Martian biological contamination may occur if astronauts breathe contaminated dust or if they contact material that is introduced into their habitat. If an astronaut becomes contaminated or infected, it is conceivable that he or she could transmit Martian biological entities or even disease to fellow astronauts, or introduce such entities into the biosphere upon returning to Earth. A contaminated vehicle or item of equipment returned to Earth could also be a source of contamination.

If an astronaut were infected by a Martian life-form, the infection could potentially be observed and treated, or at least evaluated, before the astronaut lands back on Earth. However, once an astronaut has been directly exposed to such life, it would be very difficult, to the point of being impractical, to determine conclusively that the astronaut would not pose a contamination threat to Earth's biosphere. In such an event, NASA might be faced with requiring quarantine and surveillance of returning astronauts until it is determined that a threat no longer exists.

To the degree that NASA has confidence that the life-support systems and habitats can isolate astronauts from hazardous materials, it can more aggressively place those habitats into an environment of unknown constituency. In the extreme, if 100 percent protection were assured by such isolation systems, NASA could send astronauts to an area with known biologic hazards. If 100 percent protection against exposure from the unknown cannot be assured, the hazard must be mitigated by gaining confidence that the environment on Mars poses an acceptable risk.

ENSURING THE SAFETY OF EARTH'S BIOSPHERE

While the threat to Earth's ecosystem from the release of Martian biological agents is very low, “the risk of potentially harmful effects is not zero” and cannot be ignored (NRC, 1997). In light of experience gained during Apollo missions to the Moon, a previous NRC report (NRC, 1993) concludes, “It would, however, be virtually impossible to avoid forward-contamination of Mars or back-contamination of Earth from human exploration.” This committee understands that the threat from back-contamination cannot be eliminated with all certainty, but it is confident that if NASA takes the steps outlined in this report, the threat from back-contamination will be minimized.

NASA should assume that if life exists on Mars, it could be hazardous to Earth's biosphere until proven otherwise (NRC, 1997). As such, NASA should ensure proper quarantine or decontamination of equipment that may have been exposed to a Martian life-form.

The Need for Measurements

For the purposes of identifying precursor missions that must take place prior to human exploration of Mars, the committee did not address the general question of how to detect life on Mars at every location. For instance, it does not matter from the standpoint of human safety or Earth ecosystem protection if life exists on Mars in places inaccessible to astronauts, such as in nonfriable layers of rock or in some deep sub-terranean cavern. It does matter, however, if life exists in any material that the astronauts or their spacecraft and equipment might contact, such as Martian airborne dust and surface or near-surface regolith. With these considerations in mind, the committee recommends that NASA employ the concept of zones of minimal biologic risk (ZMBRs) for astronaut exploration. These zones, operational areas on the surface of Mars, would have been predetermined, to the maximum extent practicable, to be devoid of life or to contain only life-forms that would not be hazardous to humans or Earth's biosphere.

Establishing Zones of Minimal Biologic Risk (ZMBRs)

To protect Earth from contamination by Martian life-forms aboard a returning human mission and astronauts while on the surface of Mars, NASA should first attempt to determine whether life exists (1) at the physical locations where astronauts will be operating and (2) in the Martian material to which astronauts will be exposed.

The establishment of a ZMBR might initially be based on an in situ testing protocol, to be discussed shortly, conducted prior to a human visit. Once a land-



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