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Space Sci Rev (2008) 135: 233–243
DOI 10.1007/s11214-007-9194-2
Viking Biology Experiments: Lessons Learned and
the Role of Ecology in Future Mars Life-Detection
Experiments
Andrew C. Schuerger ·Benton C. Clark
Received: 22 August 2006 / Accepted: 10 April 2007 /
Published online: 1 June 2007
© Springer Science+Business Media B.V. 2007
Keywords LR ·GeX ·PR ·GCMS ·Soil chemistry ·Microbial ecology
1 Introduction
The Viking missions to Mars landed in two areas of the northern plains, at Chryse Plani-
tia (22.5° N, 48° W) and Utopia Planitia (48° N, 226° W). Onboard the twin landers were
Biology Instruments, containing three separate experiments: the Gas-Exchange (GeX), Py-
rolytic Release (PR) and Labeled Release (LR) experiments. In addition, there was a soil
analyzer based on X-ray Fluorescence Spectrometry (XRFS) to detect the concentration
of elements in samples and, most importantly, a Gas Chromatograph / Mass Spectrometer
(GCMS) specifically designed to measure organic compounds in the soil. Together, these
instruments were used to assay soils on Mars for biological activity, the presence of organic
compounds, and the bulk elemental composition of soils.
2 Gas-Exchange (GeX) Experiment
The GeX experiment measured the compositional changes of the atmosphere in the
headspace over a humidified and then moistened regolith sample. By measuring dynamic
changes of evolved gases, the objective of the GeX was to detect the production of gases
that were derived from microbial metabolic activity from hydrated soils. The GeX instru-
ment could measure H2,N
2,O
2, CO, NO, CH4,CO
2,N
2O, and H2S; plus the inert gases
A.C. Schuerger ()
Space Life Sciences Lab, Department of Plant Pathology, University of Florida, Kennedy Space
Center, FL 32899, USA
e-mail: acschuerger@ifas.ufl.edu
B.C. Clark
Space Exploration Systems, Lockheed Martin, Denver, CO 80201, USA
e-mail: benton.c.clark@lmco.com
234 A.C. Schuerger, B.C. Clark
Ne, Ar, and Kr (Brown et al. 1978; Oyama and Berdahl 1977). Furthermore, the GeX pro-
cedure was conducted in both a “humid” mode and a separate “wet” mode in which wa-
ter vapors were first allowed to interact with regolith without directly contacting the mar-
tian samples, followed by additional injections of nutrients that then wetted the samples.
The GeX experiments were conducted at 200 mbar total pressure (composed of 7 mbar
Mars atmosphere plus added levels of CO2, He, and Kr) at 8–15°C (Klein et al. 1976;
Klein 1977,1978).
Immediately after humidification of the 1 cc sample of regolith, a large amount of O2,
and to a lesser extent CO2, were released; the evolution of these gases stabilized very quickly
and reached their maximum concentrations at 2.5 hrs (Oyama and Berdahl 1977). After the
samples were fully hydrated with the additions of nutrient media, slow decreases of CO2and
O2were observed over several sols during the continued wet mode. No further release of O2
or other potentially metabolically derived gases were observed after the soils were wetted
with the aqueous nutrients. The release of O2upon humidification had never been observed
before in prelaunch tests with terrestrial or lunar soils (Oyama et al. 1976), and was a surprise
for the Viking team (Klein 1978). One unique response was also noted when fresh nutrients
were periodically added to the GeX reaction vessel. After each subsequent addition of fresh
nutrients, the rate of CO2uptake always slowed down and became more sluggish (Klein
1978). Oyama et al. (1976) observed this response in prelaunch tests with sterile terrestrial
samples and interpreted this as an indication of a dissipative chemical reaction.
The arguments against the biological interpretation of the GeX results are: (a) the release
of O2after humidification was extremely rapid, (b) the addition of aqueous nutrients to the
samples during the wet mode resulted in no further liberation of O2,(c)O
2also was re-
leased from a sample after being “sterilized” at 145°C, and (d) the reabsorption of CO2over
time following fresh injections of media matched prelaunch tests with sterile terrestrial soils
(Klein 1978). The most likely chemical interpretation of the GeX results suggest that the
presence of a peroxide or superoxide released O2upon humidification and that metal oxides,
hydroxides, or carbonates in the samples, or created by interaction with water, resulted in a
moderately or strongly basic solution which subsequently reabsorbed the CO2(Klein 1978;
Klein et al. 1976).
3 Pyrolytic Release (PR) Experiment
The PR, or carbon assimilation, experiment tested the possibility that putative martian mi-
croorganisms could take up labeled 14CO2and 14 CO gases during either light or dark re-
actions. Martian regolith (0.25 g sample enclosed within a 4 cc incubation chamber) was
exposed to the labeled gases, with or without added water vapor, and illuminated by a simu-
lated martian spectrum (335–1000 nm) at a flux of 20% of that predicted for the Viking sites
(Horowitz et al. 1977; Klein et al. 1976; Klein 1978). The reactions were allowed to continue
for 120 hrs at temperatures between 8 and 26°C and at pressures of approximately 10 mbar.
Both light and dark-incubated tests were conducted. At the end of the incubation phases, the
chambers were vented at 120°C to separate the residual unreacted 14CO2and 14 CO gases
from potentially fixed organics formed during the light or dark reactions. The pyrolyzed or-
ganics volatilized from the soils at 625°C were passed through a gas chromatography (GC)
column maintained at 120°C which retained organic fragments larger than CH4, but allowed
the unreacted 14CO2and 14CO gases to pass. The columns contained Chromosorb P (75%)
and CuO (25%). Once the pyrolyzed organics were adhered to the GC column, it was sub-
sequently heated to 650°C in order to release the organic compounds while simultaneously
oxidizing them to CO2by reaction with the CuO.
Viking Biology Experiments: Lessons Learned and the Role 235
Results indicated that labeled 14CO2and/or 14CO gases were fixed in the martian re-
golith at very low, but significant, amounts equivalent to approximately 7 pmole of CO or
26 pmole of CO2. The reactions that fixed either 14CO2or 14CO were not inhibited after
heating the samples to 90°C, but were reduced by 90% when the samples were first “steril-
ized” at 175°C. In addition, the reactions were not affected by the addition of water vapor to
the reaction vessel, but were enhanced in the light. Although these results could be consistent
with a biological interpretation, in general there are several arguments (Horowitz et al. 1977;
Klein 1978) against such reasoning: (1) pre-heating samples to 90°C had no inhibitory ef-
fects on the reaction, and pre-heating to 175°C did not completely abolish it, (2) the presence
of H2O vapor, which would be expected to promote biological reactions, appeared to be ei-
ther inhibitory to the reactions, or had no effect (Horowitz et al. 1977), and (3) the failure
of the GCMS to detect soil organics (Biemann et al. 1976) suggests that organics do not
buildup in the regolith, and, thus, seem incompatible with the albeit small PR fixation rates
of 14CO2or 14 CO gases by soil microorganisms.
4 Labeled Release (LR) Experiment
The LR procedures for the Viking landers (Levin and Straat 1977,1979a,1979b,1981) mea-
sured evolved 14CO2gas given off by 14C-labeled carbohydrates reacting with constituents
of the martian regolith. The widespread importance of the Krebs cycle in aerobic metabolism
and the Embden–Meyerhof pathway in anaerobic metabolism makes the use of 14C-labeled
metabolites highly efficient in detecting microbial metabolism because both pathways pro-
duce carbon dioxide (Levin et al. 1964). Metabolic activity from a wide range of diverse
microorganisms (26 species) can be detected with the LR assay including: Bacillus sub-
tilis,Micrococcus spp., Pseudomonas spp., Staphylococcus epidermidis,andStreptomyces
spp. (Levin 1963). Early results with the LR procedures indicated that 1–2 µCi/ml of 14C-
labeled nutrient solution was ideal for detecting as low as 10–12 viable bacteria per ml
within 1–3 hrs (Levin 1963; Levin et al. 1959,1964). In addition, the evolved 14CO2as
measured by the LR assay was linearly correlated to cell density, such that each doubling in
cell number per sample resulted in an approximate doubling in the counts-per-minute (cpm)
from evolved 14CO2gas (Levin et al. 1959,1964).
In brief, approximately 0.5 g of Mars regolith was placed within a reaction chamber
of the Viking LR experiments, and the system closed, thus, capturing the ambient Martian
atmosphere of 95% CO2and 5% of trace gases at approximately 7 mbar. The Viking LR
reaction chamber was equilibrated to 10°C, and a series of helium and nutrient solution
injections (115 µl each) were conducted to yield a final total pressure after the first injection
of 92 mbar, and after the 2nd injection of 116 mbar (Levin and Straat, 1979a,1979b). The
evolved 14 CO2gas was recorded over several sols following each injection of nutrients.
The responses given in Fig. 1are from VL-1 (Levin and Straat 1977) and show results
from two active cycles compared to a heat-sterilization cycle which heated the sample to
160°C for 3 hrs prior to initiating the LR assay. The results gave “classic” biological re-
sponses for the two active cycles, and a negative response for the heat-sterilized cycle (cy-
cle #2). These results were accepted as strong evidence for biological activity in martian
regolith by Levin and Straat (1977,1979a,1979b,1981). The controversy on this interpreta-
tion is based on two key points: (i) following a 2nd injection of the 14C-labeled nutrient solu-
tion, the radioactivity decreased instead of increased as would be expected if the 14CO2gas
was derived exclusively from biological activity (Levin and Straat 1977), and (ii) no organics
were found in the Mars regolith with the Viking GCMS experiment (Biemann et al. 1976;
236 A.C. Schuerger, B.C. Clark
Fig. 1 Viking Lander 1 (VL-1) LR data. Soil in cycle #2 was heat-sterilized for 3 hrs at 160°C before
initiating the LR assay (from Levin and Straat 1977)
Biemann and Lavoie 1979). Although Levin and Straat have responded to these and other
criticisms (Levin and Straat 1979a,1979b,1981), the biological interpretation of the Viking
LR response is considered unverified by most of the exobiological community.
5 Gas Chromatography / Mass Spectrometer (GCMS) Experiment
Martian regolith samples were heated in a series of steps up to 500°C in the Viking GCMS
experiment to investigate the evolution of volatile compounds derived from in situ organ-
ics present in the martian regolith. It was presumed that organics would be present in the
martian regolith due to extraterrestrial sources of carbon in cosmic dust, carbonaceous me-
teorites, and comets (Biemann et al. 1976; and more recently Flynn and McKay 1990;
Bland and Smith 2000). However, results from the Viking GCMS experiments indicated
that only H2OandCO
2were detected (Biemann et al. 1976; Biemann and Lavoie 1979),
which implied that no organic molecules were present in the martian regolith at the parts
per billion level (Biemann et al. 1976). Samples were obtained from the GCMS instruments
on Viking down to a depth of no more than 10 cm. These results contributed to the strong
arguments made against the biological interpretations of the Viking biology experiments;
namely, if organics are not present in the martian surficial soils, then organic-based life must
not exist at these test sites. In retrospect, it has been shown by Benner et al. (2000)that
the Viking GCMS would not have detected certain refractile compounds such as nonvolatile
salts of benzenecarboxylic acids, which might have formed from oxidation of biological
materials. It has also been pointed out that the shunt valving used to dump relatively large
amounts of H2OandCO
2released from the soil would also have bypassed low molecular
weight organics around the detection systems.
Viking Biology Experiments: Lessons Learned and the Role 237
6 Lessons Learned from Viking Life-Detection Experiments
The presence of oxidants in the martian soils created an unexpected and unwelcome compli-
cation which was not part of the original design of the life-detection investigations. Not only
have oxidants on Mars apparently scrubbed out most, or possibly all, of the exogenously de-
livered organic compounds, but they provided reactants for the organic materials supplied to
the Viking GeX and LR experimental modules.
Ubiquitous sulfates in martian soils and the subsequently known presence of H2gas in
the martian atmosphere (Krasnopolsky and Feldman 2001) begs the question of whether
sulfate reducing microorganisms might somewhere be present on Mars. Unfortunately, this
style of metabolism was not strongly anticipated and none of the Viking life-detection ex-
periments were optimized to detect it. In particular, the incubation chambers were too small
to capture sufficient quantities of trace gases, such as H2, to assay for their efficacies as
metabolites (Clark 1979). Ironically, the Viking landers had high-pressure H2gas onboard,
but this was used only for the GCMS experiment and was not plumbed to the life-detection
modules. Thus, not all possible physiological pathways known from terrestrial microorgan-
isms were included in the Viking experiments. However, such a comprehensive approach to
a life-detection assay is a quite daunting task for a power-, volume-, and mass-limited pay-
load like the Viking Biology Instrument. As our knowledge of both Mars and of terrestrial
microbiology progresses, we should be able to design more sophisticated payloads to test
a greater diversity of microbial physiologies than were tested by Viking, but it will always
remain difficult (and perhaps impossible) to test all potential scenarios on any given mission.
Table 1lists 18 published papers that propose alternative non-biological explanations for
the Viking results. Reviewing these papers one might be left with an acute feeling that life-
detection experiments on other planetary bodies are fraught with alternative interpretations,
Tab le 1 Published non-biological explanations for the Viking results
GeX Release of O2upon humidification:
KO2;ZnO
2Ponnamperuma et al. 1977
CaO2Ballou et al. 1978
MnO2Blackburn et al. 1979
O2trapped in micropores Nussinov et al. 1978; Plumb et al. 1989
Chemisorbed H2O2Huguenin et al. 1979
O plasma Ballou et al. 1978
Activated halides Zent and McKay 1994
LR decomposition of added nutrients:
H2O2Hunten 1979; Oro and Holzer 1979
Ponnamperuma et al. 1977
Bullock et al. 1994
Peroxonitrite (NOO−
2) Plumb et al. 1989
Smectite clays Banin and Margulis 1983
O−
2Yen et a l. 2000
Lack of organics in martian soils:
UV +TiO2Chun et al. 1978;Pangetal.1982
Glow discharge from dusts Mills 1977
Feroxyhyte (δ-FeOOH) Burns 1980
UV alone Stoker and Bullock 1997
238 A.C. Schuerger, B.C. Clark
making it nearly impossible to reach a consensus on the results. However, the Viking life-
detection controversy between a biological interpretation (see papers by Levin and Straat)
and a non-biological interpretation (see papers listed in Table 1) would not have been as
difficult to resolve if all four Viking life-detection experiments (GeX, PR, LR, and GCMS)
had delivered positive, supportive, and complementary results.
Thus, perhaps the greatest lessons learned from the Viking life-detection experiments are:
(1) be prepared for unusual data in which the expectations derived from terrestrial pre-flight
experiments might not always be achieved due to unknown factors present at the planetary
test site, (2) a positive conclusion that life has been detected on another planetary body is
likely to require a wide diversity of positive results from different experimental procedures
before alternative conclusions can be ruled out, and (3) evidence for a positive detection of
life on another planet must be strong enough to rule out any abiological processes. If abi-
ological processes are as likely to explain the results as biological processes, the scientific
community will more likely use the concept of “Occam’s Razor” to accept the simpler abi-
ological explanation of the results. Thus, the burden of proof is always much greater for the
positive assertion of finding life elsewhere in the Solar System than it is for a null result.
Another example of similar lessons learned for a “life-detection claim” is the debate
on how to interpret the proposed biological signs found in the SNC meteorite ALH84001
(Thomas-Keprta et al. 2001; McKay et al. 1996). Initially, five lines of evidence were
presented by McKay et al. (1996) that supported a biological interpretation of the ob-
served objects. These were: (a) the chemistry and mineralogy of carbonate globules ob-
served in cracks within previously shocked rock, (b) presence of polycyclic aromatic hy-
drocarbons (PAHs) associated with the carbonate globules, (c) coexistence of magnetite
and iron sulfides within partially dissolved carbonates that exhibited structures similar
to inclusions found in magnetotactic terrestrial bacteria, (d) formation age of the car-
bonate globules younger than the age of the igneous rocks, and (e) SEM and TEM
images of carbonate globules with features resembling terrestrial microorganisms, bio-
genic carbonate structures, or microfossils. But since the publication of the original pa-
per in 1996, a number of other researchers have presented evidence that purely abiolog-
ical processes could explain the same results (Bradley et al. 1996; Golden et al. 2000;
Zolotov and Shock 2000). Although it is still plausible that the putative evidence for life
in ALH84001, as described by McKay et al. (1996) were created by biological processes
on Mars, it is likely that the general scientific community will remain skeptical until more
unequivocal evidence can be found in support of the biological interpretation of features in
ALH84001. In both the Viking and ALH84001 controversies, the general scientific com-
munity has opted for “abiological mechanisms” as the explanations of results because of a
built-in bias towards (a) simpler explanations (Occam’s Razor), and (b) the belief that the
burden of proof for a positive conclusion of new life must be, by necessity, very high to
avoid false positives.
The lead of the Life Detection Team on Viking, Dr. Harold Klein, had expressed the
opinion prior to the mission that a step-by-step approach to Mars, including an initial recon-
naissance of its surface state, would have been preferable to attempting detection of life on
the very first mission (Klein et al. 1976). This turned out to be more true than anyone first
imagined, and points to the importance of a thorough understanding of the chemistry, miner-
alogy, and oxidizing state of the martian surface materials as part of any future life-detection
measurements. Much has been learned, but much also remains unknown. Future missions to
the surface, such as the Phoenix, Mars Science Laboratory (MSL), and ExoMars missions
will make the first-ever direct measurements of pH, quantitative H2O content, soluble an-
ions and cations, and clay content of soils. A Mars surface sample return (MSSR) mission
Viking Biology Experiments: Lessons Learned and the Role 239
is currently on a far-distant funding horizon, and a previous finalist in the first round of
Mars Scout competitions, SCIM (Sample Collection for Investigation of Mars), was a mis-
sion which proposed to sample airborne dust and gas by skimming the middle atmosphere
of Mars and returning the samples to Earth for detailed laboratory analysis. Even if all of
these missions are successfully flown, the experimental payloads may still not perform all
the baseline measurements that will be necessary for future in situ analyses of abiotic and
putative biotic activities in martian soils.
7SoilChemistry
The surprising compositional uniformity of the soils at two Viking landing sites (Clark et
al. 1982) separated by thousands of kilometers, has been extended to all five sites now
visited by landed missions to the surface of Mars (Yen et al. 2005). This apparently uni-
versal unit of globally-distributed dust and soil particles has been produced by a combi-
nation of igneous and sedimentary processes. In spite of the extremely fine particle size
of grains in the atmosphere, with mean diameters of only a few micrometers, the soils
appear to be less chemically weathered than might be expected, with considerable com-
ponents of primary igneous minerals such as olivine and pyroxene, as well as a lack of
ferric oxyhydroxides (e.g., goethite) (Morris et al. 2006). Alteration products would be ex-
pected if aqueous processing were significant. In contradistinction to these discoveries is
the high concentrations of salts in martian soils (Clark and VanHart 1981; Yen et al. 2005;
Ming et al. 2006). However, it has been difficult to identify precise chemical forms of the
sulfates, chlorides and trace bromides that have been inferred from element enrichments of
S, Cl and Br. Detection of MgSO4mobilizations in duricrust at Viking sites (Clark et al.
1982), in trenches into Gusev plains soils (Wang et al. 2006), and in Meridiani outcrops
(Clark et al. 2005) are indicative that at least some of the sulfates in soils are in the form of
highly soluble MgSO4. Calcium and ferric sulfates have been found in certain other occur-
rences on Mars. The situation with chlorides is also problematical, and although NaCl is the
more stable form, a preponderance of Mg++ over Na+cations could implicate MgCl2.
The oxidants in martian soils (Oyama and Berdahl 1977; Hunten 1979) constituted one
of the greatest surprises of the Viking missions, but in retrospect can be explained as a small
but very important component of peroxides or possibly grain-surface oxide enrichments
which result from interaction with photochemically-produced oxidizing species in the thin
martian atmosphere which allows short wavelength solar UV to penetrate down to the sur-
face (Hunten 1979). For this reason, it is possible that the Cl and Br may be in the form
of chlorates and bromates, respectively, rather than the chlorides and bromides common on
Earth(Clarketal.2005).
This brief summary of the soil chemistries highlights the need for soil chemical analyses
carried out concomitantly with future life-detection experiments on Mars in particular, and
on all other planetary bodies in general. As discussed above, the Viking biology experiments
were fraught with difficult interpretations due in part to unexpected or inconsistent results
from the GeX, PR, LR, and GCMS experiments and, in part, due to only a rudimentary un-
derstanding of the chemistries of the soils being tested. Even today, many very fundamental
soil chemistry measurements such as pH, electrical conductivity, redox potential, bioavail-
ability of ions, and concentrations of biotoxic elements are lacking for Mars (Schuerger et
al. 2002). Most of these parameters have dramatic effects on terrestrial organisms, and are
likely to be equally important to a putative Mars microbiota. Thus, soil chemistry analy-
ses must be preformed in concert with life-detection protocols to provide the best possible
chance to accurately interpret results from the biological investigations.
240 A.C. Schuerger, B.C. Clark
8 Ecological Considerations
Several additional considerations specific to the detailed methods used for the Viking biol-
ogy experiments are warranted. First, the GeX and LR experiments were run at significantly
higher pressures (200 and ≈100 mbar, respectively) than are found on the surface of Mars.
Recent evidence suggests many common terrestrial microorganisms recovered from space-
craft might be unable to grow below 25 mbar under simulated martian conditions (Schuerger
et al. 2006; Schuerger and Nicholson 2006), and by logical extension, it might also be possi-
ble that there exists an upper pressure threshold for a putative extant martian surface micro-
biota; a threshold that might have been exceeded by the GeX and LR assays. Although it is
speculative to ascribe pressure thresholds to martian life that has not yet been observed, we
emphasize the issue of pressure to draw attention to the ecological conditions under which
a putative surface microbiota will have evolved. Thus, we suggest that future life-detection
experiments with surface samples consider that pressure may be an important ecological
factor that must be controllable in experimental hardware. Samples should be run, at least
in part, at conditions that mimic those found on the surface of present-day Mars. Further-
more, pressure has likely changed during recent obliquity cycles, rising to perhaps as much
as 25 mbar at high obliquity (Fanale et al. 1982), and, thus, pressure ranges should be tested
from at least 7–25 mbar.
The temperature ranges used in the PR, GeX, and LR experiments were all well above
0°C (+8to+15°C), and may not have reflected the temperature maximums at the two
Viking sites. Surface temperatures at both Viking sites failed to rise above 0°C during the
more than two martian years in which temperatures were measured (Kieffer 1976). Similar
to pressure, future life-detection experiments should recreate actual temperature conditions
present at the site being sampled within the incubation chambers. Both pressure and temper-
ature are briefly emphasized here, but other environmental conditions are also important in-
cluding soil pH, soil redox potential, bioavailability of ions, soil moisture, soil electrolytes,
soluble salts, and biotoxic compounds or elements in solution (Schuerger et al. 2002). In
fact, there is substantial logic behind running comprehensive soil chemistry tests at a site
prior to initiating life-detection assays in order to (a) recreate appropriate incubation condi-
tions within the life-detection payloads and (b) to enhance the interpretation of data derived
from the life-detection experiments.
For example, martian soils may be acidic, neutral or alkaline. All values of pH are pos-
sible due to the fact that most common geologic minerals are intrinsically alkaline when
reacting with H2O (from release of Mg++ and Ca++ cations), yet sulfur in the form of sul-
fates is very common on Mars. Ferric sulfates are acidic, and the formation processes for
other sulfates, from either sulfides in soils or S-containing gases released to the atmosphere,
also result in strong acidity. Thus, future experiments should not only monitor pH but also
carry sufficient reactants to modify pH to investigate optimum growth conditions for mi-
croorganisms.
Likewise, the presence of soluble salts in soils will result in different ionic strengths
depending on the relative proportions of soil and water in an incubation chamber. By mon-
itoring electrical conductivity and/or ionic content, conditions can be modified by varying
the amounts of H2O and chelators added. On Earth, various organisms are adapted for opti-
mum growth within a relatively restricted range of ionic strength, some for fresh water but
others flourishing at salt concentrations near saturation (such as peripheral biotic zones at
the Dead Sea or hypersaline regimes in salt ponds). Thus, there would be an advantage in
a life-detection payload to possess the capacity to first determine the soluble salts at a site
followed by the capability to recreate those salts and concentrations within a life-detection
incubation chamber.
Viking Biology Experiments: Lessons Learned and the Role 241
Some specific nutrients may be limiting. For example, it has been hypothesized that the
ultimate limitation on the vigor and extent of a martian ecology may be sources of nitro-
gen (Stoker et al. 1993). Although the presence of phosphorus has been well established by
in situ measurements, no techniques capable of detecting either nitrates or reduced nitro-
gen compounds in the soil have been available. These and other expected nutrients could
be added. Likewise, supplying active gases such as hydrogen (H2), methane (CH4), and/or
ammonia (NH3), none of which are currently at significant concentrations in the contempo-
raneous martian atmosphere, may stimulate metabolic responses from dormant organisms.
Finally, detailed analysis adds further doubt to the possibility of stable liquid water at the
surface of Mars (Hecht 2002; Beaty et al. 2006). Thus, the previous assays might have tar-
geted the wrong niche or may have been overly biased with respect to liquid water. Results
derived from the Viking GeX and LR experiments in particular were conducted under con-
ditions that deviated widely from the extant conditions at the martian surface. This point was
emphasized by Klein et al. (1976) when describing the PR experiments on Viking: “The (PR)
experiment is carried out under actual martian conditions, insofar as these can be attained
within the Viking spacecraft, the premise being that, if there is life on Mars, it is adapted
to martian conditions and is probably maladapted to extreme departures from those condi-
tions.” Furthermore, it is intriguing that the only Viking Biology experiment that exhibited
a weak, but significant, positive response for biology, was the PR experiment (Biemann et
al. 1976; Klein et al. 1976). Could it be possible that biology was present on Mars (i.e., the
weak PR positive result), but the enigmatic results from the GeX and LR experiments run at
higher temperatures and much higher pressures than are found at the Viking sites obscured
the biological response?
9 Conclusions
Based on what was known at the time, the Viking biology experiments were a robust set
of logical assays that failed to convince the wider scientific community that life existed at
the sampled sites. Since Viking, significant progress has been made in our understanding
of the physical, geological, and hydrological conditions on the surface of Mars; and on the
understanding of the extreme conditions in which terrestrial microbial life can survive and
grow. Although it is likely that incubation conditions for life-detection experiments on Mars
will extend into pressure, temperature, and moisture conditions not generally found on the
surface, it is not known whether these conditions will inhibit the activity of a putative Mars
microbiota. Thus, martian ecological considerations must be included in the design of future
life-detection payloads. In addition, we advocate that all life-detection experiments be ac-
companied by robust soil chemistry experiments in order to gain a concurrent understanding
of geochemical conditions in hydrated soils.
It is recognized that the list of potential experimental conditions briefly discussed above
are unlikely to be all included within a single life-detection package. However, one can eas-
ily envision several life-detection payloads on a given mission asking a range of questions
searching for an extant microbiota on Mars (e.g., there were three life-detection experiments
on Viking: PR, LR, GeX, plus organics analysis by GCMS). Thus, the primary purpose of
this brief review has been to emphasize that ecological considerations must be embraced
such that life-detection experiments are conducted, in part, under contemporaneous envi-
ronmental conditions found at the sampled site. Deviations from these norms should be
“reasonable” at first and then more extreme if life is not detected. Thus, the capacity for
maintaining a wide diversity of environmental conditions within incubation chambers in
future life-detection payloads should be emphasized.
242 A.C. Schuerger, B.C. Clark
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