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5
Assessment of the 1978 Report
REVIEW
The 1978 report of the then Space Science Board's Committee on
Planetary Biology and Chemical Evolution established a quarantine policy
for exploratory, one-way missions to Mars, Jupiter, Saturn, Uranus,
Neptune, and Titan planned for 1974 to 1994.1 The recommendation of the
1978 report was that precautionary measures be taken to minimize forward
contamination of these planets by terrestrial microorganisms so as not to
jeopardize future life-detection experiments.
The criteria used for planetary contamination prior to the 1978
report were those established by international agreement through the
Committee on Space Research (COSPAR). They stipulated that the
probability of contamination (Pc) should be less than 1 x 10-3 for each
planet. The Pc was estimated using a formula that also included the
probability of growth (Pg) of a terrestrial microorganism on each of the
planets. There was some difficulty in arriving at a sensible and useful Pg,
necessitating that the 1978 committee be charged with the task of
comprehensively evaluating Pg based on available knowledge of the
physical and chemical properties of the surface and atmosphere of each
planet and conditions that limit life as we know it.
Although the 1978 committee considered the P8 for all the planets
being considered for exploration through 1994, the current report is
limited to an evaluation of information and past recommendations for
Mars. The 1978 report attempted to evaluate the Pg for three separate
regions on Mars and included above- and below-surface subpolar areas
and the polar caps. Although the committee expressed a reluctance in
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recommending a particular value for Pg, they argued that while the Pg for
Mars is exceedingly low, the probability is not zero. Furthermore, the
Viking mission, although useful in arriving at a Pg for subpolar sites, did
not offer any insight on geochemical characteristics and the possibility of
liquid water at the polar caps. The committee recommended a Pg of less
than 10-10 for the subpolar regions of the planet within 6 centimeters of the
surface, less than 10-8 for subsurfaces in subpolar regions, and less than
10-7 for the polar ice caps. These ranges for Pg values reflect Viking data
for subpolar regions, including those results that indicated the presence of
strong oxidants, observed organic compounds, water, and the possibility
that liquid water could exist seasonally and diurnally at the polar caps. The
Pg values were arrived at subjectively and have become a matter for
debate.
It is clear that considerable uncertainty has been engendered by the
probabilistic approach to planetary protection. This concern has been
restated over the years by virtually every group that has analyzed the
problem, and indeed by NASA. Many unknowns must be factored into
such elements as the probability of growth of a terrestrial organism on the
martian surface, for example, so that estimating the potential for biological
contamination of Mars is difficult if not impossible. However, the trend is
clear: as we have learned more about Mars, our expectations regarding the
likelihood of terrestrial microbial contamination have been reduced, and
estimates of the probability of growth have been steadily lowered as a
result.
Following the 1978 report, whose recommendations were
generally accepted, NASA began to look for ways to simplify planetary
protection procedures as they applied to particular upcoming planetary
missions, and also to minimize the use of mathematical models and
quantitative analyses. These studies culminated in a report to COSPAR in
1984 that greatly deemphasized the probabilistic approach and introduced
the concept of categories based on target planet and mission type.2 This
approach directly reflects the degree of concern for a given planet in the
context of a particular type of mission.
Five categories of target planet and mission-type combinations and
their particular suggested ranges of requirements were proposed in the
1984 report, and these were accepted by COSPAR. The five categories are
summarized below; details are contained in the 1984 report (see also Table
E.1, Appendix E).
Category I missions include any mission to a target planet that is
•
not of direct interest for understanding the process of chemical evolution.
In effect, no protection of such planets (e.g., Mercury, Pluto) is warranted,
and no planetary protection requirements are imposed.
Category II missions are all types of missions to those target
•
planets that are of significant interest for understanding the process of
chemical evolution, but for which there is only a remote chance that
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contamination carried by a spacecraft could jeopardize future exploration.
The concern is primarily over unintentional impact, since these missions
are not designed to land.
Category III missions are certain types of missions (flyby and
•
orbiter) to a target planet of interest for understanding the chemical
evolution and/or the origins of life, or for which scientific opinion
suggests a significant chance of contamination that could jeopardize a
future biological experiment.
Category IV missions are certain types of missions (mostly probe
•
and lander) to a target planet of interest for understanding chemical
evolution and/or the origins of life, or for which scientific opinion
suggests a significant chance of contamination that could jeopardize future
biological experiments.
Category V missions include all Earth-return missions. The
•
concern is for the protection of the terrestrial system as well as the
scientific integrity of the returned sample.
These recommendations, made by NASA, were approved by
Subcommission F (life sciences) and subsequently by the executive
committee of COSPAR, and they have been implemented by NASA. The
task group believes that approval and implementation of these
recommended categories constitute a significant step forward in the
process of simplifying and implementing planetary protection procedures.
A goal in this report is to reassess current planetary protection
guidelines in light of new knowledge and new technology. The task group
was asked to comment only on Mars lander missions that do not involve in
situ extant life-detection experiments and has tried to do so, although it
was admittedly difficult for task group members to exclude life-detection
and sample return missions from their thinking. This group's approach,
which is somewhat different from that taken in earlier studies, is intended
to contribute to planetary studies as they relate to questions about the
origins of life, while keeping secure our profoundly important scientific
objectives.
RECOMMENDATIONS OF THE TASK GROUP
Forward Contamination
The task group views the problem of forward contamination as
separable into two principal issues. The first centers on the potential for
growth, in the martian environment, of whatever fractions of spacecraft
populations of microorganisms are able to survive transit from Earth to the
surface of Mars. The second involves importation of terrestrial organic
contaminants, living or dead, in amounts sufficient to compromise the
search for evidence of past or present life on Mars itself.
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The guidelines on probability of growth (P8) issued by the Space
Science Board in 1978 were recently reassessed in a 1991 NASA report,
Planetary Protection Issues for the MESUR Mission: Probability of
Growth (Pg).3 Comments and estimates made by the contributors point to
Pg values for terrestrial organisms on Mars that are probably lower than
the 1978 estimates. Their consensus was that an exceedingly small Pg was
necessitated by the low probability of liquid water existing on Mars and
the low probability of an appropriate terrestrial organism occupying a
particular martian environment and growing there. However, Pg was not
judged to be zero because of the possibility that suitable martian
microhabitats could conceivably exist.
Based on the findings of the MESUR mission workshop on the
probability of growth as well as on the arguments presented below, the
task group agreed that the Pg value for terrestrial organisms on Mars is so
small as to be of no consequence. Therefore, the need for severe reduction
of spacecraft bioload solely to prevent the spread of replicating terrestrial
organisms on Mars is no longer paramount. However, this is clearly not
the case as far as contamination of a possible past or extant martian
biosphere is concerned. The reduction of bioload on all lander missions to
Mars must continue to be seriously addressed. The sophistication of
current molecular analytical techniques is such that single cells are
detectable, and so the issue of spacecraft cleanliness is particularly crucial
when life-detection experiments are included in the scientific payload.
Aside from considerations related to life-detection experiments, spacecraft
cleanliness (particularly the biological-organic burden) is extremely
important (1) in order to greatly minimize the introduction of foreign
material into any site likely to be of biological interest in subsequent
missions, and (2) to minimize contamination of experimental devices that
are particularly sensitive to biological and chemical contamination (i.e.,
optic and spectrophotometric devices).
The deliberations of the task group on the issue of forward
contamination hazards posed by the planned set of U.S. and Soviet lander
missions summarized in Chapter 2 were greatly aided by NASA's 1991
report on the MESUR mission and by comprehensive briefings given by
experts on matters relevant to this issue (see workshop presentations listed
in Appendix C). These deliberations led the task group to unanimous
concurrence with the following conclusion:
Forward contamination, solely defined as contamination of the martian
environment by growth of terrestrial organisms that have potential for growth on Mars, is
not a significant hazard. However, forward contamination more broadly defined to
include contamination by terrestrial organic matter associated with intact cells or cell
components is a significant threat to interpretation of results of in situ experiments
specifically designed to search for evidence of extant or fossil martian microorganisms.4
Based on this consensus, the task group makes the following
recommendations for control of forward contamination, each tied to
specific mission objectives:
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1. Landers carrying instrumentation for in situ investigation of
extant martian life should be subject to at least Viking-level
sterilization procedures. Specific methods for sterilization are to be
determined; Viking technology may be adequate, but requirements will
undoubtedly be driven by the nature and sensitivity of the particular
experiments. The rationale for this requirement is the reduction, to the
greatest feasible extent, of contamination by terrestrial organic matter that
is deposited at the site by microorganisms or organic residues carried on
the spacecraft. This approach, when coupled with molecular analytical
methods for assessment of bioload, should allow both elimination of the
most troublesome contaminants and an inventory of those few that remain.
2. Spacecraft (including orbiters) without biological
experiments should be subject to at least Viking-level presterilization
procedures—such as clean-room assembly and cleaning of all
components—for reduction of bioload, but such spacecraft need not
be sterilized. This recommendation has important implications for the
planetary protection program in general, in that it implies that there need
be no requirement with regard to orbiter lifetimes if the orbiter is subject
to a Viking-level reduction of bioload by clean-room assembly and
cleaning.
As discussed above, the task group concurs with the conclusion,
expressed in NASA's 1991 report,5 that the probability of growth of a
terrestrial organism on present-day Mars is essentially zero. However, the
task group recommends bioload reduction for anything sent to the martian
surface. Major advances in our ability to detect cellular material have
occurred over the last decade, and future advances will undoubtedly
follow. Reducing contamination of the planet by reducing the bioload on
landed vehicles will minimize the chances of jeopardizing future
experiments designed to detect material of possible biological origin.
These conclusions and recommendations on the issue of forward
contamination are based on several considerations discussed earlier in this
report. The task group concurs with the MESUR workshop panelists'
consensus that Pg is extremely low, and probably significantly below the
upper limits estimated by the 1978 committee. Given the likelihood that Pg
is extremely low, the task group sees no utility in further attempts to
estimate its probable value in various martian environmental regimes. In
the absence of crucial data relating to the potential of terrestrial organisms
to survive and grow on Mars, such exercises are purely subjective.
Although some progress toward quantification of Pg could perhaps be
realized in welldesigned laboratory simulation experiments, the task group
is not optimistic that the central question of the presence and duration of a
liquid water phase in the near-surface martian regolith environment can be
unambiguously addressed without more information obtainable possibly
only from in situ measurements on Mars itself, or from returned
samples—or conceivably from neither.
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The task group believes that the recommendations set out above
strike an appropriate balance between the obligation for conservatism on
the issue of forward contamination insofar as Pg is concerned, and the
need to gather the data that will eventually allow that issue to be settled
definitively. It is implicit in these recommendations that the approach used
in previous attempts to calculate the probability of contamination (Pc) be
abandoned. In support of abandoning the method, the task group worked
through some sample calculations of Pc to demonstrate the nonutility of
the probabilistic approach. Pc is correctly expressed, per unit of microbial
burden, as the product of Pg and Pt, where Pt is the probability of an
organism's survival during transit from Earth surface to Mars surface. Pt is
usually expressed as Pt = P(VT) x P(UV) x P(R) x P(A) x P(SA), with
P(VT) and P(UV) representing the probabilities of an organism's surviving
exposure to space vacuum and temperature and to ultraviolet radiation,
respectively; P(R) the probability of an organism's release from a lander to
the martian surface; and P(A) and P(SA) the probabilities of an organism's
arriving at the planet and surviving atmospheric entry. Presumption of a
successful mission sets P(A) equal to 1 and P(SA) equal to near 1. Data on
P(VT) and P(UV) are lacking for most of the recently discovered highly
specialized organisms described above, but it is still possible to
conservatively estimate their product as 10-1 to 10-2 or less. (The task
group notes that appropriate laboratory simulation experiments to evaluate
these probabilities for candidate microorganisms are entirely feasible,
since both the spacecraft geometry and the characteristics of its space
environment can be well determined.) P(R) is interpreted as the
probability of release of that fraction of the total bioburden located on
surfaces in direct contact with the martian regolith. With special attention
to cleaning such surfaces, perhaps combined with prelaunch UV
irradiation, it seems feasible to reduce P(R) to 10-2 to 10-3 without total
spacecraft sterilization. Then, even with the 1978 SSB value for Pg of less
than 10-10, the product of Pg x Pt seems unlikely to exceed about 10-14 per
unit of microbial burden. This nominally allows a large bioload
approaching 1011 (say, 105 organisms per square centimeter on a
spacecraft surface area of 100 square meters) while still retaining the
COSPAR value for Pc of 10-3. The task group also notes that this bioload
is the total microbial burden. Consideration of only those species with
capabilities for surviving in the most extreme environments would reduce
Pc for them, probably by a relatively large factor. Another factor to
consider is the possibility of such extreme environments existing on Mars,
some of which may be hospitable to certain organisms. Clearly, if such
niches exist, the Pg may be greater for a population of contaminating
organisms if they are widely dispersed, thus raising the probability of their
encountering a less hostile environment.
It was the intent of the task group to illustrate the uncertainties
involved in the probabilistic approach by performing the above
calculations. With so many uncertain probabilities multiplied by each
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other, the likelihood of achieving a meaningful Pc is very low indeed.
When these problems are combined with the fact that the range of
environments on Mars is not yet known, the futility of assigning a
meaningful Pc is further exemplified.
The task group emphasizes that the philosophical intent of the
1978 committee to protect Mars from terrestrial contamination so as not to
jeopardize future life-detection experiments on Mars is still profoundly
important. Recommendation 1 above deals with the issue of contamination
by nonviable but intact cells and biochemical components from terrestrial
organisms, independent of whatever low Pg value they may have.
Back Contamination
A detailed assessment of the complex issue of sample return does
not lie within the present charge of this task group. Chapter 6 discusses
some of the martian environmental unknowns, and the data required to
address them, that will be central to evaluation of possible hazards posed
by back contamination.
SCIENTIFIC ISSUES—SUMMARY STATEMENT
As previously stated, it is the unanimous opinion of the task group
that terrestrial organisms have almost no chance of multiplying on the
surface of Mars and in fact have little chance of surviving for long periods
of time, especially if they are exposed to wind and to UV radiation.
However, current techniques to detect life, such as those that use specific
biomarkers, are much more sensitive than techniques used at the time of
the Viking mission, making contamination a serious threat to experiments
designed to look for life on Mars. With regard to this latter point, the
recommendation that landers be sterilized if they carry life-detection
experiments, but only have reduced bioloads in other instances, has long-
range strategic implications. Even if there is no organismal growth, local
contamination is to be expected around a nonsterilized spacecraft. Clearly
a lander should not return to do life-detection experiments at a site where
unsterilized spacecraft have landed previously. For these reasons, the task
group believes that it is better to err on the side of caution. Thus the task
group recommends that spacecraft be cleaned rigorously to levels that
are at least equal if not superior to Viking levels. It does not believe
that such constraints are unduly restrictive to subsequent Mars exploration.
The task group also recommends that modern methods of
bioburden assessment and tabulation be developed for spacecraft
destined for Mars missions.
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REFERENCES
1. Space Science Board, National Research Council. 1978.
Recommendations on Quarantine Policy for Mars, Jupiter, Saturn,
Uranus, Neptune, and Titan. Committee on Planetary Biology and
Chemical Evolution. National Academy of Sciences, Washington,
D.C.
2. DeVincenzi, D.L., and P.D. Stabekis. 1984. "Revised Planetary
Protection Policy for Solar System Exploration." Adv. Space Res.
4:291-295.
3. Klein, H.P. 1991. Planetary Protection Issues for the MESUR Mission:
Probability of Growth (Pg). NASA conference publication. NASA
Ames Research Center, Moffett Field, Calif.
4. See Klein, H.P., 1991.
5. See Klein, H.P., 1991.
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