A Mars Microbial Survival Model: Calculating Bioburden Reductions for Past Mars Spacecraft to Estimate Forward Contamination on Mars

Abstract Minimizing the forward contamination of Mars from terrestrial microorganisms is a key objective of planetary protection. To model the survivability of terrestrial spacecraft microorganisms under the biocidal effects of space, as well as on the surface of Mars, a Mars Microbial Survival (MMS) model was developed. The MMS model predicted the bioburden reductions at 14 historical Mars landing sites. During cruise phases, the exteriors of spacecraft aeroshells were sterilized owing to solar UVC irradiation, accumulating several Sterility Assurance Levels (SALs; defined as –12 log reductions from initial starting populations of 10 6 spores per replicate) even before arriving at Mars. On the surface of Mars, each landed spacecraft would reach one SAL for upward-facing surfaces after only one sol on the surface owing to solar UVC (200–280 nm) irradiation. Over one Mars year, all external surfaces on all landers were likely sterilized by UVC. Contributions from desiccation, low pressure, and biotoxic soils on Mars also produced small additional bioburden reductions. It is unlikely that terrestrial microorganisms will be able to survive on the external surfaces of the landed spacecraft considered in this work. Internally, heated components of the spacecraft may reach one SAL in ∼100 sols, but nonheated internal components might take as long as 25 Mars years to be sterilized.