethal (>4 logs inactivation) when compared to similar exposures at − 263°C (10 K) ( 1 log of inactivation), thus the synergistic effects are lessened as space temperatures are lowered. Generally, the effects of vacuum + UV (Nicholson et al 2000; Horneck et al 2010; Schuerger et al 2019) seem to indicate that 2 logs of increased lethality can be predicted for Bacillus spores under most conditions of either low-pressure (0.7 kPa) or vacuum (< 10 −4 Pa). The biocidal kinetics of UV irradiation (± vacuum) appear to follow a biphasic response in which the first phase generally ends after 4-logs of spore reduction, and the second phase translates to much slower inactivation rates suggesting that a small proportion of spores are either shielded from, or more refractory to, continued UV irradiation (Schuerger et al 2006, 2019).…”

“…Generally, the effects of vacuum + UV (Nicholson et al 2000; Horneck et al 2010; Schuerger et al 2019) seem to indicate that 2 logs of increased lethality can be predicted for Bacillus spores under most conditions of either low-pressure (0.7 kPa) or vacuum (< 10 −4 Pa). The biocidal kinetics of UV irradiation (± vacuum) appear to follow a biphasic response in which the first phase generally ends after 4-logs of spore reduction, and the second phase translates to much slower inactivation rates suggesting that a small proportion of spores are either shielded from, or more refractory to, continued UV irradiation (Schuerger et al 2006, 2019). However, D -values are often calculated only for the first phase of the kill-curves (i.e.…”