Potential Hazards of the Biological Environment
The committee was charged with addressing issues of biological risks on Mars from two perspectives: (1) ensuring the safety of astronauts operating on the surface of Mars and (2) ensuring the safety of Earth 's biosphere with respect to potential back-contamination by a Martian organism from returning human missions.
The probability that life-forms1 exist on the surface of Mars (that is, the area exposed to ultraviolet radiation and its photochemical products) is very small. However, as a previous NRC study (NRC, 1997) notes, there is a possibility that such life-forms exist there “in the occasional oasis,” most likely where liquid water is present, and, furthermore, that “uncertainties with regard to the possibility of extant Martian life can be reduced through a program of research and exploration.”
This charge to the committee results in a dilemma. How can NASA use human ingenuity and creativity on Mars to search for life when that life (if it exists) may pose a threat to astronaut health and safety (and therefore to the success of a human mission) as well as to Earth 's biosphere?
ENSURING THE SAFETY OF ASTRONAUTS
The committee believes that it is highly unlikely that infectious organisms are present on Mars. The same NRC study (NRC, 1997) that focuses on the possibility that Martian organisms could be agents of infectious disease also states on page 21 as follows: “The chances that invasive properties would have evolved in putative Martian microbes in the absence of evolutionary selection pressure for such properties is vanishingly small. Subcellular disease agents, such as viruses and prions, are biologically part of their host organisms, and an extraterrestrial source of such agents is extremely unlikely.” The uncertainty surrounding whether or not life exists on Mars may not be resolved until after astronauts arrive on the planet (NRC, 1997). 2 Of course, if life does exist on Mars, it could represent a biological hazard to astronauts.
Discrete entities that can actively utilize the energy and matter of their environment, react to a stimulus, and modify and propagate themselves.
Some argue that since no life-form, or no conclusive evidence of hazardous life, was identified in meteorites from Mars found on Earth, there is no biohazard threat from Mars. These SNC meteorites are, after all, handled without special precautions in the laboratory. The committee referenced the analysis of the SNC meteorites in Chapter 4 in the discussion of toxic metals on Mars. The use of SNC meteorite data is not appropriate to this argument regarding biohazards on Mars.
Extensive exposure to cosmic rays during a meteorite's travels in interplanetary space, during which the meteorite was exposed to sterilizing doses of radiation, would probably have destroyed any life contained in it (Clark, 2001).
Furthermore, the rocks at the Pathfinder landing site and the soils at all Mars landing sites have quite different compositions from these meteorites. Therefore, the known Martian meteorites are not necessarily representative of all materials on the surface of Mars.
Finally, it must be acknowledged that there have been biological upheavals through the course of paleontological history here on Earth, including millions of species extinctions, with few explana-tions of cause. It is impossible to determine the cause of these upheavals with absolute certainty, be they physical (meteorite impact) or biological (life introduced onto Earth). While the possibility that life was introduced on Earth from elsewhere is unlikely, the uncertainty makes it impossible to prove that meteorites from another planet have never had an effect on Earth 's biosphere (NRC, 1997). These considerations, and the consideration that a goal of future human missions will be to search for life-forms in Martian oases, if those oases exist, negate the argument that there is no biohazard threat from Mars based on the existence of sterile Martian meteorites.
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-
ing site is established as a ZMBR, the astronauts can land and freely operate within it.
The committee recognizes that the requirement to establish and operate in a ZMBR, while intrinsic to the study charter to manage risk to astronauts, may be in conflict with one of the primary goals of the exploration of Mars: to find extraterrestrial life.
As stated above, any and all indigenous materials from Mars must be considered as health hazards and ecohazards and be contained until proven otherwise. If life is discovered on Mars, the committee maintains that such life must be considered a risk to humans as well as Earth 's ecosystem until proven otherwise.
A large variety of chemical and microscopy analytical techniques could be used for detecting extant life, including measurements of biomass, use of specific molecular probes, and/or detection of growth, meta-bolic activity, or the presence of enzymes. A survey on life detection techniques was carried out during an April 2000 workshop sponsored by the NRC but not related to this study (NRC, 2002c). The committee believes that a search for organic carbon might be the quickest way to establish that the landing site is a ZMBR. While some have suggested the possibility of non-carbon-based life on Mars, such as silicon-based life, this committee agrees with assumptions made by previous NRC committees that should hazardous life exist on Mars it would be carbon-based (NRC, 2002a, 2002b).
The committee recommends that NASA conduct a precursor in situ experiment to determine if organic carbon is present at a location as close as reasonably possible to the landing sites selected for human missions to Mars. The committee acknowledges that it will be difficult to land exactly at the location of the anticipated human landing sites. It is beyond the scope of this report to suggest the parameters that would be used in determining the radius of a ZMBR. However, it is conceivable that NASA can establish criteria so that the results of testing one surface material (such as near-surface regolith) can be assumed to apply to all like materials, in like geologic settings, in nearby regions on Mars.
For instance, an in situ test might determine that the uppermost 10 centimeters of regolith are free of any indications of life at one location on a large flat plain on the surface of Mars. NASA could then reasonably conclude that the entire plain is likely to be free of life and therefore a ZMBR. If, on the other hand, the plain is scattered with sizable rocks, the entire plain would not be designated as a ZMBR until tests were conducted in both the open regolith and under representative rocks. In that case, the uppermost few centimeters of regolith and the material sheltered under a rock are like materials (i.e., both are regolith) but are not in like geologic settings, as one portion of the regolith is exposed and the other portion sheltered.
It is generally believed that any life-form retrievable from the surface of Mars, especially any life-form that would be a threat to humans or Earth's ecosystem, would probably be bacterial in nature and size, and possibly similar to bacterial spores. It is highly unlikely that viruses hazardous to terrestrial organisms will be present on Mars since viruses are highly adapted patho-gens that infect very specific host organisms and require those specific host organisms for replication and survival (NRC, 1997).
On Earth, certain bacteria form spores when stressed by depletion of critical growth factors such as water or other carbon/nitrogen (food) sources. Bacteria survive for long periods in the spore form until new growth opportunities are available. A typical bacterial spore is 1 to 2 microns in major dimension and is made up of about 1 × 10-12 grams of organic matter. The amount of organic material in ecosystems on Earth in nonliving form, such as foodstock or waste material, is many times greater than the amount of organic material contained in living organisms.
For purposes of illustration, making the generous assumption that there are at least 10 bacteria-size entities containing up to 1 × 10-12 grams of organic material each per gram of soil and that there is 10 times as much extraneous organic material, the concentration of organics in the soil for this life-detection threshold would still be only 0.1 ppb by weight. This is lower than the 1 ppb detection limit for organics possible with the gas chromatography-mass spectrometer instrument flown on the Viking mission (Biemann et al., 1977). The results of those tests were used by many scientists to infer that life could not be present. This life-detection threshold is controversial, however. This committee did not have the necessary expertise to criti-cally evaluate the lower limit of organic carbon needed for life detection. It therefore urges NASA to set an operational value for the life-detection threshold limit through a separate advisory process drawing on a broad range of relevant expertise.
Such a limit should be supported by a rationale for specifying the least amount of carbon that could signify the possible existence of an organism or organism
activity as discussed above. Certain classes of organic compounds in soil or other samples might be specified at different life-detection threshold levels. Amino acids, for example, would probably be considered more indicative of a possible life-form than highly refractory kerogen-like materials&—that is, mixtures of complex organic polymers of high molecular weight similar to petroleum. The presence of excess optically active organic molecules could also be considered indicative of a life-form.
These analyses should be made on a sample or samples from the surface and down to a depth at which astronauts may be exposed. Unless measurement techniques or capabilities are advanced significantly beyond current capabilities, a sample return will probably be required to establish that the planned landing sites are in a ZMBR.
As a further example (and for discussion purposes only), if NASA's mission scenario uses rockets for landing, the exhaust from the rockets will eject the regolith to some depth, exposing subsurface material that the astronauts will be walking on during EVAs. Also, some of the ejected materials will most certainty contact the outside of the return spacecraft, potentially contaminating the vehicle surfaces with some life-form that might exist below the Martian surface. If such a landing scenario is planned, the precursor organic carbon measurement must take place down to the same depth as that from which materials will be ejected.
It is conceivable that NASA might generate a passive soft-landing capability that disturbs the regolith very little. If that is the case, then the precursor in situ organic carbon test needs to be conducted only on the Mars surface and near surface.
NASA will need to establish procedures for astronauts to expand the ZMBR. One such procedure might involve the use of an iterative robotic testing process on the surface and subsurface. In the process of expanding the zones, NASA should take appropriate caution to ensure that astronauts will not come into per-sonal contact with surface and subsurface soil samples before the soil has been fully characterized with regard to the possible presence of Martian life-forms.
Some of the tests used during this pre-excursion process may be fairly straightforward and may only require that the robots determine if organic carbon is present. However, more complex experiments may require human interaction&—for example, robots might bring samples back to the habitat laboratory for more critical examination by humans. Astronauts might then use chemistry, microscopy, and biological challenge (interaction with animals, plants, and/or cell cultures) in the testing process to determine if a life-form exists and if that life-form is potentially hazardous to humans or to Earth's biosphere.
A life-detection experimental laboratory in the habitat will be a complex and critical component of Mars exploration, since it must in effect isolate the astronauts from all test materials, using appropriate biological containment techniques (Richmond and McKinney, 1999). The committee suggests that NASA plan for the development, deployment, and design of a facility for conducting life-detection tests when the astronauts land on Mars should the astronauts plan to explore new areas that have the potential to harbor life. Such a life-detection facility would be important since the precursor missions, including in situ and sample return missions, will not conclusively prove that life does not exist somewhere in an oasis on the Martian surface or subsurface. Alternatively, NASA could implement a set of safeguards to ensure that astronauts and return equipment are never directly exposed to new, uncharac-terized materials during the entire mission.
Recommendation: The committee recommends that NASA establish zones of minimal biologic risk (ZMBRs) with respect to the possible presence of Martian life during human missions to Mars. In order to do so, NASA should conduct a precursor in situ experiment at a location as reasonably close to the human mission landing sites as possible to determine if organic carbon is present. The measurement should be on materials from the surface and down to a depth to which astronauts may be exposed. If no organic carbon is detected at or above the life-detection threshold, the landing site may be considered a ZMBR. If no measurement technique can be used to determine if organic carbon is present above the life-detection threshold, or if organic carbon is detected above that threshold, a contained sample should be returned to Earth for characterization prior to sending humans to Mars.
If No Organic Carbon Is Detected
If a precursor in situ organic carbon experiment can determine that no organic carbon is present above the life-detection threshold, the committee recommends that NASA judge the near surface on Mars at the landing site to be a ZMBR. Once this initial minimal risk
zone is established, the human mission may land and operate on the surface in the region around the site of the precursor organic carbon test (with like materials, in like geologic settings) and return to Earth reasonably confident that there is no biological hazard that could harm human health or Earth's ecosystem. In this instance, no sample return is required prior to the first human visit, at least from the perspective of protecting astronauts or Earth's biosphere. It should be noted, however, that there are other scientific reasons for a sample return, as discussed in other NRC reports that call for a sample return (NRC, 2002a).
As stated above, there are currently no measurement techniques or capabilities available for such in situ testing. If such capabilities were to become available, one advantage is that the experiment would not be limited by the small amount of material that a Mars sample return mission would provide. What is more, with the use of rovers, an in situ experiment could be conducted over a wide range of locations.
Positive or Inconclusive Organic Carbon Measurement and Precursor Sample Return
If a precursor in situ organic carbon experiment indicates the presence of organic carbon on Mars above the life detection threshold, the committee recommends that a sample must be returned to Earth from the location and depth where the organic carbon is discovered if no suitable in situ life-form confirmation technologies are available, as is the case now. The returned sample should be considered hazardous, and NASA should follow the same quarantine procedures as outlined in previous NRC studies (NRC, 2002b). These quarantine procedures should be applied to all materials returned from Mars, both precursor samples and samples returned from human missions. However, NASA should also attempt to ensure that any possible life-form in the sample would survive the trip to Earth.
The location and depth of such a sample return would be dictated by NASA mission operations. If NASA determines that rocket exhaust on landing will uncover half a meter of Martian regolith and organic carbon is discovered to exist in material exposed at that depth, then a sample from that depth should be returned to Earth prior to human exploration of Mars.
If a precursor mission sample is returned to Earth subsequent to a positive organic carbon test result, the first priority should be to determine if the organic carbon constitutes a life-form. If the organic carbon is not a Martian life-form, then the location from which the sample was taken may be considered a ZMBR. On the other hand, if the organic carbon in a sample return does contain a life-form, there are varying levels of safety that can be established in the region from which the sample was taken, following a determination of whether such an organism is in fact a biohazard.
The findings and recommendations in this report are relevant to activities represented above the dotted line in Figure 5.1, a decision diagram offered by the committee as a preliminary guideline. If life is indeed discovered on Mars, the strategy for exploiting that discovery will certainly develop based upon the specific nature of the life identified. The open questions that remain below the dotted line can be answered by first testing the life-bearing sample for hazards to humans, such as infectivity, and then evaluating it for hazards to Earth's biosphere. The committee does believe that if a Martian life-form is proven to be harmless to humans and to Earth's biosphere, then the astronauts may operate in the presence of the life-form.
There has been some concern that if a sample return is required, the planning for the first human mission to Mars may be delayed until a sample can be obtained. The committee believes that, even should a sample be required because organic carbon has been found, a baseline plan for a mission to Mars and even hardware development may still proceed under the assumption that a sample return will not find anything significant enough with regard to Martian biology to invalidate the baseline mission plan.
Sample Return: Additional Benefits
The Viking lander experiments were criticized because life-detection tests gave presumably false positive experimental results in some cases and clearly unexpected results in other cases, apparently due to the complex chemistry of the Martian soil. A secondary, though not mandatory, benefit of a precursor sample return mission is that a returned sample would allow researchers to establish effective and reliable testing methods that would reduce the risk of false positive or false negative results. Soil chemical analyses conducted on returned samples would enable improved life-detection protocol testing. False positives or false negatives in testing for life on the surface of Mars could create serious problems for astronauts. For instance, if a test produces a false negative, that is, the test results indicate that Martian life is not present when it actually
is, NASA could improperly establish a ZMBR. In this case, the risk would indeed not be minimal, and astronauts could be exposed to hazardous Martian life-forms. If a false positive is returned, it may be believed that astronauts have been exposed to a Martian life-form when in reality no life-form is present.
RETURN VEHICLE CONTAMINATION
Great care must be exercised to ensure the containment of all material returned from Mars to Earth. All Martian material returned to Earth must be quarantined. There must be a sterile, intermediate transfer conducted in space that ensures that Earth's environment will not become exposed to any Martian material, including dust or soil deposits on the outside surface of the return vehicle. The protocols for such a sterile transfer will be complex and, if the transfer is unsuccessful, may require that the return vehicle be discarded in space and never returned to Earth. Ultimately, however, only contained materials should be transported back to Earth, unless sterilized first (NRC, 1997).
Other protocols may need to be established if, for unforeseen reasons, it is not possible to isolate the return vehicle. In that case, life-detection tests must be conducted within the near-surface Martian regolith that could adhere to the outside of the return vehicle when it lifts off from the Martian surface, unless precursor mission measurements (in situ or sample return) have shown that such material is not biologically hazardous.
Biemann, K., J. Oro, P. Toulmin, L.E. Orgel, A.O. Nier, D.M. Anderson, P.G. Simmonds, D. Flory, A. Diaz, D.R. Rushneck, J.E. Biller and A.L. LaFleur. 1977. “The Search for Organic Substances and Inorganic Volatile Compounds on the Surface of Mars.” Journal of Geophysical Research 82:4641-4658.
Clark, B.C. 2001. “Planetary Interchange of Bioactive Material: Probability Factors and Implications.” Origins of Life and Evolution of the Biosphere 31:185-197.
National Research Council (NRC). 1993. Scientific Prerequistes for the Human Exploration of Space. National Academy Press, Washington, D.C.
NRC. 1997. Mars Sample Return: Issues and Recommendations. National Academy Press, Washington, D.C.
NRC. 2002a. Assessment of Mars Science and Mission Priorities. National Academy Press, Washington, D.C.
NRC. 2002b. The Quarantine and Certification of Martian Samples. National Academy Press, Washington, D.C.
NRC. 2002c. Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques. National Academy Press, Washington, D.C., in press.
Richmond, J.Y., and R.W. McKinney, eds. 1999. Biosafety in Microbio-logical and Biomedical Laboratories. Department of Health and Human Services, Centers for Disease Control and Prevention and the National Institutes of Health, Washington, D.C.