Estimating the Forward Contamination of Mars with a Microbial Survival Model by Calculating Bioburden Reductions on Past Mars Spacecraft

Introduction

     Planetary Protection aims to protect both Earth and other solar system bodies from contamination by possible life forms. Protecting other bodies from terrestrial contamination (also known as forward contamination) has been a high priority for space agencies when sending spacecraft into the solar system. Even with planetary protection protocols in place to limit the prelaunch bioburden, spacecraft are being launched with terrestrial microorganisms on the surface – for example, the Perseverance Rover had a pre-launch bioburden of 3.86 x 10⁴ spores (1). This work estimates the forward contamination of Mars from terrestrial microorganisms by modelling the survivability of bacillus subtills under several biocidal effects on the Martian surface. 

Methodology

     The model used in this work, called the Mars Microbial Survival (MMS) model, is adapted from similar microbial survival models for the lunar surface (2) and interplanetary space (3). Here, we modeled the effects that UVC irradiation, low pressure and temperature synergism, biotoxic soils, and desiccation might have on Earth microorganisms on the surface of Mars at fourteen historical landing/crash sites.

     First, the biocidal effects were modelled for Mars-relevant conditions. For example, the UVC irradiation (200 – 280 nm) at the Martian surface was calculated using a radiative transfer model at the time of year for each landing site. This included time and location dependent parameters such as the UVC flux at the top of the atmosphere and the nominal dust loading conditions. The UVC irradiation incident on horizontal surfaces of each spacecraft was calculated for the first 24-hours after landing. To capture one Mars Year on the surface, the UVC irradiation received over one sol at each landing site was modelled in increments of L_s = 10, and interpolated to cover one Mars Year. In addition to the UVC light incident on horizontal surfaces, surfaces tilted 90 from the horizontal, facing north, south, east, and west were considered. The irradiation values were then converted to bioburden reductions using the method from Schuerger and Moores (2019), which adapted laboratory studies of B. subtilis into bioburden reduction plots.

 Results

 The UVC irradiation incident on horizontal surfaces for the first 24-hours on the Martian surface for eight missions is shown in Figure 1a, and the resulting bioburden reductions are shown in Figure 1b. Note that the UVC irradiation shows 24-hours starting at sunrise (Fig. 1a), but the bioburden reductions are dependent on the specific landing time of the spacecraft (Fig. 1b).

Figure 1: (a) UVC irradiation on flat surfaces for the first sol on Mars at 8 landing sites. (b) Bioburden reductions from UVC after one sol on the surface

     The biphasic bioburden reduction as reported in (2) is seen in Figure 1b. Upon landing, all spacecraft except Pathfinder, which landed in the middle of the night, underwent a rapid inactivation of microbes from UVC exposure. After the initial inactivation of approx. ‒4 logs in < 2 hrs, a slower decrease began at approximately –4.1 log reductions. For Pathfinder, the initial, rapid inactivation occurred when the sun rose approximately 10 hours after landing. Horizontal plots in Fig. 1b indicate no bioburden losses during night-time hours.  After the first 24 hours on Mars, sun-facing surfaces of each spacecraft had a bioburden reduction of approx. –20 logs, indicating UVC rapidly sterilized external surfaces of each spacecraft.

    Figure 2 shows the bioburden reductions from UVC after one Mars Year on the surface for fourteen spacecraft. After one year on the surface, every spacecraft accumulated thousands of log-reductions on horizontal and upward facing surfaces. Despite the proximity to the northern and southern poles, Mars Phoenix Lander and Mars Polar Lander still underwent at least –4000 log reductions on sun-facing surfaces. We found that surfaces tilted away from the horizontal had a smaller bioburden reduction but were still able to accumulate several thousand log-reductions on every surface over one Mars Year.

Figure 2: Bioburden reductions from UVC inactivation on horizontal surfaces after one year on the surface of Mars

     On the internal components of the spacecraft, low-pressure and temperature synergism rapidly inactivated microbes. If the internal temperature of a spacecraft was kept at 313 K, a –12 log reduction (referred to as the Sterility Assurance Level) was reached within a few sols. Colder components, such as those kept around 233 K, had a –2 log bioburden reduction after one Mars Year (668 sols), meaning colder components are sterilized much more slowly than warmer components. Below 210 K, when low-pressure and temperature cease to have synergism---and low-pressure acts alone as the dominant biocidal effect---the time to one Sterility Assurance Level is greater than 25 Mars Years. Only Mars 2, Mars 3, and Mars 6 have been on the surface for the time required to sterilize the insides of the spacecraft from low-pressure alone.

     The results from the MMS imply that there are likely terrestrial microorganisms that have not yet been inactivated on the surface of Mars today. However, due to the highly biocidal nature of UVC irradiation, the outsides of the spacecraft are likely sterilized. Microbes within the spacecraft themselves, or microbes that dislodged upon landing and became covered under UVC-attenuating regolith, are likely to not yet be affected by the biocidal factors of the Martian surface.  With these results, the MMS helps provide a first-order analysis on the likelihood of forward contamination at historical landing sites on Mars, which may be applied to future landing sites of interest.

References

(1) Cooper M, Chen F, Guan L, Hinzer AA, Kazarians G, Ly C, et al. Planetary Protection Implementation and Verification Approach for the Mars 2020 Mission. Astrobiology. 2023 Aug;23(8):825–34.

(2) Schuerger AC, Moores JE, Smith DJ, Reitz G. A Lunar Microbial Survival Model for Predicting the Forward Contamination of the Moon. Astrobiology. 2019 Jun;19(6):730–56.

(3) Moores JE, Schuerger AC. A Cruise-Phase Microbial Survival Model for Calculating Bioburden Reductions on Past or Future Spacecraft Throughout Their Missions with Application to Europa Clipper. Astrobiology. 2020 Dec;20(12):1450–64.