Dispersal of bacterial spores away from spacecraft on the Martian surface is unlikely to contaminate subsequent life-detection samples

Abstract

Mitigating the forward contamination of Mars in the vicinity of a landed spacecraft is a significant concern for future life-detection experiments. Here we model the dispersal of bacterial cells or spores (henceforth spores) from a single dispersal event at landing that injects 100 viable spores to a height of 100 m followed by settling of the spore plume under no-wind or windy scenarios. Subsequent to the single dispersal event, it is assumed that all spores remain viable during dispersal, during their residence time on the surface terrain and throughout the eventual collection of regolith or rock samples. The highest risk for recovering one viable spore after dispersal is in a circle with a radius of 10 m, under a no-wind scenario, and approaches a probability ( P ) of ≤ 1 × 10 ⁻³ . If the spores are lofted with a wind at 1 m/s, the P values for recovering one spore range between 1 × 10 ⁻⁵ at 20 m and 5 × 10 ⁻⁸ at 500 m downwind, respectively. As the wind speeds increase to 10 m/s, the dispersal cones increase downwind to over 7000 m and their aspect ratio narrows significantly. At 10 m/s, P approaches ≤ 1 × 10 ⁻⁹ for recovering one viable spore at 7000 m. If a rover is landed and the traverse is orthogonal to the downwind spore dispersal plume, P = 0, except where sampling vibrations of the rover arm might create secondary dispersal events. Results suggest that the risks of false positives in life-detection experiments from dispersed spores are very low.