Mars 2020 Mission Biological Return Sample Contamination Control Approach and Verification

The Mars 2020 Perseverance rover is equipped with a Sample Caching System (SCS) designed to collect and cache Martian core and regolith samples for return to Earth. To ensure the integrity of these samples, the mission requirements for each encapsulated sample for return is less than one earth-sourced viable organism (Level 1 requirement) and more than a 99.9% probability of being free of any earth-sourced viable organism (Level 2 requirement). To satisfy the stringent biological contamination requirements in support of return sample science (RSS) investigations, special bioburden mitigation and reduction approaches were developed and implemented for SCS hardware that will directly contact or be in close proximity to the Martian samples. Here, we describe the implemented approaches for microbiological contamination reduction and mitigation, detail the processes of the SCS aseptic assembly, and report the estimated viable organism (VO) for each returned sample. We found that our conservative estimate (CE) of the computed probability of a single VO in the returned sample is more than one order of magnitude lower than the biological contamination requirement while the best estimate (BE) exceeds two orders of magnitude.火星2020毅力号火星车搭载了样品缓存系统（Sample Caching System，SCS），该系统专为收集并缓存火星岩心与风化层样品以供返回地球而设计。为保障此类样品的完整性，任务针对每份待返回封装样品制定了两项生物污染要求：一级要求为地球来源活生物体数量不超过1个；二级要求为样品不含任何地球来源活生物体的概率不低于99.9%。为满足支撑返回样品科学（Return Sample Science，RSS）研究的严苛生物污染管控要求，研发并实施了针对直接接触或紧邻火星样品的样品缓存系统硬件的生物负载缓解与降低方案。本文详述了已落地的微生物污染降低与缓解方案，拆解了样品缓存系统的无菌装配流程，并报告了每份返回样品的预估活生物体（Viable Organism，VO）数量。经分析发现，单份返回样品中存在1个活生物体的计算概率的保守预估（Conservative Estimate，CE）比生物污染要求低一个数量级以上，而最优预估（Best Estimate，BE）则比该要求低两个数量级以上。