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Viking Biology Experiments: Lessons Learned and the Role of Ecology in Future Mars Life-Detection Experiments


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The Viking missions to Mars landed in two areas of the northern plains, at Chryse Planitia (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), Pyrolytic 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.
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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
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, CO, NO, CH4,CO
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
B.C. Clark
Space Exploration Systems, Lockheed Martin, Denver, CO 80201, USA
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:
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
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.
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
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-
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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... Therefore, it is critical that growth experiments be conducted under conditions present at the sample site. The Viking lander gas-exchange and labeled release experiments were conducted under suboptimal Martian conditions without full consideration of the diversity of microbial physiologies ( Schuerger and Clark 2008). As such, it is unclear whether the failure to detect clear signals was due to assay conditions or the absence of life. ...
... As such, it is unclear whether the failure to detect clear signals was due to assay conditions or the absence of life. In the future, experimental design should be based upon (1) chemical analyses of returned samples, (2) Mars surface conditions (temperature, pressure, and atmosphere), and (3) our knowledge about the physiology and metabolism of microorganisms under similar conditions on Earth ( Schuerger and Clark 2008). Table 2.21 summarizes the type of samples that should be collected, and the associated measurements required in order to carry out Investigation Strategy 2.3C and move toward an assay of possible biosignatures of present Martian life. ...
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This report requested by the International Mars Exploration Working Group (IMEWG). Return of samples from the surface of Mars has been a goal of the international Mars science community for many years. Affirmation by NASA and ESA of the importance of Mars exploration led the agencies to establish the international MSR Objectives and Samples Team (iMOST). The purpose of the team is to re‐evaluate and update the sample‐related science and engineering objectives of a Mars Sample Return (MSR) campaign. The iMOST team has also undertaken to define the measurements and the types of samples that can best address the objectives. Seven objectives have been defined for MSR, traceable through two decades of previously published international priorities. The first two objectives are further divided into sub‐objectives. Within the main part of the report, the importance to science and/or engineering of each objective is described, critical measurements that would address the objectives are specified, and the kinds of samples that would be most likely to carry key information are identified. These seven objectives provide a framework for demonstrating how the first set of returned Martian samples would impact future Martian science and exploration. They also have implications for how analogous investigations might be conducted for samples returned by future missions from other solar system bodies, especially those that may harbor biologically relevant or sensitive material, such as Ocean Worlds (Europa, Enceladus, Titan) and others.
... The most promising extraterrestrial sites that could host extant microbial life are subsurface oceans on jovian satellite Europa or saturnian satellite Enceladus and also regolith on Mars (Schulze-Makuch et al., 2015;Cockell et al., 2016) (Schuerger & Clark, 2008;Schulze-Makuch et al., 2015). Consequently, the question about life on Mars remains unanswered (Bianciardi et al., 2012;Levin & Straat, 2016;Guaita, 2017). ...
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Life-detection experiments carried out in extraterrestrial locations provided inconclusive results whether processes observed were biological or chemical. In this study, the typical effect of temperature on metabolic rates is described and a life-detection method that is easy to perform is proposed. The method comprises observing changes in microbial metabolic rates after temperature shift. The method was demonstrated by experiments on aquatic microorganisms in the Gulf of Gdansk (Baltic Sea). First experiment, in which temperature was shifted within the temperature range encountered at the sampling site, demonstrated a typical Q10 coefficient (2.84). The experiment in which temperature was shifted beyond the environmental temperature range provided an unexpectedly low Q10 coefficient (1.44), which indicated that excessive temperature exerted an inhibitory effect on metabolism. This response is not expected for chemical reactions, but it is typical for biological processes. In summary, a pair of properly-tailored experiments permitted separating biological and chemical reactions.
... The results of the Viking biology experiments were equivocal-some aspects of the results were consistent with what would be expected if extant life were present in the samples, but other results were not, thus allowing for multiple interpretations (Levin and Straat, 1981;Zent and McKay, 1994;Klein, 1998;Houtkoope and Schulze-Makuch, 2007). Subsequently, scientists proposed explanations for the Viking biology experiment results that do not require the presence of life (Horowitz et al., , 1977Klein et al., 1976;Oyama et al., 1977;Ponnamperuma et al., 1977;Klein, 1978;Oyama and Berdahl, 1979;Huguenin, 1982;Plumb et al., 1989;Quinn and Zent, 1999;Schuerger and Clark, 2008;Navarro-González et al., 2010;Quinn et al., 2013;Georgiou et al., 2017). ...
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On November 5-8, 2019, the "Mars Extant Life: What's Next?" conference was convened in Carlsbad, New Mexico. The conference gathered a community of actively publishing experts in disciplines related to habitability and astrobiology. Primary conclusions are as follows: A significant subset of conference attendees concluded that there is a realistic possibility that Mars hosts indigenous microbial life. A powerful theme that permeated the conference is that the key to the search for martian extant life lies in identifying and exploring refugia ("oases"), where conditions are either permanently or episodically significantly more hospitable than average. Based on our existing knowledge of Mars, conference participants highlighted four potential martian refugium (not listed in priority order): Caves, Deep Subsurface, Ices, and Salts. The conference group did not attempt to reach a consensus prioritization of these candidate environments, but instead felt that a defensible prioritization would require a future competitive process. Within the context of these candidate environments, we identified a variety of geological search strategies that could narrow the search space. Additionally, we summarized a number of measurement techniques that could be used to detect evidence of extant life (if present). Again, it was not within the scope of the conference to prioritize these measurement techniques-that is best left for the competitive process. We specifically note that the number and sensitivity of detection methods that could be implemented if samples were returned to Earth greatly exceed the methodologies that could be used at Mars. Finally, important lessons to guide extant life search processes can be derived both from experiments carried out in terrestrial laboratories and analog field sites and from theoretical modeling.
... The results of the Viking mission's Labeled Release (LR) experiment, which were baffling, suggestive of both life and nonlife, provide an especially salient illustration of the difference between searching for alien life and searching for biologically promising anomalies. The LR biology experiment used a carbohydrate solution radioactively labeled 726 CLELAND with 14 C which was known to be metabolizable by a wide variety of cultivatable bacteria; see Schuerger and Clark (2008) for a review of the Viking biology experiments and a history of the debate over the results. If there were microbes in the martian soil, the LR team conjectured that they would behave like these bacteria and metabolize some of the organic compounds in the nutrient solution, releasing radioactively labeled 14 CO 2 gas in the process. ...
According to the 2015 Astrobiology Strategy, a central goal of astrobiology is to provide a definition of life. A similar claim is made in the 2018 CRC Handbook of Astrobiology. Yet despite efforts, there remains no consensus on a definition of life. This essay explores an alternative strategy for searching for extraterrestrial life: Search for potentially biological anomalies (as opposed to life per se) using tentative (vs. defining) criteria. The function of tentative criteria is not, like that of defining criteria, to provide an estimate (via a decision procedure) of the likelihood that an extraterrestrial phenomenon is the product of life. Instead, it is to identify phenomena that resist classification as living or nonliving as worthy of further investigation for novel life. For as the history of science reveals, anomalies are a driving force behind scientific discovery and yet (when encountered) are rarely recognized for what they represent because they violate core theoretical beliefs about the phenomena concerned. While the proposed strategy resembles that of current life-detection missions, insofar as it advocates the use of a variety of lines of evidence (biosignatures), it differs from these approaches in ways that increase the likelihood of noticing truly novel forms of life, as opposed to dismissing them as just another poorly understood abiological phenomenon. Moreover, the strategy under consideration would be just as effective at detecting forms of life closely resembling our own as a definition of life.
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There are three groups of scientists dominating the search for the origin of life: the organic chemists (the Soup), the molecular biologists (RNA world), and the inorganic chemists (metabolism and transient-state metal ions), all of which have experimental adjuncts. It is time for Clays and the Origin of Life to have its experimental adjunct. The clay data coming from Mars and carbonaceous chondrites have necessitated a review of the role that clays played in the origin of life on Earth. The data from Mars have suggested that Fe-clays such as nontronite, ferrous saponites, and several other clays were formed on early Mars when it had sufficient water. This raised the question of the possible role that these clays may have played in the origin of life on Mars. This has put clays front and center in the studies on the origin of life not only on Mars but also here on Earth. One of the major questions is: What was the catalytic role of Fe-clays in the origin and development of metabolism here on Earth? First, there is the recent finding of a chiral amino acid (isovaline) that formed on the surface of a clay mineral on several carbonaceous chondrites. This points to the formation of amino acids on the surface of clay minerals on carbonaceous chondrites from simpler molecules, e.g., CO2, NH3, and HCN. Additionally, there is the catalytic role of small organic molecules, such as dicarboxylic acids and amino acids found on carbonaceous chondrites, in the formation of Fe-clays themselves. Amino acids and nucleotides adsorb on clay surfaces on Earth and subsequently polymerize. All of these observations and more must be subjected to strict experimental analysis. This review provides an overview of what has happened and is now happening in the experimental clay world related to the origin of life. The emphasis is on smectite-group clay minerals, such as montmorillonite and nontronite.
The Quest for a Universal Theory of Life - by Carol E. Cleland September 2019
Cambridge Core - History of Astronomy and Cosmology - The Quest for a Universal Theory of Life - by Carol E. Cleland
To return samples from Mars is a major goal of the scientific community studying the origin of life, looking for extraterrestrial forms of life, and studying the planets and their evolution. NASA and ESA are preparing such a mission which is currently categorized by COSPAR as restricted Earth return. This will strongly constrain the management of the returned samples until they are certified safe. This certification will be obtained after preliminary studies conducted in specialized and dedicated facilities. This paper describes the main rationale to build and operate, at minimum, two different facilities capable of handling such samples and to perform the tests and investigations required.
Most organic matter (OM) on Earth occurs as kerogen‐like materials, that is naturally formed macromolecules insoluble with standard organic solvents. The formation of this insoluble organic matter (IOM) is a topic of much interest, especially when it limits the detection of compounds of geomicrobiological interest. For example, studies that search for biomarker evidence of life on early Earth or other planets usually use solvent‐based extractions. This leaves behind a pool of OM as unexplored post‐extraction residues, potentially containing diagnostic biomarkers. Since the IOM has an enhanced potential for preservation compared to soluble OM, analysing IOM‐released biomarkers can also provide even deeper insights into the ecology of ancient settings, with implications for early Earth and Astrobiology investigations. Here, we analyse the prokaryotic lipid biosignature within soluble and IOM of the Taupo Volcanic Zone (TVZ) silica sinters, which are key analogues in the search for life. We apply sequential solvent extractions and a selective chemical degradation upon the post‐solvent extraction residue. Moreover, we compare the IOM from TVZ sinters to analogous studies on peat and marine sediments to assess patterns in OM insolubilisation across the geosphere. Consistent with previous work, we find significant but variable proportions—1%–45% of the total prokaryotic lipids recovered—associated with IOM fractions. This occurs even in recently formed silica sinters, likely indicating inherent cell insolubility. Moreover, archaeal lipids seem more prone to insolubilisation as compared to the bacterial analogues, which might enhance their preservation and also bias overall biomarkers interpretation. These observations are similar to those observed in other settings, confirming that even in a setting where the OM derives predominantly from prokaryotic sources, patterns of IOM formation/occurrence are conserved. Differences with other settings, however, such as the occurrence of archaeol in IOM fractions, could be indicative of different mechanisms for IOM formation that merit further exploration.
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Executive Summary Return of samples from the surface of Mars has been a goal of the international Mars science community for many years. Affirmation by NASA and ESA of the importance of Mars exploration led the agencies to establish the international MSR Objectives and Samples Team ( iMOST ). The purpose of the team is to re‐evaluate and update the sample‐related science and engineering objectives of a Mars Sample Return ( MSR ) campaign. The iMOST team has also undertaken to define the measurements and the types of samples that can best address the objectives. Seven objectives have been defined for MSR , traceable through two decades of previously published international priorities. The first two objectives are further divided into sub‐objectives. Within the main part of the report, the importance to science and/or engineering of each objective is described, critical measurements that would address the objectives are specified, and the kinds of samples that would be most likely to carry key information are identified. These seven objectives provide a framework for demonstrating how the first set of returned Martian samples would impact future Martian science and exploration. They also have implications for how analogous investigations might be conducted for samples returned by future missions from other solar system bodies, especially those that may harbor biologically relevant or sensitive material, such as Ocean Worlds (Europa, Enceladus, Titan) and others. Summary of Objectives and Sub‐Objectives for MSR Identified by iMOST Objective 1 Interpret the primary geologic processes and history that formed the Martian geologic record, with an emphasis on the role of water. Intent To investigate the geologic environment(s) represented at the Mars 2020 landing site, provide definitive geologic context for collected samples, and detail any characteristics that might relate to past biologic processes This objective is divided into five sub‐objectives that would apply at different landing sites. Characterize the essential stratigraphic, sedimentologic, and facies variations of a sequence of Martian sedimentary rocks. Intent To understand the preserved Martian sedimentary record. Samples A suite of sedimentary rocks that span the range of variation. Importance Basic inputs into the history of water, climate change, and the possibility of life Understand an ancient Martian hydrothermal system through study of its mineralization products and morphological expression. Intent To evaluate at least one potentially life‐bearing “habitable” environment Samples A suite of rocks formed and/or altered by hydrothermal fluids. Importance Identification of a potentially habitable geochemical environment with high preservation potential. Understand the rocks and minerals representative of a deep subsurface groundwater environment. Intent To evaluate definitively the role of water in the subsurface. Samples Suites of rocks/veins representing water/rock interaction in the subsurface. Importance May constitute the longest‐lived habitable environments and a key to the hydrologic cycle. Understand water/rock/atmosphere interactions at the Martian surface and how they have changed with time. Intent To constrain time‐variable factors necessary to preserve records of microbial life. Samples Regolith, paleosols, and evaporites. Importance Subaerial near‐surface processes could support and preserve microbial life. Determine the petrogenesis of Martian igneous rocks in time and space. Intent To provide definitive characterization of igneous rocks on Mars. Samples Diverse suites of ancient igneous rocks. Importance Thermochemical record of the planet and nature of the interior. Objective 2 Assess and interpret the potential biological history of Mars, including assaying returned samples for the evidence of life. Intent To investigate the nature and extent of Martian habitability, the conditions and processes that supported or challenged life, how different environments might have influenced the preservation of biosignatures and created nonbiological “mimics,” and to look for biosignatures of past or present life. This objective has three sub‐objectives: Assess and characterize carbon, including possible organic and pre‐biotic chemistry. Samples All samples collected as part of Objective 1. Importance Any biologic molecular scaffolding on Mars would likely be carbon‐based. Assay for the presence of biosignatures of past life at sites that hosted habitable environments and could have preserved any biosignatures. Samples All samples collected as part of Objective 1. Importance Provides the means of discovering ancient life. Assess the possibility that any life forms detected are alive, or were recently alive. Samples All samples collected as part of Objective 1. Importance Planetary protection, and arguably the most important scientific discovery possible. Objective 3 Quantitatively determine the evolutionary timeline of Mars. Intent To provide a radioisotope‐based time scale for major events, including magmatic, tectonic, fluvial, and impact events, and the formation of major sedimentary deposits and geomorphological features. Samples Ancient igneous rocks that bound critical stratigraphic intervals or correlate with crater‐dated surfaces. Importance Quantification of Martian geologic history. Objective 4 Constrain the inventory of Martian volatiles as a function of geologic time and determine the ways in which these volatiles have interacted with Mars as a geologic system. Intent To recognize and quantify the major roles that volatiles (in the atmosphere and in the hydrosphere) play in Martian geologic and possibly biologic evolution. Samples Current atmospheric gas, ancient atmospheric gas trapped in older rocks, and minerals that equilibrated with the ancient atmosphere. Importance Key to understanding climate and environmental evolution. Objective 5 Reconstruct the processes that have affected the origin and modification of the interior, including the crust, mantle, core and the evolution of the Martian dynamo. Intent To quantify processes that have shaped the planet's crust and underlying structure, including planetary differentiation, core segregation and state of the magnetic dynamo, and cratering. Samples Igneous, potentially magnetized rocks (both igneous and sedimentary) and impact‐generated samples. Importance Elucidate fundamental processes for comparative planetology. Objective 6 Understand and quantify the potential Martian environmental hazards to future human exploration and the terrestrial biosphere. Intent To define and mitigate an array of health risks related to the Martian environment associated with the potential future human exploration of Mars. Samples Fine‐grained dust and regolith samples. Importance Key input to planetary protection planning and astronaut health. Objective 7 Evaluate the type and distribution of in‐situ resources to support potential future Mars exploration. Intent To quantify the potential for obtaining Martian resources, including use of Martian materials as a source of water for human consumption, fuel production, building fabrication, and agriculture. Samples Regolith. Importance Production of simulants that will facilitate long‐term human presence on Mars. Summary of iMOST Findings Several specific findings were identified during the iMOST study. While they are not explicit recommendations, we suggest that they should serve as guidelines for future decision making regarding planning of potential future MSR missions. The samples to be collected by the Mars 2020 (M‐2020) rover will be of sufficient size and quality to address and solve a wide variety of scientific questions. Samples, by definition, are a statistical representation of a larger entity. Our ability to interpret the source geologic units and processes by studying sample sub sets is highly dependent on the quality of the sample context. In the case of the M‐2020 samples, the context is expected to be excellent, and at multiple scales. (A) Regional and planetary context will be established by the on‐going work of the multi‐agency fleet of Mars orbiters. (B) Local context will be established at field area‐ to outcrop‐ to hand sample‐ to hand lens scale using the instruments carried by M‐2020. A significant fraction of the value of the MSR sample collection would come from its organization into sample suites, which are small groupings of samples designed to represent key aspects of geologic or geochemical variation. If the Mars 2020 rover acquires a scientifically well‐chosen set of samples, with sufficient geological diversity, and if those samples were returned to Earth, then major progress can be expected on all seven of the objectives proposed in this study, regardless of the final choice of landing site. The specifics of which parts of Objective 1 could be achieved would be different at each of the final three candidate landing sites, but some combination of critically important progress could be made at any of them. An aspect of the search for evidence of life is that we do not know in advance how evidence for Martian life would be preserved in the geologic record. In order for the returned samples to be most useful for both understanding geologic processes (Objective 1) and the search for life (Objective 2), the sample collection should contain BOTH typical and unusual samples from the rock units explored. This consideration should be incorporated into sample selection and the design of the suites. The retrieval missions of a MSR campaign should (1) minimize stray magnetic fields to which the samples would be exposed and carry a magnetic witness plate to record exposure, (2) collect and return atmospheric gas sample(s), and (3) collect additional dust and/or regolith sample mass if possible.
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Robotic spacecraft are launched with finite levels of terrestrial microorganisms that are similar to the microbial communities within facilities in which spacecraft are assembled. In particular, spores of mesophilic aerobic Bacillus species are common spacecraft contaminants considered most likely to survive interplanetary transfer to Mars. During the cruise phase to Mars, and then again during surface operations, microbial bioloads are exposed to a diversity of biocidal factors that are likely to render the microbial species either dead or significantly inhibited from active metabolic activity and replication. We report here, for the first time, that interactive effects of low pressure, low temperature, and high CO 2 atmospheres approaching conditions likely to be encountered on the martian surface strongly inhibit the growth and replication of seven common Bacillus spp. isolated from spacecraft. Tests were conducted within a small glass bell-jar system maintained in a low-temperature microbial incubator. Atmospheric pressures were controlled at 1013 (Earth-normal), 100, 50, 35, 25, or 15 mb, and temperatures were maintained at 30, 20, 15, 10, or 5 • C. Experiments were carried out for 48 h or 7 days under either Earth-normal O 2 /N 2 or pure CO 2 atmospheres. Results indicated that low pressure, low temperature, and high CO 2 atmospheres, applied separately or in combination, were capable of inhibiting the growth and replication of B. pumilus SAFR-032, B. pumilus FO-36B, B. subtilis HA-101, B. subtilis 42HS-1, B. megaterium KL-197, B. licheniformis KL-196, and B. nealsonii FO-092 under simulated martian conditions. Endospores of all seven Bacillus spp. strains failed to germinate and grow at 25 mb at 30 • C. Although, vegetative cells of these strains exhibited a slightly greater ability to replicate at lower pressures than did endospores, vegetative cells of these species failed to grow at pressures below 25 mb. Interactive effects of these environmental parameters acted to generally increase the inhibitory nature of the low-pressure conditions on growth and replication of the seven Bacillus spp. tested.
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Current planetary protection (PP) protection policy designates a categorization IVc for spacecraft potentially entering into a “special region” of Mars that requires specific constraints on spacecraft development and operations. National Aeronautics and Space Administration (NASA) requested that Mars Exploration Program Analysis Group (MEPAG) charter a Special Regions–Science Analysis Group (SR-SAG) to develop a quantitative clarification of the definition of “special region” that can be used to distinguish between regions that are “special” and “non-special” and a preliminary analysis of specific environments that should be considered “special” and “non-special.” The SR-SAG used the following general approach: Clarify the terms in the existing Committee on Space Research (COSPAR) definition; establish temporal and spatial boundary conditions for the analysis; identify applicable threshold conditions for propagation; evaluate the distribution of the identified threshold conditions on Mars; analyze on a case-by-case basis those purported geological environments on Mars that could potentially exceed the biological threshold conditions; and, furthermore, describe conceptually the possibility for spacecraft-induced conditions that could exceed the threshold levels for propagation. The following represent the results of the SRSAG study in which “special regions” are more practically defined, including a comprehensive distillation of our current understanding of the limits of terrestrial life and their relationship to relevant martian conditions. An analytical approach is presented to consider special regions with current and future improvements in our understanding. The specific findings of the SR-SAG reported in the executive summary are in bold.
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The Mössbauer spectrometer on Spirit measured the oxidation state of Fe, identified Fe-bearing phases, and measured relative abundances of Fe among those phases for surface materials on the plains and in the Columbia Hills of Gusev crater. Eight Fe-bearing phases were identified: olivine, pyroxene, ilmenite, magnetite, nanophase ferric oxide (npOx), hematite, goethite, and a Fe3+-sulfate. Adirondack basaltic rocks on the plains are nearly unaltered (Fe3+/FeT < 0.2) with Fe from olivine, pyroxene (O1 > Px), and minor npOx and magnetite. Columbia Hills basaltic rocks are nearly unaltered (Peace and Backstay), moderately altered (WoolyPatch, Wishstone, and Keystone), and pervasively altered (e.g., Clovis, Uchben, Watchtower, Keel, and Paros with Fe3+/FeT ∼ 0.6-0.9). Fe from pyroxene is greater than Fe from olivine (O1 sometimes absent), and Fe2+ from O1 + Px is 40-49% and 9-24% for moderately and pervasively altered materials, respectively. Ilmenite (Fe from Ilm ∼3-6%) is present in Backstay, Wishstone, Keystone, and related rocks along with magnetite (Fe from Mt ∼10-15%). Remaining Fe is present as npOx, hematite, and goethite in variable proportions. Clovis has the highest goethite content (Fe from Gt = 40%). Goethite (α-FeOOH) is mineralogical evidence for aqueous processes because it has structural hydroxide and is formed under aqueous conditions. Relatively unaltered basaltic soils (Fe3+/FeT ∼ 0.3) occur throughout Gusev crater (∼60-80% Fe from O1 + Px, ∼10-30% from npOx, and ∼10% from Mt). PasoRobles soil in the Columbia Hills has a unique occurrence of high concentrations of Fe3+-sulfate (∼65% of Fe). Magnetite is identified as a strongly magnetic phase in Martian soil and dust.
The Viking Landers were unable to detect evidence of life on Mars but, instead, found a chemically reactive soil capable of decomposing organic molecules. This reactivity was attributed to the presence of one or more as-yet-unidentified inorganic superoxides or peroxides in the martian soil. Using electron paramagnetic resonance spectroscopy, we show that superoxide radical ions (O2 –) form directly on Mars-analog mineral surfaces exposed to ultraviolet radiation under a simulated martian atmosphere. These oxygen radicals can explain the reactive nature of the soil and the apparent absence of organic material at the martian surface.
PRELIMINARY results of the Viking Lander 1 (VL-1) biology experiments1 revealed that humidification of the martian soil sample in the gas exchange experiment (GEX) released substantial amounts of carbon dioxide and oxygen, as well as detectable amounts of nitrogen and argon or carbon monoxide. We have reviewed the available flight data and found that, when the amounts of evolved gases were plotted in an adsorption potential plot, the amount of evolved oxygen was anomalously high compared to the other gases. This paper also describes simulation experiments with a model Mars soil provided by the Viking Inorganic Analysis Team (ICAT) and treated with a radiofrequency (RF) glow discharge in a simulated martian atmosphere. The findings indicated that the GEX simulation procedure released oxygen, carbon dioxide, and nitrogen in amounts comparable to that seen in the experiment on Mars.
The value of the sorbtional specific surface of the martian soil (from CO2 evolution in GEX (gas exchange experiments) of Viking craft) is more than an order of magnitude greater than the value of its geometrical specific surface (from granulometry). An hypothesis is therefore proposed here to explain the microporous structure of the soil grains. Absence of O2 and CO2 in GCMS (gas chromatography–mass spectrometry) (heating up to 500 °C) gives some indication of the closeness of the pores. The origin of such soil structure and the filling of pores with CO2 and O2 due to the effects of various forms of radiation are discussed. The similarity between kinetics in LR (labelled release) and GEX as well as their correspondence with the filtration curve for water vapour migrating through the soil sample suggest that both the formation and the diffusion of the gases are rapid processes. Displacement desorption by water vapour by simultaneous opening of the pores due to the Rebinder effect, is suggested as the natural mechanism for outgassing in the GEX and LR ‘Viking’ experiments.
Although some data from the Viking Lander biology experiments can be interpreted as indicative of biological activity, the existence of organisms in the martian soil samples is considered unlikely because of the non-detection of organic compounds in the sample1. Viking gas chromatography–mass spectrometer analysis detected no organic molecules above a concentration of parts per 109 (ref. 2). We consider here why no organic molecules were detected at the landing sites, whether the sterility of the two sites is representative of the entire planet and if there are locations on Mars more conducive to the formation and preservation of organics. We first simulate the destruction of organic compounds in Mars-like laboratory conditions, and then examine whether the destructive mechanism would operate planetwide; and second re-examine UV and IR reflectance spectra of Mars for any evidence of organic molecules, and in doing so set an upper limit on the organic carbon content of average martian soil. The results reveal that the average martian soil is organic-poor and makes an unfavourable habitat for life forms based on carbon chemistry. There is no reason to believe that organic molecules are preferentially preserved anywhere on the planet.
A REMARKABLE characteristic of those samples of the martian soil which have so far been analysed is the absence of carbonaceous matter down to the parts per billion (109) level1. As well as a lack of endogenous organic material there is no sign of the component expected from infalling carbonaceous meteorites. The inorganic particles constituting the fine martian soil seem to be extremely ‘clean’—far cleaner than terrestrial desert or Antarctic analogues. I suggest here that glow discharges generated by friction within dust clouds might explain this apparent absence of carbonaceous matter. In addition glow discharges might account for some reactions noted in the Viking biological experiments.
Microorganisms deep in the Martian soil could derive energy indirectly from the sun via chemical reactions involving atmospheric photolysis products of the solar ultraviolet flux. The Viking discovery of a chemically uniform regolith which, though poor in organics, is rich in sulfur-containing compounds suggests reaction sequences in which sulfur is recycled through reduced and oxidized states by biologically catalyzed reactions with photochemically-produced atmospheric constitutents. One candidate reaction, reduction of soil sulfate minerals by molecular hydrogen, is already exploited on earth by bacteria of the ubiquitous and tenaciousDesulfovibrio genus.