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D
Excerpts from the 1978 Report*
*Reprinted from the Space Science Board, National Research Council, 1978.
Recommendations on Quarantine Policy for Mars, Jupiter, Saturn, Uranus, Neptune, and
Titan. National Academy of Sciences, Washington, DC.
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2
Recommendations on Quarantine
Policy for Mars Based on the Current
Viking Findings
The current NASA policy on the likelihood of growth of terrestrial
microorganisms on Mars is based on the December 14, 1970, Space
Science Board report, Review of Sterilization Parameter Probability of
Growth (Pg).
The report established the minimum conditions necessary to define
a microenvironment on Mars that would support growth of the most
"hardy terrestrial organisms." The conditions established were the
following:
(a) Water activity ( w) 0.95.
(b) Temperature 0°C for at least 0.5 h/day.
(c) Nutrients: At least small amounts of water-soluble nitrogen,
sulfur, phosphorus, carbon (and/or light). pH values
between 5 and 8.
(d) Attenuation of uv flux by more than 103.
(e) Antinutrients-absence of antimetabolites.
All the above conditions must occur simultaneously, or nearly so.
The report then proceeded to estimate the value of Pg, the
"estimated probability that growth and spreading of terrestrial organisms
on the planet surface will occur." The estimated value of Pg was 3 x 10-9,
with less than one chance in a thousand that it exceeded 1 x 10-4. For the
Viking project, NASA adopted a value of Pg = 10-6, some three orders of
magnitude more favorable to growth than the best estimate of the review
committee, but still two orders of magnitude less than the extreme upper
limit. The adoption of this value required terminal heat sterilization of the
entire Viking Lander but not of the Orbiter. The value remains NASA
policy to date.
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I. VIKING FINDINGS PERTINENT TO QUARANTINE
Estimating the likelihood of the growth of terrestrial organisms on
Mars requires a comparison between the known physical and chemical
limits to terrestrial growth and the known and inferred conditions present
on or just below the Martian surface. Table 1 makes that comparison in
abbreviated form. Appendix A discusses in fuller form the inferences that
can be drawn from the Viking findings about those physical and chemical
characteristics of the Martian surface that are pertinent to the question of
the growth of terrestrial microorganisms.
Orbital measurements have covered appreciable fractions of the
planet's surface, but the two Landers (VL-1 and VL-2) have sampled only
a few square meters of the surface at two subpolar sites. The biologically
relevant experiments were conducted on soil samples acquired during the
Martian summer and early fall from as deep as 6 cm below the surface. (In
March 1977 a sample was acquired from a depth of 20 cm, but as of April
1977 an inorganic analysis is the only experiment that has been
performed.) Nevertheless, certain extrapolations relevant to the quarantine
question can be made with various degrees of confidence to other regions
of the planet, to greater depths, and to other seasons of the year.
TABLE 1 Limits for Growth of Terrestrial Organisms
1970
Conditions on Marsa
Factor Refs. 2 and 3
Study
Water activity ( w) >0.9 0 to 1
0.95
Water (liquid) — Required Not detected
Temperature >-15°C +20 to -143°C (see text)
0°C
pH 5-8 <11.5 Not known
Ultraviolet
0.1 J cm-2 0.04 J cm-2 min-1
radiationb
Ionizing radiationb — <500 rad/yrc
2-4 Mrad
Nutrients See text and Refs. 2 and 3 Organic compounds ppb;
most required elements
detected (see text)d
Antimetabolites None present Strong oxidants present (see
text)d
a
Cf. Reference 1; uv flux data from Reference 18.
b
Limit for survival. Limits for growth are not known.
c
See p. 11.
d
At VL-1 and VL-2 sites.
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II. CONCLUSIONS ON THE LIKELIHOOD OF THE GROWTH
OF TERRESTRIAL ORGANISMS ON MARS
We turn now to a reassessment of Pg, the likelihood of the growth
of terrestrial organisms on Mars. We will consider three regions
separately: (1) subpolar areas within a few centimeters of the surface, (2)
subpolar regions more than a few centimeters below the surface, and (3)
the residual polar caps. Finally, we will discuss briefly, the likelihood that
terrestrial organisms could survive transport at or above the surface from
one region to another.
A. Subpolar Regions within about 6 Centimeters of the Surface
Our conclusion is that no terrestrial organisms could grow within a
few centimeters of the surface in the regions lying between the two
residual polar caps. We base this judgment on the following of the Viking
findings:
— The presence in VL-1 and VL-2 sample of strong oxidants.
— The absence of detectable organic compounds, which (a) attests
to the power of the oxidants and (b) renders unlikely the existence of the
specific types of organic compounds required for terrestrial heterotrophic
organisms.
— The inability of physical shielding by a rock to eliminate the
oxidants.
Our conclusion is reinforced by three additional factors that were
well known before the mission:
— The unlikelihood of organisms being deposited in regions that
receive sufficient visible light to support photosynthetic autotrophy
without at the same time receiving lethal fluxes of ultraviolet radiation. *
— The exceedingly low probability for the existence of liquid
water with activity ( w) high enough to support terrestrial growth.
— The fact that, even if liquid water were present, vegetative cells
would be subjected to daily cycles of injurious freezing; and only
vegetative cells can grow.
*
Although unlikely, the probability is not zero. Sagan and Pollack10 have calculated that, although the uv
flux is attenuated several millionfold at 0.8 cm below the Martian surface, the flux of visible light would
still be 3.8 x 102 erg cm-2 sec-1 at that depth.
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It is highly likely that the surface conditions enumerated above
at the VL-1 and VL-2 sites prevail over the subpolar regions of the
planet. This conclusion is based on
1. The similarity in the findings at two widely separated points
for the elemental composition of the regolith and for the results of the
organic analysis and the gas-exchange experiments.
2. The strong probability that the oxidants are derived from
atmospheric reactions or atmosphere-regolith reactions. Accordingly, it
is difficult to conceive of regions that would be accessible to terrestrial
microorganisms and at the same time be capable of excluding the
atmosphere.
3. The fact that the Infrared Thermal Mapper (IRTM) has
mapped a sizable fraction of the Martian surface without detecting
thermal heterogeneities significantly more favorable to terrestrial
growth than those that we have reviewed in Appendix A.
Viking has provided much information that was either not
known beforehand or was known only with considerable uncertainty.
None of this new information suggests that the Martian surface is less
harsh to terrestrial microorganisms than was thought prior to Viking. *
On the other hand, two pieces of information indicate that it is harsher
than was thought previously: the lack of detectable organic compounds
and the presence of strong oxidants even in regions physically shielded
from uv.
Our conclusion is that no terrestrial organism could grow under
the conditions found by Viking to prevail on subpolar surfaces at the
landing sites and none could grow under the conditions that are highly
likely to prevail throughout the entire subpolar region. Few if any
terrestrial organisms could grow in contact with even one of the
adverse conditions cited, much less grow when exposed to all of them
simultaneously. Although we cannot absolutely rule out the existence
of oases capable of supporting terrestrial life, we believe, for the
reasons cited, that the likelihood of their existence is extremely low.
Unfortunately, we know of no quantitative basis for assigning a
numerical probability to "extremely low" when no oasis has been
detected and when the weight of evidence is that none can exist. And
yet a numerical value for Pg is required in order to determine what
*
The demonstration by Viking that the atmosphere contains nitrogen answers an important question that
was unknown previously. However, the ignorance prior to Viking of the existence of nitrogen was not a
significant factor in prior estimates of the probability of growth of terrestrial organisms.
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procedures are needed to reduce the microbial burden on future spacer
rift to Mars to levels that fulfill current COSPAR quarantine policy.
Reluctantly, then, we recommend for these purposes, and these
purposes alone, that NASA adopt a value of Pg < 10-10 for the subpolar
regions of the planet within 6 cm of the surface. ** This number, which
is more than four orders of magnitude below the current value of Pg,
reflects the fact that Viking has found the conditions to be considerably
harsher to terrestrial life than was heretofore assumed and has obtained
evidence that renders the existence of oases far less likely than was
heretofore assumed.
B. Regions More than 6 Centimeters below the Surface of Subpolar
Regions
As mentioned, Viking conducted biology experiments and
organic analysis on samples obtained from depths of 4-6 cm. Greater
depths would be required. to reduce or eliminate the lethal surface
conditions. The depths required are unknown chiefly because the
relation between depth and the presence of oxidants is unknown.
However, the maximum temperature falls rapidly with depth. In the
northern hemisphere, even at a depth of 4 cm, the maximum
temperature is estimated to be 20° below the minimum confirmed
growth temperatures (-15°) observed for terrestrial organisms
(Appendix B). By a depth of 24 cm, the maximum temperature is
estimated to be -50°C, some 35° below the minimum confirmed
terrestrial growth temperature. In the southern hemisphere, the
maximum temperature at a depth of 24 cm is estimated to be -35°C, still
20° below the minimum terrestrial growth temperature.4,8
At increased depths there is an increased likelihood of
encountering ice, the existence of which would enhance the possibility
of liquid water. But water that is liquid below -20°C and is in
equilibrium with ice has an activity ( w) below that which will support
the growth of any known terrestrial organism capable of growing under
the partial pressure of oxygen on Mars (Appendix B, Figure B.2).9
Thus, temperature alone seems an absolute barrier to the growth
of any terrestrial organisms at depths below a few tenths of a meter.
But again, sufficient uncertainties exist to preclude an absolute
statement to this effect; viz.,
**We obtain this value by estimating probabilities of < 10-2 for the presence of liquid water of high enough
aw, < 10-1 for the ability to survive multiple freezing and thawing, < 10-1 for the avoidance of lethal uv
<<10-2 for the presence of organic compounds of appropriate types in appropriate concentrations, <<10-3
for the absence of powerful oxidants, and 0.1 that the deposited microorganism is an anaerobe.
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— Although the surface temperatures are derived directly from
the orbital infrared measurements and are consistent with the direct
meteorological measurements at the landing sites 1.5 m above the
surface, the estimates of subsurface temperatures require assumptions
about the thermal diffusivity of the soil. The range of error is estimated
by Kieffer8 to be 5°C. This error would not be sufficient to change our
conclusions, but larger errors are conceivable.
— There could exist heterogeneities below the resolving power
of the IRTM (a minimum of 2 km) that have higher temperatures.
— Although there is extensive information on the minimum
growth temperatures of terrestrial microorganisms, the remote
possibility exists that some unknown organism has a growth minimum
below -15°C. We view this as extremely remote because, as indicated
in Appendix B, the number of species capable of growth diminishes
drastically as the temperature is lowered below 0°C. Furthermore,
growth below -15°C is tantamount to growth in 8 osmolal solute,
conditions that even at ordinary temperatures preclude the growth of all
except halophiles and osmophiles.
— There is the remote possibility that there exists somewhere a
narrow zone of subsurface that is deep enough to preclude oxidants and
shallow enough to have temperatures high enough to support growth.
Although these uncertainties prevent us from concluding that
the possibility for growth is zero, we are still forced to conclude that
subsurfaces of Mars are exceedingly harsh for terrestrial life.
Accordingly, for the specific purpose of determining quarantine
requirements for future Martian missions, we recommend that NASA
adopt a value of Pg < 10-8 for subsurfaces in the subpolar regions of
the planet.
C. The Residual Polar Caps
The arguments just presented for subsurface regions generally
apply to the residual polar caps as well. As in the subsurface regions,
the temperatures mapped by the IRTM are too low to permit the growth
of known terrestrial organisms. However, thermal heterogeneities h
have been detected. The maximum temperatures observed (237 K) are
not high enough to permit the growth of earth organisms, but their
presence raises the remote possibility that there exist .other undetected
heterogeneities for which the temperature does rise high enough. But
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warmer regions will also be drier regions, because the increased vapor
pressure associated with higher temperatures would cause water to
distill rapidly from these regions and freeze out at the cold trap
furnished by the remainder of the residual cap. The water ice itself in
the residual caps constitutes a possible source of liquid water, provided
that special conditions were present to permit that ice to liquefy rather
than to sublime (e.g., freezing point depression by electrolytes). But
even then, as in the case of subsurfaces, the temperatures would be too
low to permit the growth of terrestrial organisms.
The polar regions would not be immune from the atmospheric
oxidants, but chemical interactions between atmosphere and ice might
be different from chemical interactions between atmosphere and
regolith.
Our conclusions about the likelihood of growth in the residual
polar caps are similar to those reached in Section B above for
subsurface subpolar regions—it is extremely low. Nevertheless,
because there is more uncertainty about the physical and chemical
conditions at the residual polar caps, we believe that these regions
should be handled with prudence and recommend that they be assigned
a value of Pg <10-7.
D. Transport from Subpolar Regions into the Residual Polar Caps
or into Putative Oases
There is little likelihood that any terrestrial organism could
survive a voyage on or above the surface requiring more than a few
minutes. First, the uv flux on the surface of Mars is 4 X 10-2 J cm-2 min-
1
, and that flux would kill the most resistant of terrestrial
microorganisms in a few minutes (upper terrestrial limit 0.1 J/cm2)
(Table 1). Second, organisms protected from the direct exposure to the
uv by a layer of soil particles would nevertheless be in contact with the
oxidants in those soil particles.
One consequence of these lethal conditions is that our
recommended value of <10-7 for Pg in the residual polar caps applies
only to terrestrial organisms that are released directly in that region.
The Pg for organisms transported into the polar caps from the subpolar
regions would be orders of magnitude lower. Similarly, even if Mars
were to possess oases that were hospitable to terrestrial life, few if any
terrestrial organisms would survive a surface or aerial trip to the oasis
and few if any would ever survive an escape from the oasis.
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III. LIMITS TO THE GROWTH OF TERRESTRIAL LIFE
VERSUS THE QUESTION OF INDIGENOUS LIFE ON MARS
The evidence that leads us to the conclusion that terrestrial
microorganisms have little and in most regions of the planet no
probability of growth does not rule out the possibility that indigenous
life forms may exist currently on Mars or may have existed sometime in
the past. The limiting conditions listed in Table 2 for terrestrial life are
not the limits for conceivable life elsewhere.
There is fairly wide agreement that life, if it exists elsewhere, is
based on carbon chemistry and that it requires nitrogen; organic
compounds of high information content, energy, and substrates to
permit the synthesis of the organic compounds; and liquid water.
Although, as discussed, organic compounds and liquid water have not
been detected on Mars, there is no basis for precluding their existence.
There is, moreover, strong evidence that liquid water in large quantities
existed in the Martian past.
It might be argued that, if indigenous life forms do exist, they
themselves could constitute micro-oases for the growth of terrestrial
organisms. We consider this unlikely. For example, a Martian organism
growing in thermal equilibrium with its surroundings at -40°C is
conceivable. However, to do so, a spherical organism 2 x 10-4 cm in
diameter, for example, encased in efficient insulation 1 mm thick
would have to assimilate and burn about 1000 times its steady-state
concentration or organic compounds per second to maintain the 40-
degree differential. The probelm would be only slightly less serious in a
macroscopic Martian organism. Analogous difficulties arise in
postulating that the organic compounds in putative Martian biota would
be compatible with and utilizable by the enzyme systems of terrestrial
microorganisms.
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TABLE 2 Estimated Contributions to Pg for Jupiter and Saturn
1974 1976
Jupiter Uranus
Factor Report Report Comments
Temperature 1 1 Assumed between -20 to 100°C
Pressure 1 1 Not a critical parameter for
microbiology
Radiation 1 Not Deleterious
specified
but <1
Liquid H2O 1 1 Assumed
10-1 <10-3a
Nutrients Organics, ions - aqueous solution
10-1 10-1
Anaerobiosis About 0.10 of the earth's microbes
are anaerobes, but these are
unlikely to be spacecraft
contaminants
10-2 <10-4b
NH3
<10-3a
Growth in Not Completion of life cycle in the
aerosols specified atmosphere has never been
reported for any earth organisms
10-3 <10-3
Convection to All models predict that organisms
lethal will be carried from water levels to
temperatures lethal depths; the times required
are somewhat model dependent
10-7 <10-14
TOTALS
a
Based on more detailed analyses.2,3
b
New information, e.g., Reference 22.
IV. CONCLUSIONS PERTINENT TO THE CURRENT VIKING
ORBITERS
As of August 1977, two years have elapsed since the unsterilized
Orbiters were launched from earth. Any organisms on the outer surface of
the Orbiter have surely been killed by uv irradiation. Most organisms in
the interior of the Orbiter have been subjected to moderate temperatures
(10 to 38°C), high vacuum, and some ionizing radiation.11 Although the
cell dehydration associated with the high vacuum would be lethal to a
fraction of the microbial population, many (perhaps 1 to 10 percent) would
likely survive.6,12,13 Some protons from galactic cosmic rays and solar
flares would strike organisms in the interior, but the dose would be
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appreciably less than 500 rad/year,11,14 and many microorganisms can
survive such doses. (The flux of solar protons far exceeds that from
galactic source, but the great bulk of the solar protons have energies of
MeV,11 and such protons are only capable of penetrating 0.1 mm of
material with a density of 1, e.g., water.14) Conservatively, then, one
cannot assume that the microbial burden within the Orbiter has decreased
by more than 1 or 2 orders of magnitude since launch.
In spite of the expected survival of a fraction of the original burden
of terrestrial microorganisms, our new estimates of the values of Pg lead to
the conclusion that COSPAR requirements for planetary quarantine will
not be compromised by lowering the periapsis of the Orbiters to 300 km.
Indeed, with the new values for Pg, still lower periapses for unsterilized
Martian orbiters may well be compatible with COSPAR requirements.
NASA will probably wish to determine these minimum orbital altitudes
before assessing and designing Mars follow-on missions in detail.
V. QUARANTINE STRATEGY FOR FUTURE MISSIONS TO THE
MARTIAN SURFACE
Our Committee has recommended that the next phase in the
biological exploration of Mars should be to acquire and characterize soil
samples from areas likely to contain sediments and ice-regolith interfaces.1
Locating these areas and locating sites that are shielded from the powerful
atmospheric ultraviolet radiation and the powerful surface oxidants will
require subsurface sampling by a soft lander, by penetrators, or by both.
The samples acquired from the subsurface of Mars should be characterized
with respect to organic compounds, carbon and sulfur isotope ratios, the
amount and state of water, the presence of water-soluble electrolytes, and
the existence of nonequilibrium gas compositions. The greater the extent
to which samples possess these characteristics the greater the priority for
the initiation of a second phase of post-Viking biological exploration of
Mars—a detailed search for evidence of present or past life on Martian
samples returned to earth.
With respect to quarantine considerations for the mission that
conducts the first exploratory phase, our estimates for the values of Pg lead
to the conclusion that terminal heat sterilization would not be required in
the case of a nominal soft landing in the subpolar regions (Section A) and
possibly in other cases as well. However, we would have no objections to
sterilization provided that it has no impact on the scientific payload of the
landers and that it does not increase the mission cost. (We have been
informed by representatives of NASA that this may be the case.)
Decisions on scientific payloads for the missions should be based on their
scientific quality and cost effectiveness. We would object to the
elimination of an experiment or the degradation of its performance
because of the imposition of unessential sterilization requirements.
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In the report Post-Viking Biological Investigations of Mars,1 we
stated that we consider metabolic-type life-detection experiments on the
surface of Mars to be of low priority scientifically. Nevertheless, NASA
may decide to include them. If so, a limiting factor with respect to the
allowable microbial burden on a soft lander would likely become the
avoidance of contaminating the metabolic experiment by terrestrial
microorganisms.
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(Appendix D—continued)
Appendix A:
Findings from Viking Pertinent to the
Possible Growth of Terrestrial
Microorganisms on Mars*
I. DEFINITIVE FINDINGS FROM VIKING
A. Water
The gas chromatograph-mass spectrometer (GCMS) has
detected less than 0.1 percent water in soil samples (several tenths of
a percent in one sample collected from beneath a rock). The current
belief is that this water represents mineral hydrate water of moderate
or low thermal stability. Neither this instrument nor the others on the
lander were designed to detect free liquid water, nor have they done
so.
Unfortunately, Viking carried no instrument to measure
relative humidity. However, indirect evidence (e.g., cloud
formation) indicates that saturation does occur in the atmosphere.
The probability for the existence of liquid water anywhere on
the planet remains low. The surface temperatures and atmospheric
pressures preclude the existence of pure bulk liquid water under
equilibrium conditions. However, there continue to be three remote
possibilities for the existence of liquid water: (1) water adsorbed to
subsoil, (2) water that is liquid by virtue of kinetic factors slowing
*
Pertinent references not cited here will be found in Reference 1.
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the approach to equilibrium (i.e., conditions under which diffusion
of water is slower than diffusion of heat), and (3) water that has its
chemical potential (and hence freezing point) lowered by the
presence of dissolved solutes. The solutes could be one or more of
the several salts that are almost certainly present. The eutectic points
of salts like CaCl2 MgCl2, and K2CO3 are below -30°C; hence their
presence would permit stable liquid water down to these
temperatures. The electrolyte concentrations, however, would be
multimolar.
Another argument against the existence of liquid water at the
landing sites is the findings of the Labelled Release and Gas
Exchange biology experiments. In both cases, the initial addition of
water vapor or liquid water to the soil samples dissipated the
reactants so that further additions produced no further reactions
(release of 14CO2 and release of oxygen in the two experiments,
respectively). Presumably, therefore, no reactions at all would have
been observed if the soil itself had been exposed to high-activity
liquid water just prior to the acquisition of samples by the Landers.
B. Temperature
The maximum temperatures observed at the surface of the
landing sites during the summer-autumn observation period were -2
to -3°C. This is below the minimum growth temperature of most
terrestrial microorganisms, although, as discussed later and in
Appendix B, a few terrestrial organisms can grow at temperatures as
low as -14°C. In the southern hemisphere of Mars, the maximum
summer surface temperatures may reach 20°C.4
At night, even in summer, the temperature drops to -83°C.
Dry bacterial and fungal spores could survive many cycles of such
freezing, but hydrated and germinated spores or vegetative cells of
most terrestrial species could not.5-7 And any terrestrial
microorganism that is to grow on Mars must by definition be in the
vegetative state to carry out such growth.
C. Lack of Detected Organic Compounds
No organic compounds other than traces attributable to
terrestrial contaminants have been detected in regolith samples
analyzed by the GCMS. If volatizable organic compounds were
present in the samples, they were either present in concentrations
below the parts per billion range (the detection limit of the
instrument) or they were totally restricted to substances like methane
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with molecular weights of less than 18, which were undetectable or
detectable only at reduced sensitivities.
The inability to detect organic compounds does not, of
course, prove that none was present. But even if trace amounts of
organic compounds are in fact present in the soil of the landing sites,
the probability is remote that these would provide a nutrient medium
that could be used by terrestrial microorganisms (see Reference 3
for further discussion).
D. Elemental
The biologically vital element nitrogen has now been shown
to be in the Martian atmosphere. Calcium, sulfur, magnesium,
chlorine, and probably potassium and phosphorus have also been
detected in soil samples. All six are essential to terrestrial living
systems. Instrument limitations precluded the detection of sodium,
but there is no reason to believe that it is not present, although
probably only in low concentrations.
One striking finding is that the elemental composition of the
samples was nearly identical at the two widely separated landing
sites.26 This similarity indicates that the fine-grained material in at
least the upper surfaces of the regolith has been thoroughly mixed
over large regions of the planet-presumably as the result of wind
action.27
E. Oxidants
Two lines of evidence indicate that strong oxidants are
present in at least the top few centimeters of the regolith at the
landing sites. The first line of evidence comes from orbital
measurements of the atmosphere and from modeling. One model.
predicts the existence of active strongly oxidizing species, especially
hydrogen peroxide: Second, the gas-exchange experiment (GEX) on
the Viking landers showed the release of up to nearly a micromole
of oxygen when samples were humidified with water and warmed to
~10°C.
The GEX experiment suggests that oxidants are present to at
least the 4-6-cm depth from which samples were acquired. The
experiment also showed that the oxidants were present in samples
collected from beneath a rock, a rock that presumably had laid
undisturbed for many years. Finally, the experiment showed that the
oxidants were present at both landing sites.28
The oxidants are believed to be responsible for the lack of
detectable organic compounds, i.e., they have decomposed them.
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II. EXTRAPOLATION FROM THE VIKING FINDINGS TO
THE PLANET'S SURFACE AS A WHOLE, TO REGIONS
BELOW THE SURFACE, AND TO OTHER SEASONS OF
THE YEAR
Surface temperatures in the Martian winter will drop far
lower than those experienced during the Lander experiments.
Estimates from the infrared thermal mapper (IRTM) indicate that the
maximum surface temperature will fall below -15°C (the minimal
terrestrial growth temperature—see Appendix B) for more than half
the Martian year at the VL-2 site (48° N) and further north. At VL-1
(22° N) the maximum surface temperature will just about reach -
15°C in the winter.8 (Orbital IRTM measurements during winter will
become available during the ensuing months.)
In considering extrapolations from the findings of VL-1 and
VL-2 on surface chemistry, we note that, although the two landing
sites (22° N and 48° N) are separated by some 1500 km in latitude
and 176 degrees-in longitude, the results of the gas-exchange (GEX)
and labeled release (LR) biology experiments and of the organic and
inorganic analyses at the two sites were either similar or essentially
identical*. Strong similarities were evident as well from the imaging
experiment and from the atmospheric analyses. As noted, the results
of the GEX, LR, and GCMS experiments are consistent with the
presence of powerful oxidants in the surface samples. Since these
oxidants are almost certainly derived from atmospheric
photochemical reactions or from chemical reactions between
atmospheric species and the regolith, there is every reason to expect
that they will be globally distributed in the Martian surface, except
possibly in the residual polar caps.
Certain extrapolation can also be made to depths below the
4-6 cm sampled by the Landers.
A. Water
Several Viking experiments have confirmed or strengthened
the inference that large amounts of water are locked beneath the
surface in the form of ice. Subsurface liquid, water is conceivable;
however, because of the low temperatures at subsurfaces (see
below), the existence of liquid water in an equilibrium state would
*
Samples from the two sites in the Pyrolytic Release (PR) experiment, however, responded differently to
the addition of water vapor.25 The experimenters suggest that this reflects differences in the properties of
the soil at the two sites, but they draw no inferences as to the nature and degree of the differences.
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require multimolar concentrations of electrolytes (see Appendix B,
Table B.1).
B. Temperature
The maximum summer temperatures some 6 cm below the surface
at the VL-1 and VL-2 sites are estimated from the IRTM measurements to
be -35°C.8 This temperature is 20° below the minimum confirmed growth
temperature for terrestrial microorganisms. It is even below the lowest
growth temperature ever claimed in published reports. At a depth of 24
cm, the maximum summer temperatures at the VL-1 and VL-2 sites are
estimated to be -50°C, or 35° below the minimum confirmed terrestrial
growth temperature. In the southern hemisphere as a result of the
eccentricity of the Martian orbit, the maximum surface-.temperatures
between latitudes 5° and 45° are about 15° warmer than at the present
landing sites. As a result, at subsurface depths sufficient to damp out
diurnal variations, the maximum summer temperature is calculated to be
about -35°C, still some 20° below the minimum confirmed terrestrial
growth temperature.4
C. Ultraviolet Light
As shown in Table 1, the flux of ultraviolet radiation impinging on
the Martian surface would be rapidly lethal to any terrestrial organism.
However, the uv flux is sharply attenuated below the surface. For
example, Sagan and Pollack10 estimate an attenuation of several
millionfold at a depth of 0.8 cm.
D. Oxidants and Organic Compounds
Since the oxidants in the regolith are almost certainly derived from
atmospheric processes, their concentrations ought to diminish with depth
below the surface. But the relationship between depth and concentration is
unknown. Presumably at least some of the oxidant species are diffusable,
for they were present in the soil samples collected from beneath the rock
at the VL-2 site.
Since the lack of detectable organic compounds within 4-6 cm of
the surface seems due to the presence of the oxidants, the likelihood of
organic compounds ought to increase with depth. (Organic matter must be
present at least transiently on the Martian surface, if from no other source
than the infall of carbonaceous chondrites.)
Although the Martian surface is strikingly similar at two widely
separated points when viewed close-up from the two Landers, the surface
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is strikingly heterogeneous when viewed from orbit. Still, there is no
evidence that any of the heterogeneities represent oases that possess
characteristics more favorable to terrestrial life than those already
enumerated. One dramatic class of heterogeneities, for example, is the
huge channels that were almost certainly formed by flowing liquid water.
But these channels are too old (probably 109 years) to have much
bearing on their current suitability for the growth of terrestrial organisms,
except that they might possibly contain concentrated deposits of
electrolytes and organic compounds.
The orbital infrared temperature and water-vapor measurements
also show heterogeneities, but again none of those detected have
properties significantly more favorable to terrestrial life than do the larger-
scale features. The resolving power of the IRTM is 0.3°, which translates
to 8 km at the normal periapsis of 1500 km and 1.6 km for the now
lowered periapsis of Orbiter 1 (300 km). Smaller oases with respect to
some of the biologically relevant factors are conceivable (e.g., higher
temperatures on south-facing slopes in the northern hemisphere; higher
temperatures because of heat absorbed by dark objects). It is difficult,
however, to conceive of any oasis on the surface of subpolar regions that
would be accessible to terrestrial organisms and yet not contain the
atmospherically induced oxidants. As mentioned, the subrock sample at
VL-2 indicates that some of the oxidants can diffuse in the regolith.
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(Appendix D—continued)
Appendix B
Minimum Temperature for
Terrestrial Microbial Growth
The most thorough review known to us of the minimum
growth temperatures of terrestrial microorganisms is that of
Michener and Elliott.15 A histogram summarizing their findings on
reports of growth below 0°C is shown in Figure B.1. Many of these
reports are based on incubation times of over a year. We separate
bacteria from fungi because the latter are nearly all aerobic and
would be incapable of growing at Martian partial pressures of
oxygen. The single case of a bacterium growing below -12°C was a
report of growth at -20°C. Neither it nor the three reports of fungal
growth below -12°C have been confirmed. Michener and Elliott
point out that "The best evidence that growth does not generally
occur in foods in this temperature range [i.e., <-17°C] is that billions
of cartons of frozen food have been stored at or near -18°C without
reported microbial spoilage."
A more recent study by Fennema et al.16 confirms Michener
and Elliott's conclusion that microbial growth in foods does not
occur at -18°C.
This inability of organisms to grow below about -15°C is
consistent with the known physical state of aqueous solutions at
these temperatures. As Table B.1 shows, when solutions of sodium
chloride in water, for example, are equilibrated at various subzero
temperatures, the concentrations in the unfrozen portions exceed 4
molal below -15°C. For solutes in general, the concentrations of
solutes in the unfrozen portions of solutions are given by vm =
T/ 1.86 where is the osmotic coefficient, v the number of species
into which the solute dissociates, and m is the molality.17 Aside from
the toxic effects to nearly all microorganisms of such high
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FIGURE B.1 Reported cases of microbial growth below 0°C. (Adapted
from Reference 15.)
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FIGURE B.2 Water activity w = Pice/PH2O of a solution in equilibrium
with ice as a function of temperature.
concentrations of electrolytes, the high concentrations also depress the
water activity (αw) below the value permitting the growth even at optimal
temperatures of all microorganisms save halophilic and osmophilic forms.
As shown in Table B-1 and Figure B-2, the values of aw at -14, -16, -18,
and -20°C are 0.87, 0.85, 0.84, and 0.82, respectively.
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