Understanding of pathogenesis and the nature of biological epidemics has expanded significantly in recent years.^2,3 However, the potential for large-scale pathogenic effects arising from the release of small quantities of pristine martian samples is still regarded as being very low. Significant changes have been made in requirements for containing both known pathogens and novel, or unknown, biological materials, and there have been major improvements in containment design, laboratory practices, and operational oversight.^4,5,6 Numerous reports for planning a Mars sample return mission have acknowledged that biocontainment requirements and planetary protection controls will be integrated as essential elements for handling and testing returned samples.^7,8,9,10

As reviewed in Chapter 3, extreme environments on Earth have not yet yielded any examples of life forms that are pathogenic in humans. However, it is worth noting in this context that interesting evolutionary connections between alpha proteobacteria and human pathogens have recently been demonstrated for natural hydrothermal environments on Earth,¹¹ suggesting that evolutionary distances between nonpathogenic and pathogenic organisms may be quite small in some instances. It follows that, since the potential risks of pathogenesis cannot be reduced to zero,¹² a conservative approach to planetary protection will be essential, with rigorous requirements for sample containment and testing protocols.

New discoveries in environmental microbiology continue to expand understanding of the taxonomic and metabolic diversity of the microbial world, yet much remains unknown.¹³ It is worth noting, however, that extreme environments on Earth have not yet yielded any examples of life forms that are disruptive to ecosystem functions. The risks of environmental disruption resulting from the inadvertent contamination of Earth with putative martian microbes are still considered to be low. But since the risk cannot be demonstrated to be zero, due care and caution must be exercised in handling any martian materials returned to Earth. The demand for a conservative approach to both containment and test protocols remains appropriate.

Although negative effects from nonreplicating biological materials (e.g., toxins and other metabolic by-products) are possible, they are unlikely to be responsible for large-scale pathogenic effects.¹⁴ Nonetheless, they are important as potential biohazards that must be considered when designing protection for the workers who will handle returned martian materials. Operationally, the committee anticipates that existing regulatory frameworks (e.g., that of the Occupational Safety and Health Administration and the Centers for Disease Control and Prevention), coupled with rigorous laboratory biosafety controls, will be incorporated into future discussions of handling and testing protocols and other operations used in the analysis of returned martian materials.

Martian meteorites hold additional importance for planetary protection considerations, beyond the information they convey about environmental conditions on Mars (see Chapter 2). If life originated on Mars and still persists there today, it is possible that over geological time, organisms may have been intermittently delivered to Earth from Mars via impact ejection, a process known as panspermia.¹⁵ Thus, it is appropriate to ask if this natural transfer of materials between Mars and Earth (and vice versa) may have caused large-scale effects for Earth’s environments in the past. If large-scale effects have not demonstrably occurred in the past, can the presence of martian meteorites on Earth be used to argue that there are no back-contamination concerns associated with a Mars sample return mission?