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Summary and Recommendations
INTRODUCTION
Task, Approach, and Scope of Report
Whenever Earth-originating spacecraft intrude on the atmosphere
or surface of other solar system bodies or return to Earth from one of these
bodies, there is a risk of contamination by foreign substances or
organisms. In the case of in situ exploration of other bodies, a major
concern is disruption of scientific findings by imported material. In the
case of back contamination (return to Earth of extraterrestrial material),
there is concern over the possible release into the biosphere of potentially
harmful organisms or substances.
Since 1967, a policy of planetary protection has been in place in
order to control contamination of planets by terrestrial microorganisms
and organic constituents during planetary missions. In the United States,
the policy is implemented by the National Aeronautics and Space
Administration (NASA). It is accepted as official policy by the Committee
on Space Research (COSPAR) of the International Council of Scientific
Unions. The policy lays out a framework of specific planetary protection
guidelines for implementing procedures for future missions. Through
COSPAR, review and analysis of the policy have been ongoing and have
resulted in periodic revisions in light of new information obtained from
planetary exploration.1,2
In addition, the United States is a signatory to an international
treaty that declares in part that "States Parties to the treaty shall pursue
studies of outer space . . . so as to avoid their harmful contamination and
also adverse changes in the environment of the Earth. . . ."3
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The Space Studies Board (SSB) of the National Research Council
has served as NASA's primary advisor concerning planetary protection (or
quarantine) for many years. The board, through its Committee on
Planetary Biology and Chemical Evolution, has published a number of
reports and letters concerning planetary protection (or quarantine) in
response to NASA requests.4-12 Most recently, NASA's planetary
protection officer requested that, prior to the 1992 COSPAR meeting, the
board make recommendations regarding planetary protection policy for
upcoming Mars missions (Appendix A). In response to this request, the
board formed the ad hoc Task Group on Planetary Protection, made up of
planetary scientists, biochemists, ecologists, and microbiologists who
specialize in studying life in extreme environments such as the polar
regions and deep oceans and lakes (Appendix B). The task group hosted a
workshop in September 1991 at which extensive briefings on planned and
contemplated Mars missions and the many aspects of Mars science and
survival of Earth organisms were reviewed and discussed in detail
(Appendix C). Scientists from Europe and the former USSR made
presentations concerning their current views and approaches to planetary
protection. These presentations and discussions, along with a reassessment
of the SSB's 1978 report, Recommendations on Quarantine Policy for
Mars, Jupiter, Saturn, Uranus, Neptune, and Titan13 (excerpted in
Appendix D), form the basis for this report. Additional information
considered by the task group is given in Appendix E.
In keeping with NASA's request, the task group focused on making
recommendations concerning the protection of Mars from forward
contamination (i.e., contamination of the martian environment by
terrestrial organisms) during upcoming missions by both the United States
and the former Soviet Union. In so doing, it distinguished between
missions whose goals include reconnaissance and measurement and those
that specifically include experiments to detect life. The task group also
discussed what additional knowledge will be needed in order to assure that
future recommendations regarding contamination of Earth from Mars
(back contamination) might be made with a higher degree of certainty than
is now possible.
Following a short introduction to the rationale underlying planetary
exploration (Chapter 1) is a brief summary of approved and contemplated
missions to Mars (Chapter 2). Chapter 3 briefly reviews the state of
knowledge in several areas pertinent to the problem of planetary
protection, including chemical and physical properties of Mars, and
Chapter 4 discusses the limits of life on Earth and the abilities of known
terrestrial organisms to withstand extreme environmental conditions, as
well as new approaches to detecting life forms. Chapter 5 includes a
review and comments—made in light of current knowledge—on the
recommendations made in Recommendations on Quarantine Policy for
Mars, Jupiter, Saturn, Uranus, Neptune, and Titan. Updates to the
recommendations made in 1978 are also given in Chapter 5. Chapter 6
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gives additional recommendations concerning collection of essential data,
spacecraft sterilization and bioburden assessment, and future research, as
well as legal and societal issues and NASA's overall planetary protection
program.
Background
Understanding the origin and evolution of life has been an
important goal of NASA; studies in this area generate some of the more
interesting scientific questions for all mankind. One promising approach to
understanding life's origins is that of searching for life elsewhere,
primarily on other planets, where physical, hydrological, and geochemical
properties might favor (or might have favored in the past) the existence of
replicating biotic systems like those found on Earth. Historically, Mars has
been the planet of choice for understanding life's origins.
With the technological advances that accompanied the advent of
spacecraft exploration, our ability to conduct detailed studies of planets in
the solar system improved dramatically. As our knowledge of present
conditions on the surface of Mars has increased, there has been a
concomitant decrease in any expectation that life as we know it could exist
on the surface of the planet. At the same time, it is important to remember
that (1) Viking lander sites have not been representative of the entire
planet and (2) the early state of Mars seems to have differed quite
markedly from its present state and may have been characterized by the
presence of abundant liquid water and a more substantial atmosphere.
Future life-detection missions to Mars must include investigation of other
more biologically relevant, desirable sites where evidence of the survival
of either molecular or morphologically preserved cells or cell components
may exist.
As in the past, it is necessary to continue to take precautions to
ensure planetary protection, both from forward and back contamination.
With respect to forward contamination, NASA's historic concern has been
to preserve pristine conditions on the planets for future experiments with
biological and organic constituents that might lead to insights concerning
the origin and evolution of life in the cosmos. Knowledge has increased
substantially since the Viking mission. Recommendations for planetary
protection that guided the Viking mission may not be relevant to missions
being flown today or to those planned for the future. As more information
is acquired about a given extraterrestrial body, assessment of the amount
of planetary protection needed to protect that body from contamination
should change accordingly. The process must be iterative and must allow
for altering the techniques used to ensure protection as we learn more
about planetary conditions and the probability of contamination.
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FUTURE MISSIONS
At this time, there are two approved missions to Mars: the U.S.
Mars Observer mission to be launched in October 1992 and the Soviet
Mars 94/96 mission. Both NASA and the European Space Agency (ESA)
are studying a network mission that involves placing numerous small
stations on the surface of the planet. In addition, both the United States
and the former Soviet Union have been studying various rover and sample
return missions for some time. These missions, which will gradually
improve our knowledge of the environmental parameters of Mars and
enhance our ability to select and protect appropriate landing sites, are
discussed in detail in Chapter 2.
SURFACE ENVIRONMENT OF MARS
Despite an incomplete understanding of the surface environment of
Mars, it is generally agreed that conditions are extremely inhospitable to
terrestrial life. Various aspects of the surface environment have relevance
to the issue of forward contamination, including both growth on Mars of
organisms from Earth and the lifetime of bioorganic matter deposited on
the martian surface. Chapter 3 of this report reviews the state of
knowledge regarding the martian surface, including its chemistry, solar
radiation flux, temperature, water, volcanism, and past climate conditions.
LIMITS OF LIFE ON EARTH: EXPANSION OF
THE MICROBIAL WORLD AND DETECTION OF LIFE
Life in Extreme Environments
The Task Group on Planetary Protection assessed past reports and
current views on the range of environmental conditions believed to exist
on Mars and unanimously agreed that it is extremely unlikely that a
terrestrial organism could grow on the surface of Mars. It is clear that the
most extreme environments on Earth where organisms can replicate are
considerably less extreme than the environments that are known to occur
over most of the martian surface. Particularly important in this regard are
the high levels of ultraviolet radiation, the thin atmosphere, the extremely
low maximum temperatures, and the absence of liquid water on the
surface.
Based on current knowledge of conditions on Earth that limit cell
growth and on the best estimates of surface conditions on Mars, the task
group concluded that no known terrestrial organisms could grow on the
martian surface. However, this does not imply that life does not exist
anywhere on the planet. There is far too little information to assess the
possibility that life may exist in subsurface environments associated with
hydrothermal activity or in selected microenvironments free from the
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harsh conditions previously mentioned, or to conclude that organisms
resembling terrestrial life forms did not evolve on Mars.
The task group concentrated on the problem of forward
contamination by intact cells or components of cells that could be detected
by sophisticated molecular methods in future expeditions designed to look
for evidence of extant or past life on Mars. Planning for present and future
missions to Mars must include awareness of new results obtained from
studies of extreme environments as well as the inevitable extension of the
limits of environments where growth and survival can take place.
Advances in understanding the microbiology of extreme environments
have been accompanied by advances in the development of new methods
and considerably more accurate and sensitive instruments for detecting the
presence of life and life-related molecules and for identifying their
evolutionary relatedness.
Nevertheless, it is not a straightforward matter to define the ranges
of physical and chemical conditions on Earth in which organisms can
grow, replicate, or survive for extended periods. During the 13 years since
the SSB's last report on planetary protection, Recommendations on
Quarantine Policy for Mars, Jupiter, Saturn, Uranus, Neptune, and Titan,
bacteria have been detected or isolated from many of Earth's hostile
environments—the dry, extremely cold subsurfaces and interiors of rocks
in the dry valleys of the Antarctic, hot environments associated with
submarine and terrestrial volcanoes and geothermal systems, and deep
subsurface sediments and aquifers. Chapter 4 includes a review of these
organisms.
Life Detection and Bioburden Determination
for Planetary Protection
Techniques for assessing the existence of microorganisms have
advanced dramatically since pre-Viking days. These advances will have a
strong impact both on bioburden assessment procedures and on future life-
detection experiments. New methods have been developed with
increasingly greater sensitivity and specificity. The task group strongly
recommends that efforts be made to explore current analytical
methods for use in bioburden assessment and inventory procedures
before spacecraft assembly and launch.
In addition to epifluorescent microscopic techniques for directly
counting viable cells, many other new methods have been developed, such
as the polymerase chain reaction, allowing greatly increased sensitivity of
detection by enzymatically amplifying specific biomarkers of even a
single cell to detectable levels. The appeal of these techniques is their
extreme sensitivity. In many cases, single cells can be detected and
identified with confidence.
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ASSESSMENT OF THE 1978 REPORT
Review
Recommendations on Quarantine Policy for Mars, Jupiter, Saturn,
Uranus, Neptune, and Titan, the 1978 report by 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. The
task group's assessment of this report is limited to an evaluation of
information and past recommendations concerning Mars. After the 1978
report was issued, NASA began to look for ways to simplify planetary
protection procedures as they applied to particular upcoming planetary
missions, and to minimize the use of mathematical models.
Prior to the 1978 report, the criteria used for determining
categories of planetary contamination were those established by
international agreement through COSPAR. They stipulated that the
probability of contamination (Pc) should be less than 1 x 10-3 for each
planet. Considerable uncertainty was engendered by this probabilistic
approach to planetary protection. Concern related to this point has been
expressed over the years by virtually every group that has analyzed the
problem, and indeed by NASA. Although the probability of depositing a
microbe or some organic material indicative of life is very high (microbes
and organic contaminants have almost certainly been deposited by past
missions), our expectations regarding the likelihood of permanent
contamination as a result of microbial growth (expressed as the probability
of growth, Pg) have been steadily reduced as we have learned more about
Mars.
The NASA studies that followed the 1978 report culminated in a
1984 report to COSPAR that greatly deemphasized the probabilistic
approach and introduced the concept of target planet and mission-type
categories.14 This approach, which is reviewed in Chapter 5, directly
reflects the degree of concern for a given planet, in the context of a
particular type of mission.
Recommendations of the Task Group
The task group views the problem of forward contamination as
separable into two principal issues: (1) the potential for growth of
terrestrial organisms on Mars and (2) the 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.
The guidelines concerning probabilities of growth (Pg) issued by
the Space Science Board in its 1978 report were recently reassessed in a
1991 NASA report.15 Comments and estimates made by the participants
illustrate a consensus that the Pg values for terrestrial organisms on Mars
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are probably lower than the 1978 estimates. However, this observation
does not alter the case as far as contamination of a possible past or extant
martian biosphere is concerned. Prudence dictates that bioload reduction
on all lander missions to Mars must continue to be seriously addressed.
The issue of spacecraft cleanliness is particularly crucial when life-
detection experiments are included in the scientific payload.
The deliberations of the task group were greatly aided by the
MESUR mission workshop that resulted in the above-mentioned 1991
report. That report, together with the comprehensive briefings given by
experts on relevant matters, led the task group to concur unanimously with
the following conclusion from the MESUR workshop:
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.
Based on the MESUR group's consensus and the task group's
agreement with it, the task group makes the following recommendations
for control of forward contamination, each tied to specific mission
objectives.
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 objective of this requirement is the reduction, to the
greatest feasible extent, of contamination by terrestrial organic matter
and/or microorganisms deposited at the landing site.
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 bioload reduction, but such spacecraft need not be
sterilized. Table 1.1 in Chapter 1 summarizes Viking-level procedures,
and Appendix E includes a detailed description of the procedures.
The task group sees little utility in further attempts to estimate
actual probability-of-contamination values in various martian
environmental regimes. In the absence of crucial data relating to the
survivability and growth potential of terrestrial organisms on Mars, such
exercises are purely subjective. The task group emphasizes that the
philosophical intent underlying the 1978 report—to protect Mars from
terrestrial contamination so as not to jeopardize future experiments aimed
at detecting martian life—is still profoundly important.
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ADDITIONAL RECOMMENDATIONS
Recommendations for Research
The task group strongly recommends that a sequence of
unpiloted missions to Mars be undertaken well in advance of a piloted
mission. Any future changes in recommendations to ensure planetary
protection, especially for piloted or sample return missions, will depend on
the acquisition of new data. With regard to these missions, the task
group recommends that a broad spectrum of martian sites be
examined, with emphasis on measurements that provide data most
likely to contribute to models that provide for a better understanding
of the probability of life on Mars and where best to go to find it.
Until such data are available, it will be impossible to make
informed decisions concerning landings for in-depth biological study.
Such data will also greatly affect the ability to make future decisions
concerning the rigor required for spacecraft cleanliness and possible
sterilization.
Location of martian lander sites should take into account our
rudimentary but growing understanding of Mars and our extensive
knowledge of the basic requirements of life. It is also important to
consider the subsurface of Mars. Within a site, it may prove important to
plan for data collection that probes below the readily accessible surface, in
order to obtain information on subsurface environments.
Microenvironments—whether on the surface or in isolated vents, cracks,
or layers of the subsurface—may exist now or may once have existed at
some time in the past. Properly designed experiments may be able to
address the issue of spatial and (perhaps) temporal heterogeneity and its
possible relationship to our ability to evaluate the biotic and abiotic status
of a given site.
Collection of appropriate data should allow the scientific
community to amend planetary protection policy recommendations for
back contamination, perhaps resulting in recommendations similar to the
alterations in procedures for assessing forward contamination suggested
by this task group. The determination of current or inferred past
geophysical conditions on Mars may help identify locations where life-
detection missions should be sent.
Recommendations Regarding Assessment of Spacecraft Bioload
The task group's recommendation to reduce bioload on all
spacecraft and to sterilize those spacecraft used in life-detection missions
assumes the use of Viking procedures. However, the task group
recommends that the Viking protocols for assessment of spacecraft
bioloads be upgraded to include state-of-the-art methods for the
determination of bioload. It is critical that methods for assessing bioload
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be compatible with methods used to detect life, with methods for both
assessment and detection reflecting the same limits and sensitivity. Data
on bioloads of Viking components and spacecraft are not relevant to
current life-detection procedures. Modern methods of bioburden
assessment should be developed for and applied to spacecraft destined for
future Mars missions, especially those carrying in situ extant life-detection
experiments. Although immediate use of these techniques is not a feasible
goal, the development of the methodology in anticipation of future life-
detection missions is absolutely essential.
Recommendations Concerning Other Issues
Piloted Versus Unpiloted Missions
Missions carrying humans to Mars will contaminate the planet. It
is therefore critical that every attempt be made to obtain evidence of past
and/or present life on Mars well before these missions occur. The issues of
forward and back contamination have societal, legal, and international
implications. These implications are serious, and they deserve discussion
and attention.
Societal Issues
A substantial number of active national and international
organizations are on the alert for environmental abuse. There is every
reason to take seriously the concern (already expressed in some cases)
about contamination of Mars and almost certainly about the issue of back
contamination of Earth by martian samples. Although public concern over
such issues is often sincere and productive, it at times becomes distorted
and exaggerated in the media, leading to public misunderstanding and
opposition. The task group recommends that NASA inform the public
about current planetary protection plans and provide continuing
updates concerning Mars exploration and sample return.
Legal Issues
There are also legal issues that must be addressed, involving
international restrictions as well as federal, state, and local statutes that
may come into play. There are currently no binding international
agreements concerning forward or back contamination. The task group
recommends as essential that efforts be made (1) to assess the legal
limits (and implied liabilities) in existing legislation that relates to
martian exploration and (2) to pursue the establishment of
international standards that will safeguard the scientific integrity of
research on Mars. Furthermore, the task group recommends that
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NASA make a strong effort to obtain international agreement for a
planetary protection policy.
NASA Planetary Protection Program
Although a planetary protection officer currently exists at NASA,
there is no budgeted program (as there was during the Viking Program) to
implement needed planetary protection research, a public education
program, examination of legal and international issues, and the like. The
task group recommends that NASA redefine the responsibilities and
authority of its planetary protection officer and provide sufficient
resources to carry out the recommendations made in this report.
SUMMARY OF RECOMMENDATIONS
All of the recommendations put forward by the task group in this
report are summarized below. Each is discussed further in the full report in
the chapter(s) indicated.
1. Efforts should be made to adopt current molecular analytical
methods for use in bioburden assessment and inventory procedures for
spacecraft assembly and launch for future missions, and also to develop
new methods for the same purposes (Chapters 4 and 5).
2. 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 (Chapter 5).
3. 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 bioload
reduction, but such spacecraft need not be sterilized (Chapter 5).
4. A sequence of unpiloted missions to Mars should be undertaken
well in advance of a piloted mission (Chapter 6).
5. A broad spectrum of martian sites should be examined with
emphasis on measurements that provide data most likely to contribute to a
better understanding of the probability of life on Mars and where best to
go to be able to detect it (Chapter 6).
6. The Viking protocols for assessment of spacecraft bioloads
should be upgraded to include state-of-the-art methods for the
determination of bioload (Chapter 6).
7. NASA should inform the public about current planetary
protection plans and provide continuing updates concerning Mars
exploration and sample return (Chapter 6).
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8. It is essential to assess the legal limits (and implied liabilities) in
existing legislation that relates to martian exploration and to pursue the
establishment of international standards that will safeguard the scientific
integrity of research on Mars (Chapter 6).
9. NASA should make a strong effort to obtain international
agreement for a planetary protection policy (Chapter 6).
10. NASA should redefine the responsibilities and authority of its
planetary protection officer and provide sufficient resources to carry out
the above recommendations (Chapter 6).
REFERENCES
1. DeVincenzi, D.L., and P.D. Stabekis. 1984. "Revised Planetary
Protection Policy for Solar System Exploration." Adv. Space Res.
4:291-295
2. DeVincenzi, D.L. 1990. "Planetary Protection Issues and the Future
Exploration of Mars." Adv. Space Res. December preprint.
3. United Nations. 1967. Treaty on Principles Governing the Activities of
States in the Exploration and Use of Outer Space, Including the
Moon and Other Celestial Bodies. U.N. Document No. 6347,
January.
4. Space Science Board, 1967, "Study on the Biological Quarantine of
Venus," report of an ad hoc panel, January 9, National Academy of
Sciences, Washington, D.C.
5. Space Science Board, 1970, "Review of Sterilization Parameter
Probability of Growth (Pg)," report of an ad hoc review group, July
16-17, National Academy of Sciences, Washington, D.C.
6. Space Science Board, 1976, "On Contamination of the Outer Planets by
Earth Organisms," report of the Ad Hoc Committee on Biological
Contamination of Outer Planets and Satellites, Panel on
Exobiology, March 20, National Academy of Sciences,
Washington, D.C.
7. Space Science Board, 1976, "Recommendations on Quarantine Policy
for Uranus, neptune, and Titan," report of the Panel on
Exobiology, May 24, National Academy of Sciences, Washington,
D.C.
8. Space Science Board, National Research Council, 1977, Post-Viking
Biological Investigations of Mars, Committee on Planetary
Biology and Chemical Evolution, National Academy of Sciences,
Washington, D.C.
9. 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.
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10. Letter report from SSB Chairman Thomas Donahue to NASA Office
of Space Science and Applications Associate Administrator Burton
I. Edelson regarding planetary protection policy, November 22,
1985 (unpublished).
11. Letter report from Committee on Planetary Biology and Chemical
Evolution to Arnauld E. Nicogossian, director, Life Sciences
Division, NASA, regarding the planetary protection categorization
of the Comet Rendezvous-Asteroid Flyby mission, May 16, 1986
(unpublished).
12. Letter report from Committee on Planetary Biology and Chemical
Evolution to John D. Rummel, chief, Planetary Quarantine
Program, Office of Space Science and Applications, NASA,
regarding a formal recommendation on planetary protection
categorization of the Comet Rendezvous-Asteroid Flyby mission
and the Titan-Cassini mission, July 6, 1988 (unpublished).
13. See Space Science Board, National Research Council, 1978.
14. See DeVincenzi and Stabekis, 1984.
15. 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.
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