Methodology is proposed to assess the probability that the planet Mars will be contaminated by terrestrial microorganisms aboard a spacecraft. The present NASA methods are extended to permit utilization of detailed information on microbial characteristics, the lethality of release and transport mechanisms, and of other information about the Martian environment. Different types of microbial release are distinguished, and for each release mechanism a probability of growth is computed. Using this new methodology, an assessment was carried out for the 1975 Viking landings on Mars. The resulting probability of contamination for each Viking lander is 6 x 10 to the -6 power, and is amenable to revision as additional information becomes available

Judd, B. R.

North, D. W.

Pezier, J. P.

English

NASA Technical Reports Server

COSPAR IDENTIFICATION V.4.3
i5 6 To be presented at theS .XVII COSPAR Meeting, W.G.5D
> June 21, 197 4
Sao Paulo, Brazil
Methodology for Assessing the
Probability of Contaminating Mars
D. Warner North, Bruce R Judd, and Jacques P. Pezier
•* .Stanford Research Institute
" Menlo Park, California 94025
SNew methodology is proposed to assess the probability that the planet
Mrs will be contaminated by terrestrial microorganisms aboard a space-
a craft. Present NASA methods are based on the Sagan-Coleman formula,
which states that the probability of contamination is the product of
Sthe expected microbial release and a probability of growth. The proposed
0o H P new methodology extends the Sagan-Coleman approach to permit utilization
of detailed information on microbial characteristics, the lethality of
U - release and transport mechanisms, and of other information about the
S Martian environment. Three different types of microbial reease arerl I H O
distinguished, and for each release mechanism a probability of growth
w e is computed.
Using this new methodology an assessment has been carried out for the
1975 Viking landings on Mars. The resulting probability of contamination0
0-6;
for each Viking lander, 6 x 106, is based on expert judgment and is
VV amenable to.revision as additional information becomes available.U H cn
https://ntrs.nasa.gov/search.jsp?R=19750005702 2020-03-23T00:21:07+00:00Z
1. Introduction
The biological contamination of Mars is a complex issue involving
a great variety of scientific, engineering, and policy considerations.
In many areas the information available is limited. Nonetheless,
NASA is committed to a planning process derived from the 1966 COSPAR
resolution that is based on assessment of the probability of planetary
contamination. The task facing the authors in a recent project for
NASA was to criticize the existing assessment methodology, develop
appropriate new methodology, and apply the methodology to the 1975
Viking lander. While the application deserves further review from the
scientific community, the results indicate a probability of contamination
well below the mission constraint of 10-4 imposed by NASA. We believe
that the new methodology represents a significant advance over present
NASA practice in assessing the probability of planetary contamination.
2. Review of Sagan-Coleman Approach
The probabilistic model of planetary contamination advanced by
Sagan and Coleman [2] was the stimulus and theoretical foundation for
the 1966 COSPAR resolution. Their procedure used the approximation
P(C) - E(N) P(G),
where
A detailed discussion of this research is available in the project final
report [1] .
2
C - the event that Mars will be contaminated by terrestrial
organisms aboard the spacecraft
N = the number of viable terrestrial organisms (VTOs) released to
the Martian environment or into its atmosphere from the
spacecraft (a random variable)
E(N) - k P(N-k), the expected (or mean) number of VTOs released
kno
G the event that a single VTO will grow, meaning that it would
survive and multiply in the Martian environment.
This formula forms the basis for the assessments of the probability
of planetary contamination as they are currently carried out by NASA.
The most serious problem in the use of the Sagan-Coleman formula
is that P(G) and E(N) have been used to refer to a randomly selected
organism, with no specification of the type of organism or how and
where it is introduced into the Martian environment. This approach
ignores important available information and places an exceedingly
difficult task on the scientific experts who are asked to assign P(G).
By a relatively straightforward extension of the Sagan-Coleman approach,
the problem can be overcome.
3. Distinguishing among Organisms
To survive and reproduce on Mars a microbe should be facultatively
anaerobic, that is, able to reproduce in the absence of oxygen. All
terrestrial life requires water in a usable form. Water usable by
microbes is unlikely to exist on Mars at temperatures significantly
3
above O*C, so the microbe should be facultatively psychrophilic, that
is, able to reproduce in a temperature range of O0C or below. For
our analysis we assumed that only organisms that are facultatively both
anaerobic and psychrophilic contribute significantly to the probability
of contamination.
The UV radiation flux on the Martian surface is strong enough to
kill any unprotected terrestrial microorganisms in minutes. A micro-
organism implanted directly into Martian soil will have a far better
chance of surviving than a microbe that rests for many days on the
exterior surface of the spacecraft. We therefore distinguished
three mechanisms for release of organisms into the Martian environment:
* direct implantation of a microbe into Martian soil
* release by aeolian erosion, presumably during a Martian
dust storm, and
e release from the surface of the spacecraft into the Martian
atmosphere due to mechanical vibration, thermal effects, or
other means.
Dependence on landing site was not included in our analysis, but if
future information indicated that the availability of usable water on
Mars varies significantly with the release location, the model could
be expanded to include this dependence.
The importance of organism characteristics was noted six years ago
by Sagan, Levinthal, and Lederberg. [3]
4
4. New Assessment Methodology
An overview of the model for assessing the probability of
planetary contaaination is shown in Figure 1. The model is composed
of four submodels that describe successively (1) the bio-burden on
the Viking Lander, (2) microbial release mechanisms, (3) transport in
the Martian environment, and (4) the resistance of terrestrial microbes
to the Martian environment and the availability of nutrients needed
for microbial reproduction on Mars. Communication among the submodels
is through the expected number of VTOs that undergo various specific
events, such as release from the spacecraft.
The overall output from the model is the expected number of
organisms that reproduce on the planet. Contamination is defined as
reproduction on the planet by one or more organisms. Since the
expected number of organisms that reproduce on the planet is much less
than unity, we can interpret this output quantity as the probability
of contamination of Mars.
The first submodel provides the subsequent submodels with the
number of microorganisms existing on the Viking Lander when it lands
on Mars. This biological load is characterized not only by the type
of microorganism but also by its location on the lander. Included
in this submodel are the number and type of organisms at various
locations prior to sterilization, the reduction in bio-burden effected
by the sterilization requirements, possible recontamination, and
increase or decrease of the microorganism population during transit
to Mars. Assessment of the probability of contaminating Mars
5
BIO-BURDEN RELEASE TRANSPORT REPRODUCTION
External implantation
STERILIZATION LANDING MODE EXISTENCE OF RESISTANCE
USABLE WATER TO
Covered Reach ENVIRONMENT
Usable
Erosion Water Growth PROBABILITY
RECONTAMINATION RELEASE TRANSPORT OF
Mated MECHANISM MECHANISM CONTAMINATION
Vibration NUTRIENTS
RELEASE TRANSPORTBIOEXPERIMENT Encapsulated LEA TALT
LETHALITY LETHALITY
Bioexperiment Contamination
Arrows represent transfer of viable terrestrial organisms (VTOs).
FIGURE 1 MISSION CONTAMINATION MODEL
from contamination of the bioexperiments on the lander is calculated
directly; the contribution from this term is not significant.
The Release Submodel uses the bio-burden profile as input. It
represents explicitly the uncertainty in the landing mode (hard or
soft) and the release mechanism: implantation, erosion, or vibration.
The lethality of each mechanism is assessed, and the number of VTOs
released by each mechanism is calculated.
Unless a microbe from the lander is directly implanted in a
hospitable water microenvironment, Martian winds or other transport
mechanisms are needed to transport it there. However, the microbe
may be killed or immobilized by UV radiation or other causes during
transit. These transport and lethality processes have been represented
by a dynamic probabilistic model, specifically, a Markov process.
Each of the three release mechanisms corresponds to a separate
starting state in this process. The output from the Transport Sub-
model is the expected number of VTOs reaching a microenvironment
with usable water.
Finally, given that a VTO has reached a hospitable water
microenvironment, we examine the circumstances required for its
reproduction. The organism must be facultatively anaerobic and able
to reproduce at temperatures near or below OOC. It must also be able
to acquire the nutrients necessary for reproduction. The output
from the Reproduction Submodel is the number of organisms expected
to reproduce in the Martian environment.
7
5. Results of the Analysis
Given the present state of scientific information, the probability
of biological contamination by each of the two Viking landers is
-66 x 10 6 . This value is approximately a factor of 16 below the mission
-4
constraint of 1 x 10 imposed by NASA.
Figure 2, which reproduces the structure of the model presented in
Figure 1, indicates the crucial variables and the major intermediate
results at each point in the model. The expected number of VTOs trans-
ferred from one submodel to the next is indicated on each arrow linking
the components. Also, the box representing each submodel contains a
list of the critical variables pertaining to that part of the model.
Another important result, not apparent in Figure 2, is that the
probability of growth of a microbe varies widely with its release
mechanism. A VTO released by implantation is not immediately exposed
to the UV radiation and has a probability of growth of 2.8 x 10-5
At the other extreme, a microbe released by erosion must survive
transport in a Martian dust storm and is 100 times less likely to
grow and reproduce than a microbe released by implantation; its proba-
bility of growth is 2.8 x 10 -7  Microbes released by vibration have
an intermediate chance of surviving. Since they were initially located
on exposed surfaces and are released in a viable state, they must
already be shielded from UV radiation,. However, because they fall
on surface of the Martian soil, they have less chance of reaching a
microenvironment with usable water than do microbes that are implanted
directly into the soil. The probability of growth for microbes re-
x 10-6leased by vibration is about 5.3 -6 . These findings clearly
8
1BO-BURDEN RELEASE TRANSPORT REPRODUCTION
11
External 0.045
SENSITIVE SENSITIVE Implantation SENSITIVE SENSITIVE
VARIABLES VARIABLES VARIABLES 1.2 x 10-3  VARIABLES
16 Reach
* External, Covered * Erosion lethality 14.9 0 Availability of Usable * Percent facultatively 6 x 10 6
covered and for encapsulated Erosion' usable water Water psychrophilic and Growth PROBABILITY
encapsulated 9 microorganisms Degree of UV D anaerobic OF
bio-burden Mated * Probability of hard shielding * Availability of CONTAMINATION
landing and fracturing •11 nutrients necessary 6 x 1
20,000 raity of release Vibration for microbial growth
Encap sulated Lethality of releaseEncapsulated by vibration
* Fraction of external
bio-burden viably
implanted
Bioexperiment Contamination: Probability of 10 -
Numbers on arrows give the expected numbers of VTOs
FIGURE 2 MISSION CONTAMINATION MODEL RESULTS
indicate the importance of conditioning the probability of growth
on the release mechanism.
6. Insights from Sensitivity Analysis
The above results reflect the present state of scientific informa-
tion, which is characterized by large uncertainties. The sensitive
variables, for which the uncertainty has a significant effect on the
probability of contamination, are given in Figure 2.
The probability of contamination is especially sensitive to two
characteristics of the transport submodel: the probability of finding
water on Mars and the lethality of transport. The two variables of
the Reproduction Submodel are also critical, If growth is to occur;
(1) the microbe must be resistant to the Martian environment, and (2)
it must find appropriate nutrients. Experimentation in microbiological
laboratories could address the question of whether different micro-
organisms surviving the dry heat sterilization cycle could reproduce
in a Martian microenvironment if supplied with usable water and UV
protection. Unfortunately, little attention has been given to this
research until recently [4].
10
72 Overall Sensitivity to Further Information
To determine the sensitivity of the overall assessment to additional
information, an approximate calculation was performed in which 16 of
the input variables in the assessment model were considered uncertain.
Additional information would cause these inputs to be revised. We
modeled the effect of additional information by assigning probabilities
to the eventuality that the variables would take on higher or lower
values than the nominal values used in arriving at the assessment of
6 x 10-6. The resulting calculations showed a probability of a few
percent that the constraint of 10-4 might be violated and a probability
of 50 percent that the probability of contamination would be revised
106to less than 10 on the basis of the additional information.
8. Conclusion
We have shown how available scientific information can be
structured into a model for assessing the probability of contamination.
We believe that the use of a formal model as a basis for planning"
quarantine policy represents a substantial advance over NASA's current
approach, which relies on the single parameter P(G). By providing a
detailed structural basis for assessing the probability of microbial
growth, the model facilitates critical review and revision by the
community of scientists concerned with planetary quarantine.
We have recommended to NASA that the current procedure of deter-
mining mission sterilization requirements on the basis of a single
probability of growth be replaced by a procedure that distinguishes
11
among types of organisms, types of release mechanisms, and other
characteristics that affect whether an organism released from a
spacecraft will reproduce in the environment of another planet.
Important decisions will be made in the coming years about
missions to the outer planets and the return of a surface sample
from Mars. It would be highly desirable to have a comprehensive
decision framework to address the question of quarantine strategy.
The suitability of decision analysis concepts to the quarantine
problem has already been pointed out [2]; the methodology and pro-
cedures have been applied to similar complex problems in space
project planning [5], [6], [7], and other large-scale scientific
research programs [8]. The formulation should make explicit the
interaction between quarantine procedures and spacecraft cost and
reliability, value judgments about microbial proliferation, and
the goals of the planetary exploration program.
12
References
[1] B.R. JUDD, D.W. NORTH, and J.P. PEZIER, Final Report, SRI Project
MSU-2788, NASA Contract NASW-2535, Stanford Research Institute (1974).
[2] C. SAGAN and S. COLEMAN, Astronautics and Aeronautics, 3, 22(1965).
[3] C. SAGAN, E.C. LEVINTRAL, and J. LEDERBERG, Science, 159, 1191(1968).
[4]. T.L. FOSTER, Report No. 3, Hardin-Simmons University (1973).
[5] J.E. MATHESON and W.J. ROTHS, paper presented at National Symposium,
Saturn/Apollo and Beyond, American Astronautical Society, 11 June 1967.
[6] J.E. MATHESON and A.B. POLLARD, Final Report, SRI Project 6152,
Stanford Research Institute (1967).
[7] M.M. MENKE and B.R. JUDD, Final Report, SRI Project MSD-2006, JPL
Contract 953 435 under NAS-7-100, Stanford Research Institute (1973).
[8] R.A. HOWARD, J.E. MATHESON, and D.W. NORTH, Science, 176, 1191(1972).
13
ILLUSTRATIONS
1. MISSION CONTAMINATION MODEL. . . . . . . . . . .. 6
2. MISSION CONTAMINATION MODEL RESULTS. . . .. . . . . . 9
14


New methodology for assessing the probability of contaminating Mars

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