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Organics are expected to exist on Mars based on meteorite infall, in situ production, and any possible biological sources. Yet they have not been detected on the martian surface; are they there, or are we not capable enough to detect them? The Viking gas chromatograph-mass spectrometer did not detect organics in the headspace of heated soil samples with a detection limit of parts per billion. This null result strongly influenced the interpretation of the reactivity seen in the Viking biology experiments and led to the conclusion that life was not present and, instead, that there was some chemical reactivity in the soil. The detection of perchlorates in the martian soil by instruments on the Phoenix lander and the reports of methane in the martian atmosphere suggest that it may be time to reconsider the question of organics. The high-temperature oxidizing properties of perchlorate will promote combustion of organics in pyrolytic experiments and may have affected the ability of both Phoenix's organic analysis experiment and the Viking mass spectrometer experiments to detect organics. So the question of organics on Mars remains open. A primary focus of the upcoming Mars Science Laboratory will be the detection and identification of organic molecules by means of thermal volatilization, followed by gas chromatography-mass spectrometry--as was done on Viking. However, to enhance organic detectability, some of the samples will be processed with liquid derivatization agents that will dissolve organics from the soil before pyrolysis, which may separate them from the soil perchlorates. Nonetheless, the problem of organics on Mars is not solved, and for future missions other organic detection techniques should therefore be considered as well.
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Review Article
Organics on Mars?
Inge L. ten Kate
Organics are expected to exist on Mars based on meteorite infall, in situ production, and any possible biological
sources. Yet they have not been detected on the martian surface; are they there, or are we not capable enough to
detect them? The Viking gas chromatograph–mass spectrometer did not detect organics in the headspace of
heated soil samples with a detection limit of parts per billion. This null result strongly influenced the inter-
pretation of the reactivity seen in the Viking biology experiments and led to the conclusion that life was not
present and, instead, that there was some chemical reactivity in the soil. The detection of perchlorates in the
martian soil by instruments on the Phoenix lander and the reports of methane in the martian atmosphere suggest
that it may be time to reconsider the question of organics. The high-temperature oxidizing properties of per-
chlorate will promote combustion of organics in pyrolytic experiments and may have affected the ability of both
Phoenix’s organic analysis experiment and the Viking mass spectrometer experiments to detect organics. So the
question of organics on Mars remains open. A primary focus of the upcoming Mars Science Laboratory will be
the detection and identification of organic molecules by means of thermal volatilization, followed by gas
chromatography–mass spectrometry—as was done on Viking. However, to enhance organic detectability, some
of the samples will be processed with liquid derivatization agents that will dissolve organics from the soil before
pyrolysis, which may separate them from the soil perchlorates. Nonetheless, the problem of organics on Mars is
not solved, and for future missions other organic detection techniques should therefore be considered as well.
Key Words: Mars—Organic degradation—Search for Mars organics—Laboratory investigations—Interpretation
of planetary mission data. Astrobiology 10, 589–603.
1. Introduction
Organic molecules have been detected in the interstellar
medium (Ehrenfreund and Charnley, 2000; Millar, 2004;
and references in both), comets (see Crovisier, 2004, for a re-
view), meteorites (Botta and Bada, 2002; Sephton, 2002; and
references in both), and interplanetary dust particles (Flynn,
1996), and are highly abundant on Earth. Mars seems to be a
next logical place to search for organic material in that Mars
experienced a long history of bombardment (Marcus, 1968;
Werner, 2008) by comets, meteorites [an estimated yearly
accumulation rate of 510
to 510
meteorites greater than
10 g in mass per square kilometer (Bland and Smith, 2000)],
and dust particles. The total mass accretion rate by comets,
meteorites, and dust particles is estimated between 1.810
and 410
per year (Flynn and McKay, 1990), which
corresponds to an annual arrival of organic material at the
martian surface on the order of 10
kg (Flynn, 1996). Another
possible source of organic matter is through production on, or
beneath, the martian surface. Several mechanisms for en-
dogenous production of organic material on early Earth have
been suggested, which may have played a role on early Mars
as well, such as lightning, coronal discharge, UV radiation,
and atmospheric shocks (Chyba and Sagan, 1992). Even
though the presence of organics on Mars seems likely, based
on the above, organic matter has not yet been detected on the
martian surface. Is it there at all, or are investigators simply
incapable of detecting it? In this paper, I offer an overview of
the search for organics on Mars by the Viking mission and, to
a lesser extent, the Phoenix mission; the conclusions drawn, in
particular, from the Viking mission; and the ways in which
these conclusions have shaped laboratory studies on organics
under martian environmental conditions.
2. Before the Viking Mission
By the early 1970s, several missions had visited the planet
Mars with varying results. Data obtained by Mariner 4 in
1964 and Mariners 6 and 7 in 1969 caused pessimism about
the possibility of life on Mars (Klein, 1974). However, the
Mariner 9 orbiter and its terrestrial observations showed that
(1) Mars is a geologically active planet; (2) the 3–10 mbar
NASA Goddard Space Flight Center, Greenbelt, Maryland, and Goddard Earth Science and Technology Center, University of Maryland,
Baltimore County, Baltimore, Maryland.
Volume 10, Number 6, 2010
ªMary Ann Liebert, Inc.
DOI: 10.1089/ast.2010.0498
atmosphere is primarily composed of CO
with small
amounts of CO, O
O, and some other traces gases, in-
cluding O (Barker, 1972); (3) there is 10–20 mm precipitable
water; and (4) the surface contains about 1% H
O (Mariner
Mars, 1973; Oro
´and Flory, 1973). Present martian atmo-
spheric chemical conditions are believed to be hostile with
respect to abiotic organic synthesis, but conditions may have
been favorable in the distant past (Fanale, 1971). Laboratory
simulations have indicated that relatively simple organic
compounds could be produced by the direct interaction of
UV radiation with the martian atmosphere (Young et al.,
1965) in the presence of catalytic surfaces (Hubbard et al.,
1971, 1973). Tyson and Oyama (1973) showed that CO
be incorporated into already present amino acids under
simulated martian radiation conditions. More-complex or-
ganics are known to be present in carbonaceous chondrites
(Anders, 1964; Oro
´, 1972; Nagy, 1975), and Dycus (1969)
calculated the mass loss of meteorites that occurs while they
travel through the atmosphere and the mass of meteorites
after impact on the martian surface, his findings implying
that it is likely organic matter has been delivered to the
martian surface over the course of billions of years. Since the
Mariner missions were not designed to look for organic
material or small forms of life (such as microorganisms) and
could only scan the surface remotely, the Viking mission was
launched in 1975 to land on Mars and analyze the surface.
3. The Viking Mission
The Viking mission consisted of two spacecraft, Viking 1
and Viking 2, each composed of an orbiter and a lander and
carrying a range of instruments designed for analysis of soil,
rock, and atmosphere (Soffen, 1977). On July 20, 1976, the
Viking 1 lander landed at Chryse Planitia (22.488N, 49.978W).
The Viking 2 lander landed at Utopia Planitia (47.978N,
225.748W) on September 3, 1976. Biemann et al. (1977) de-
scribed the aim of the Viking mission as follows:
One of the major goals of the Viking mission was to find
out whether or not organic compounds exist on the sur-
face of the planet Mars and, if they do exist, to determine
their structures and measure their abundances. This
seemed important because we hoped that the nature of
martian organic molecules would provide a sensitive in-
dicator of the chemical and physical environment in
which they were formed. Furthermore, we hoped that the
details of their structures would indicate which of many
possible biotic and abiotic syntheses are occurring on
For this purpose, the Viking landers were equipped with a
biological and a molecular analysis experiment (Soffen,
3.1. The Viking molecular analysis experiment
The molecular analysis experiment, also known as the
Viking gas chromatograph–mass spectrometer (GCMS), was
designed to heat soil samples to 2008C, 3508C, and 5008C,
and analyze the gases that were released upon heating. The
molecular analysis experiment had the following goals
(Anderson et al., 1972):
(1) A qualitative and semi-quantitative determination of
the organic compounds present in the top 10 cm layer of
the surface of Mars; (2) qualitative and quantitative
composition (including isotope abundances) of the at-
mosphere at the surface; (3) semi-quantitative determi-
nation of water (and its physical state, if possible) in the
surface; and (4) whatever information on the inorganic
(mineralogical) composition of the surface that can be
obtained in the course of the measurements performed to
satisfy (1)–(3) above.
The released gases were subsequently analyzed by a gas
chromatographic column-detector system, a mass spec-
trometer with a range of 12–200 dalton, or a combination of
both in which the mass spectrometer served as a detector for
the gas chromatograph. Direct atmospheric measurements
were made with the mass spectrometer (Anderson et al.,
1972). The detection limits of the GCMS were in the parts per
billion range for compounds containing more than two car-
bon atoms and in the parts per million range for compounds
containing one or two carbon atoms (Biemann, 1979). In the
four samples taken from surface and subsurface material
from both landing sites, water (0.1–1.0 wt%), carbon dioxide
(0.05–0.6 ppm), and some organics were detected, including
benzene and toluene. Furthermore, Viking 1 detected traces
of chloromethane at 15 ppb at 2008C and Viking 2 detected
dichloromethane at 0.04–40 ppb at 200–5008C. However,
these chlorohydrocarbons were all considered to be terres-
trial contaminants, although they had not been detected
at those levels in the blank runs. Therefore, the conclusion
was drawn that no organic compounds of martian origin
were detected (Biemann et al., 1976, 1977).
3.2. The Viking biology investigations
The Viking biology instrument was equipped with three
experiments that could test four different hypotheses: (i) an
active martian metabolism is limited by the availability of
water; (ii) biological activity is best detected under condi-
tions that approximate those on Mars; (iii) an active martian
metabolism (the chemical reactions in a cell that convert
‘‘food’’ into energy) is heterotrophic (in need of an external
source of ‘‘food’’) and uses a very dilute aqueous solution
of simple organic compounds; and (iv) an active martian
metabolism is heterotrophic and uses a concentrated mixture
of many organic compounds (Klein, 1977). The Viking bi-
ology package included a gas exchange (GEx) experiment
(Oyama and Berdahl, 1977) that measured the production of
, and O
and the uptake of CO
by soil
samples; a pyrolytic release (PR) or carbon assimilation ex-
periment (Horowitz et al., 1977) that measured the incorpo-
ration of carbon-14 (
C, radioactive carbon) from
CO or
into organic matter; and a labeled release (LR) ex-
periment (Levin and Straat, 1977) that measured the pro-
duction of
C labeled gas upon addition of nutrient containing
C labeled organics. Table 1 gives an overview of the three
experiments and their results.
The GEx experiment consisted of an 8.5 cm
sample cell that
could be heated and two gas chromatographic columns with
two thermal conductivity detectors, which could measure
, Ne, N
, Ar or CO, NO, CH
, Kr, CO
O, and H
(Oyama et al., 1976; Oyama and Berdahl, 1977). Soil samples
of *1cm
were incubated in the presence of Mars atmo-
sphere in the cell that was filled up to 200 mbar with a test
gas composed of additional CO
, Kr, and He. This higher
pressure was necessary for the gas to be sampled into the gas
chromatographic columns. The experiment tested two hypo-
theses and an aqueous nutrient solution described in Oyama
et al. (1976). The first hypothesis was based on the assump-
tion that nutrients were already present in the martian soil
and that martian organisms only needed water to grow.
Therefore, in the humid, non-nutrient mode a small amount
(*0.5 cm
) of the nutrient solution was added in such a way
that the solution would not touch the soil but would only
saturate the atmosphere with water. This way the soil would
only be humidified by the atmospheric water in the cell, and
no interaction with the nutrients would take place. The at-
mosphere was then analyzed to measure the generation and
loss (desorption) of gases. Nonbiological activity was dif-
ferentiated from biological activity by repetitive charges of
fresh medium and prolonged incubation cycles (Oyama,
1972). The second hypothesis assumed that organisms on
Mars are heterotrophic. This was tested in the wet mode,
where enough (*2cm
) nutrient solution was added to the
soil sample to wet the soil and let it interact with the nutrient.
The samples were incubated for 0.1 to 103 sols (martian days;
0.1 to 106 Earth days). During the incubation, the gas chro-
matographs analyzed the atmosphere in the cell to measure
the production of CO
, and O
and the uptake
of CO
by soil samples (Oyama and Berdahl, 1977). The re-
sults showed that, upon both humidification and wetting of
the soil samples, an immediate release of N
, and Ar
was measured. The release of N
, and Ar was associ-
ated with surface desorption caused by water, and the O
release with decomposition of superoxides (O
2) inferred to
be present in the soil (Ballou et al., 1978). When fresh nutrient
solution and test gas were added to the wet samples, only
additional CO
was released, but at a rate that decreased
with each recharge (Oyama and Berdahl, 1977). If microor-
ganisms had been present, a recharge of the samples with
fresh nutrient solution would have led to a similar gas pro-
duction, assuming they had survived the first dose of nu-
trient. This CO
evolution in the wet samples was, therefore,
thought to have come from the oxidation of organics present
in the nutrient by g-Fe
in the surface samples (Oyama and
Berdahl, 1977). A control experiment with a sterilized sam-
ple, which was heated to 1458C for 3.5 h, was conducted and
also showed the release of O
. From these results, the overall
conclusion clearly excluded a biological explanation of the
results (Oyama and Berdahl, 1977).
The PR experiment or carbon assimilation was designed to
detect life in the martian soil by measuring the incorpora-
tion of carbon-14 from
CO or
into organic matter
Table 1. The Viking Biology Experiment
Experiment Measurement Nutrients added
added Illumination Results
Incorporation of
C from
CO or
organic matter
None None Light and
C yield
None Trace Light and
Heating to 908C has
hardly any effect.
Heating to 1758C
reduced yield by
Gas Exchange Production of CO
, and O
the uptake of CO
by soil samples
None Moist Dark Release of some CO
, and Ar
Rapid release of O
upon humidification
Concentrated solution
of organic and
inorganic compounds
Wet Dark
Same as moist
Additional CO
upon recharge
Labeled Release Production of
gas upon addition of
nutrient containing
C-labeled organics
Dilute solution
of simple organic
Moist Dark
C-bearing gas produced
Heating to 188C had
no effect.
Heating to 40–508C
slowed production.
Heating to 1608C stopped
Storage at 108C for
4 months stopped
Adapted from Klein, 1977.
Horowitz et al., 1972.
Hubbard, 1976.
Horowitz et al., 1976, 1977.
Oyama and Berdahl, 1977.
Levin and Straat, 1976a.
Levin and Straat, 1976b.
Levin and Straat, 1977.
Levin and Straat, 1979.
(Horowitz et al., 1972). The experiment was based on the
hypothesis that potential martian life, like terrestrial life, is
carbon based and exchanges carbon with the atmosphere. The
experiment consisted of a closed sample cell with a window
and a heater, a trapping column, and a radiation detector. A
sample was inserted into the sample cell together with cap-
tured martian atmosphere, which was enriched with *20 ml
CO and
(ratio 2:98). Some samples were illumi-
nated with a xenon lamp during incubation to simulate sun-
light, while others were left in the dark. After an incubation
time of 120 h, the cell was heated to 1208C and vented to
remove the nonreacted
CO and
. The cell was then
heated to 6358C to pyrolyze potential organic compounds and
CO and
that was adsorbed onto the soil. This
gas passed through a column, which trapped potential or-
ganics, but the adsorbed
CO and
, the ‘‘pyrolysis
,’’ continued into the radiation detector for analysis. By
heating the trap to 7008C, the ‘‘trapped organics’’ were re-
leased and oxidized into CO
, and subsequently analyzed in
the radiation detector as well. If
CO and
were as-
similated by martian microorganisms, they would appear in
the analysis of the ‘‘trapped organics’ (Horowitz et al., 1972,
1977). The results showed that the amount of fixed carbon-14
was small compared to terrestrial standards and that illumi-
nated samples seemed to have produced the highest carbon-
14 yields. Furthermore, multiple samples from the same scoop
of soil were analyzed: one sample directly after retrieval and
the others after a period of storage at elevated temperatures.
Data from these samples showed that heating the surface
material to 908C for nearly 2 h had no effect on the reaction,
but heating to 1758C for 3 h reduced the reaction by nearly
90% (Horowitz et al., 1976, 1977; Hubbard, 1976). Initially, the
results of the PR experiments were explained as weak but
significant presumptive positives (Horowitz et al., 1976).
However, later laboratory experiments designed to reproduce
and further explain these results appeared to rule out a bio-
logical explanation. In combination with results from the other
biological as well as the molecular analysis experiments, it
was concluded that it is unlikely that the reaction is biological
(Horowitz et al., 1977).
The LR experiment was based on radiorespirometry, a
technique in which a
C radioactively labeled nutrient so-
lution is added to a sample, in this case martian soil, in a
closed environment (sample cell), and any radioactive car-
bon dioxide subsequently detected in the atmosphere in the
cell is used as evidence for the presence of life (see below).
The experiment consisted of four incubation cells, a nutrient
reservoir, and two solid-state beta detectors, and was de-
signed to measure carbon-14 produced by potential micro-
organisms (Levin and Straat, 1976a). The assumptions on
which the experiment was based were (Levin, 1972) (a) al-
though possible life on Mars may not be limited to micro-
organisms, the latter must be present to accomplish the
biodegradation required for recycling of the organic matter;
(b) the biochemical reactions at the cellular level are aqueous;
(c) the organisms assimilate compounds from their envi-
ronment and produce gas as an end product; (d) such com-
pounds include any or all of the following: relatively simple
carbon molecules, ions, sulfate. First LR results showed—at
both landing sites—rapid evolution of radioactive counts
upon addition of the nutrient to a fresh surface sample (Le-
vin and Straat, 1976b). Analysis of a second set of samples
showed a similar behavior. In summary, the initial produc-
tion of gas from the LR nutrient was uniform; when the
reaction approached completion, addition of more nutrient
resulted in a net loss of the carbon-14 labeled gas. Further-
more, when the samples were heated to moderate tempera-
tures (40–508C) the reaction slowed down, whereas raising
the temperature to 1608C caused the reaction to end. In
contrast, exposure to 188C for 2 sols (1 sol ¼1 martian
day &24.6 Earth hours) did not appear to affect the reaction
(Levin and Straat, 1977). The final experiments, which in-
volved two soil samples that were stored at the spacecraft at
*108C for several months, showed that the reactions had
strongly diminished (Levin and Straat, 1979). The first con-
clusion drawn from these experiments was that ‘‘the results
are consistent with a biological response and also greatly
narrow the number of possible chemical reactants’’ (Levin
and Straat, 1977). Later, laboratory experiments showed that
addition of hydrogen peroxides might have led to similar
results as measured on the martian surface. These laboratory
results, however, were only obtained in the presence of
certain minerals, some of which are not found on Mars, and
assumed a much higher hydrogen peroxide content than had
been measured up to then in the martian atmosphere. This
led to the conclusion that ‘‘the presence of a biological agent
on Mars must still be considered’’ (Levin and Straat, 1981).
3.3. Overall conclusions
The main conclusions drawn from the experiments were
that there is neither organic material nor life present on the
surface of Mars at the two Viking landing sites. ‘‘The molecular
analysis experiment has demonstrated the absence of organic
compounds in the surface material at two Viking sites. This
fact has had a very important impact on the interpretation of
the data received from the biology experiments (Klein, 1977;
Klein et al., 1976) and put some boundaries on the chemical
and physical environment at the surface’’ (Biemann, 1979). The
biology experiments were then interpreted by Klein (1977) as
follows: ‘‘For each experiment, except for the LR experiment,
we must conclude that there were no organisms present within
the limits of detectability for these experiments and that all of
the observed reactions for these were the results of nonbio-
logical phenomena.’’ However, Klein (1977) then continued,
‘‘we must not overlook the fact, in assessing the probabilities of
life on Mars, that all of our experiments were conducted under
conditions that deviated to varying extents from ambient
martian conditions, and while we have accumulated data,
these and their underlying mechanisms may all be coincidental
and not directly relevant to the issue of life on that planet.’’
4. Now What?
Although there were many indications that pointed to-
ward the presence of at least organic material on the surface
of Mars (as described in the Introduction), Viking did not
find them. Absence of evidence, however, is not evidence of
absence, and a wealth of papers has argued for reasons of
nondetection of organics by this mission. These papers can
roughly be divided into two categories, (1) studies that fo-
cused on the ability of the Viking instruments and techniques
to detect organic matter on the martian surface, and (2)
laboratory simulations in which the effects of Mars’ envi-
ronmental conditions, as well as soil composition and min-
eralogy, on organic matter are studied (see Table 2 for an
overview). In 2009, two significant events brought new evi-
dence to the search for organics on Mars.
(a) Instruments on the Phoenix lander detected perchlo-
rates in the martian soil (Hecht et al., 2009).
(b) New results (Mumma et al., 2009) on the presence of
methane in the martian atmosphere confirmed earlier
observations (Formisano et al., 2004; Geminale et al.,
2008) that this methane varies both geographically and
seasonally, implying a faster production and destruc-
tion rate than assumed before.
These events, described in more detail in Section 5, spurred a
new wave of laboratory investigations. In Section 6, I will
first describe the studies that focus on the ability of the Vi-
king instruments and techniques to detect organic matter on
the martian surface. In Section 7, I will give an overview of
the laboratory studies in which organic material was inves-
tigated under simulated martian conditions; and, in Section
8, I will make some concluding remarks. Another category of
research that evolved after Viking focused on microorgan-
isms and potential life on Mars. These studies fall outside the
scope of this paper; for overviews of this category see, for
example, Jensen et al. (2008) and Schuerger and Clark (2008).
5. Methane and Perchlorates
5.1. Methane
In 2004, Formisano et al. (2004) and Krasnopolsky et al.
(2004) published works on the detection of methane in the
martian atmosphere, although they used different methods
and obtained different mixing ratios. Formisano et al. (2004)
measured a variable amount of 0–30 ppbv, with a global
average of 10 5 ppbv from orbit (the Planetary Fourier
Spectrometer, on board Mars Express), whereas Krasno-
polsky et al. (2004) measured 10 3 ppbv from an Earth-
based telescope. The photochemical lifetime of methane was
modeled to be 300–600 years (Wong et al., 2003), a lifetime
also used by Krasnopolsky et al. (2004) to explain their ob-
servations. The potential variation in the observations of
Formisano et al. (2004) is, however, more difficult to explain
photochemically and points toward localized sources or
sinks. Additional Planetary Fourier Spectrometer observa-
tions showed an even stronger seasonal variation, which
could not be explained photochemically (Geminale et al.,
2008). Earth-based observations by Mumma et al. (2009)
confirmed this seasonal variation and showed a mixing ratio
of nearly 0 ppb in the early northern spring to 45 ppb in the
late northern summer. Assuming a cyclic, and not sporadic,
seasonal cycle, Mumma et al. (2009) derived a production
(from equinox to summer) and loss (from summer to equi-
nox) rate of methane of 0.5 kg s
. However, photochemistry
alone is not enough to explain the seasonal variation of
methane. Modeling results have suggested that dust-induced
electrochemistry can significantly increase the destruction
rate of methane, both via direct dissociation and the en-
hanced production of OH- and H- (Farrell et al., 2006). At
electric fields above 10 kV m
, the destruction of methane is
more efficient through electrochemistry than photochemis-
try. Both destruction processes have, however, been sug-
gested not to be effective enough to explain the seasonal
variation, which suggests an extraordinarily harsh environ-
ment for the survival of organics on the planet (Lefe
`vre and
Forget, 2009). Methane-producing processes at work on the
martian surface may explain the methane observation by
Mumma et al. (2009). The presence of biology was suggested
by Krasnopolsky et al. (2004) as a possible methane-producing
process, but there are numerous abiological processes to
consider as well. Recent volcanism (Neukum et al., 2004)
could be a minor source, although only a few hot spots have
been observed by the Thermal Emission Imaging System
(THEMIS) on the Mars Odyssey orbiter (Christensen et al.,
2003, 2005), and sulfur dioxide, the gaseous counterpart of
volcanic methane, has only been observed in trace amounts
(Krasnopolsky, 2005). Another process could be the serpen-
tinization of ultramafic crust whereby methane is formed in
the presence of limited amounts of water and CO
, with FeO
as a catalyst (Palandri and Reed, 2004). Serpentinization re-
quires temperatures of 40–908C, which are believed to occur
at a depth of as little as 2 km, and stable liquid water, which
occurs at depths of 2–20 km (Oze and Sharma, 2005; Atreya
et al., 2007). Exogenous sources of methane, from comets and
meteorites, have also been considered (Court and Sephton,
2009). Court and Sephton (2009) used pyrolysis to simulate
the ablation and pyrolysis of carbonaceous micrometeorites
upon atmospheric entry and showed that this process pro-
duces twice as much methane as the measured free methane
present in the CM2 carbonaceous chondrite, Murchison. This
source, however, produces less than 10 kg of methane an-
nually, a mass far below that required to maintain the
abundance of methane observed in the atmosphere of Mars.
5.2. The Phoenix lander and the detection
of perchlorates
In the late northern spring, the Phoenix lander touched
down on Mars on May 25, 2008, inside the arctic circle at
68.228N, 234.258E, on a valley floor covered by the Scandia
Formation, a deposit that surrounds the northern margin of a
shield volcano named Alba Patera (Smith et al., 2009). The
main goal of Phoenix was to verify the presence of subsur-
face water ice (Smith et al., 2008). With regard to the search
for organics on Mars, the most important instrument on
Phoenix was the Thermal and Evolved Gas Analyzer (TEGA)
instrument. TEGA consists of a Thermal Analyzer containing
eight separate sample ovens, and a mass spectrometer with a
mass range of 2–140 dalton, called the Evolved Gas Analyzer
(Boynton et al., 2001). Five samples and two blanks were
analyzed by TEGA during the Phoenix mission (Arvidson
et al., 2009). The most interesting constituents found in these
samples were CaCO
(Boynton et al., 2009), H
O (Smith et al.,
2009), and O
(Hecht et al., 2009). TEGA did not detect any
organic material (Ming et al., 2009; Sutter et al., 2009). A
second instrument on the Phoenix lander was the Wet
Chemistry Laboratory (Hecht et al., 2008; Kounaves et al.,
2009), which was designed to add water and specified salts
to a sample of the martian soil and then measure the sam-
ple’s dissolved ionic components and properties; its chemical
and mineralogical composition, including sulfates; and its
acidity, metal content, and redox pairs (Smith et al., 2008).
One of the most interesting finds of the Wet Chemistry Lab-
oratory was the detection of perchlorates (ClO
4) in the
martian soil, at least some of which were in the form of
Table 2. Laboratory Simulations on Organics on Mars
Atmospheric composition (%) Solar radiation
Year Sample
Temperature (8C) Pressure
(mbar) CO
Ar O
He (nm) Light source
addition Oxides Reference
1979 Adenine, glycine,
on quartz
Quartz tubes 10 to 25
0.001 - 100 - - - 200–300 Mercury-
- - Oro
´and Holzer,
1979 Olivine Pyroxene Mars
22 to 0 1000
- - - - 100 - - <67.8 mbar (to
be frozen
et al., 1979
1982 Murchison
Pyrex flasks R.T.
3.4 - - - 100 - UV-visible
- - Pang et al., 1982
1997 Palagonite Glycine Mars Jars R.T. 100 95.59 - 4.21 0.11 - 210–710 Xenon - - Stoker and
Bullock, 1997
1998 Tholins
Humic acid
23 to þ10
------ - - H
et al., 1998
2000 Labradorite 30 6 Mars gas
254 (peak) Mercury - - Yen et al., 2000
0.34; 3.4;
- - - 100 -
2005 Glycine D-alanine Mars
R.T. 410
- - - - - 190–325 Deuterium - - ten Kate et al.,
120–180 Hydrogen
2006 Glycine D-alanine Mars
63 and R.T. 10
and 7 99.9 - - - - 190–325 Deuterium - - ten Kate et al.,
2006 Salten Skov
JSC Mars-1
25 *10
- - - - - 190–325 Deuterium - - Garry et al.,
63 7 99.9
2006 Amino acids Pyrex glass
R.T. 1,000
- 100 - - - 0.50–
5.42 MGy
Co - - Kminek and
Bada, 2006
2008 ATP Mars
80, 10,
and þ20
7.1 Mars gas
200–280 Xenon-arc - - Schuerger
et al., 2008
2009 Brines with amino
acids, with and
without iron
Silica glass
vessels in
135 to þ40 7–15 95.3 2.7 1.6 0.13 - 250–700 Xenon-arc In suspension - Johnson and
Pratt, 2010
2009 Aqueous solutions
of organics on
R.T. 196 1,000
air 355 Nd:YAG
laser (6 ns)
In suspension Goethite,
Shkrob and
2009 Carboxylic
Glass jar 65 10
- - - - - 190–250 Xenon-arc - - Stalport et al.,
2010 Phenanthrene,
and benzoic
10 R.T. 6.9 95.5 2.7 1.6 0.13 - 200–280
1,500 V
0.03% in
- Hintze et al.,
2010 Aqueous
of organics
on minerals
196 1000
100 355 Nd:YAG
(6 ns)
et al., 2010
Adapted from Jensen et al., 2008.
Constant temperature.
Ambient terrestrial atmospheric pressure.
R.T. ¼room temperature *208C.
Applied doses: 0.50, 1.00, 2.05, 5.42 MGy.
or Ca(ClO
(Hecht et al., 2009). These findings
were further endorsed by the detection of O
by TEGA,
which evolved at temperature ranges consistent with the
thermal decomposition of perchlorates (Hecht et al., 2009).
This perchlorate finding is particularly interesting because,
although perchlorate does not readily oxidize organics under
martian conditions, the low water activity associated with
such a strongly desiccating substance may inhibit many
forms of life. The high-temperature oxidizing properties of
perchlorate will, however, promote combustion of organics
in pyrolytic experiments (Hecht et al., 2009), which could
have affected the ability of both Phoenix’s TEGA experiment
and the Viking mass spectrometer experiments to detect or-
ganics (Biemann, 2007; Ming et al., 2009).
6. The Suitability of Thermal Volatilization
Gas Chromatography–Mass Spectrometry
for the Detection of Organics on Mars
After the results from the Viking mission were analyzed
and did not directly show the presence of organics or life on
the surface of Mars, a debate ensued as to whether the in-
struments on Viking were the best choice for finding or-
ganics on Mars, either from an instrument sensitivity point of
view or a technique point of view. Biemann (1979) stated the
following: ‘‘First, the question arises, whether the instru-
ments indeed worked properly. Fortunately, experimental
data exists which demonstrates this proper function beyond
any doubt.’’ However, he also added that
certain types of organic material can not be detected
by the molecular analysis (thermal volatilization GCMS)
experiment, which consisted of heating the sample to
5008C in several steps followed by the analysis of organic
compounds evolved by volatilization or pyrolysis. Any
compound that would be stable at 5008C such as highly
cross-linked polymers, would probably not produce de-
tectable material. Compounds of the opposite behavior,
namely one that decomposes upon heating into certain
molecules, which the Viking GCMS can not detect, would
also remain unnoticed (Biemann, 1979).
Benner et al. (2000) suggested that organic compounds that
arrive on Mars through meteoritic delivery, such as alkanes,
alkylbenzenes, naphthalene and higher polycyclic aromatic
hydrocarbons, kerogen, and amino acids and hydroxyacids,
are most likely to be converted to carboxylic acid derivatives.
These compounds would not be easily detected by thermal
volatilization GCMS because they are either nonvolatile or
generate CO
, CO, and H
O, which are present in the mar-
tian atmosphere and therefore difficult to distinguish. At-
tempts to reproduce the Viking LR results led to conclusions
that the mechanism for the decomposition of organics in
Yungay soils is different from the processes observed in the
Viking LR experiment (Quinn et al., 2007) but also that a
mysterious oxidant in the martian soil that evolved oxygen
when humidified in the LR experiment might have been
of biological origin (Houtkooper and Schulze-Makuch,
2007). Viking GCMS-like experiment results (Navarro-
´lez et al., 2006) suggest that the presence of iron oxides
or their salts, or both, in other analyzed soil samples (e.g.,
jarosites from Rı
´o Tinto and Panoche Valley) caused the in-
trinsic organic material to oxidize to carbon dioxide (CO
which drastically attenuates the detection of organics. An-
other result from this study, which was that small amounts
of organics would fall below the detection limit of the Viking
GCMS, was refuted, however, because the equipment used
was a thousand times less sensitive than the Viking GCMS
and the experimental methods different from the Viking
methods (Biemann, 2007). In
˜iguez et al. (2009) further in-
vestigated the usefulness of thermal volatilization for organic
detection on Mars and found that there are two sources of
strong oxidizers in palagonite soils: hydroxyl radicals and
oxygen atoms. These strong oxidizers completely decom-
posed low levels of stearic and mellitic acids present in the
soil samples, which suggests that thermal volatilization
may not be the best method for organic detection on Mars.
´lez et al. (2009) showed that, when organics
are present in the soil at levels above 1500 ppm, several
characteristic organic fragments can be detected by thermal
volatilization mass spectrometry; however, when the levels
are below <150 ppm, thermal volatilization oxidizes them,
and no organic fragments are released. The most recent ex-
periments that have taken into account both Viking and
Phoenix data showed that, upon heating of Atacama soils
mixed with 1 wt% of magnesium perchlorate, all the organics
present were decomposed to water and carbon dioxide, but
that a small amount was chlorinated and formed chlor-
omethane and dichloromethane at 5008C (Navarro-Gonza
et al., 2010). ‘‘Re-interpretation of the Viking results therefore
suggests 0.1% perchlorates and 1.5–6.5 ppm organic carbon
at the landing site 1, and 0.1% perchlorates and 0.7–2.6 ppm
organic carbon at the landing site 2’’ (Navarro-Gonza
´lez et al.,
7. Laboratory Investigations of Organics on Mars
A few years after Viking, several processes were sug-
gested that could have led to the nondetection of organic
material. These processes can roughly be divided into four
categories: (i) the potential presence of some oxidizing agents
in the martian soil, (ii) the potential effects of electrification
and glow discharge, (iii) degradation by UV and other ra-
diation, and (iv) the photocatalytic effects caused by inter-
action of radiation with mineral surfaces. Table 2 shows an
overview of the studies performed that involve either one or
a combination of these effects.
7.1. Oxidization
The presence of an oxidizing source in the martian soil could
have destroyed organics. Klein (1979) suggested that a
thermally labile oxidant such as H
, which is known to
destroy organics, could have caused the evolution of CO
the LR experiments by reacting with the formate (CHOO
in the nutrient solution that was part of the experiment. This
could be produced due to photochemical reactions in
the martian atmosphere (Hunten, 1979) but also by frost
weathering (interaction of minerals with H
O frost) of olivine
(Huguenin et al., 1979; Huguenin, 1982). Oyama and Berdahl
(1979) determined that g-Fe
, which has been suggested to
be present on the martian surface (Hargraves et al., 1976),
could both oxidize organics directly as well as catalyze oxi-
dation through H
. This could explain the slow CO
duction in the LR and GEx experiments and the CO
production in the LR experiment. Ponnamperuma et al.
(1977) had earlier obtained similar results by showing
production upon adding Viking nutrient mixture to g-Fe
On the other hand, there were also arguments against H
Levin and Straat (1981) proposed that H
reacted also with
compounds in nutrients other than only formate, which
suggests that the H
hypothesis did not account for the
fact that only one compound in LR was oxidized to CO
Furthermore, they found that there should by at least 2 wt%
of H
in the soil to reproduce the LR results. This was
considered to be doubtful because there was hardly any
detected in the atmosphere (Hanel and Maguire, 1980,
in Levin and Straat, 1981), and H
has a short lifetime
s) against UV destruction.
A different approach suggested that the GEx and LR re-
sults may have been caused not by oxidation of organics but
by the presence of clays, such as smectites, in the martian
soil. These clays may have absorbed compounds from the
atmosphere that were released in the GEx experiment, and
their catalytic surfaces may have helped in decomposing the
formate in the LR nutrient, which would have released
the atmosphere in the LR experiment (Banin and Rishpon,
1979; Banin and Margulies, 1983).
In 1994, Zent and McKay offered that in the most realistic
model the martian soil contains oxidants produced by het-
erogeneous chemical reactions with a photochemically pro-
duced atmospheric oxidant. However, most compounds
with the capacity to oxidize H
, as seen in the GEx
experiment, are thermally labile or unstable against reduc-
tion by atmospheric CO
, and the oxidants most often sug-
gested to explain the LR experiment, including H
, are
expected to decompose rapidly under martian UV (Zent and
McKay, 1994). The interaction between absorbed H
O, (ferric
and ferrous) iron, and H
leads to very efficient radical
production through the (photo-)Fenton reaction (Spacek
et al., 1995; Southworth and Voelker, 2003; Mo
¨hlmann, 2004;
Zhang et al., 2005), although this mechanism still needs the
presence of H
. Other oxidizing mechanisms that are not
based on H
have been proposed as well. Oxygen radicals
2) can be formed by the interaction of UV with Mars-
analog minerals both at martian temperatures (308C) and
room temperature in the presence of oxygen, either in a
Mars-like mixture or as pure oxygen with a partial pressure
comparable to that of the oxygen in the martian atmosphere
(Yen et al., 2000). In Mars analog environments on Earth, for
example, the Atacama Desert, dry acids can be produced
photochemically in the atmosphere from SO
and NO
cursors, which then adsorb onto dust and soil surfaces. High
relative humidity at night can then trigger oxidative acid soil
reactions. These soil acids are expected to play a significant
role in the oxidizing nature of the soils, the formation of
mineral surface coatings, and the chemical modification of
organics in the surface material (Quinn et al., 2005). Garry
et al. (2006) reached a similar conclusion based on experi-
ments that showed a decrease in intrinsic amino acid content
of cold martian soil analogues (608C) upon UV irradiation
in a CO
atmosphere with trace amounts of residual water.
In contrast, experiments (Mancinelli, 1989; McDonald et al.,
1998) have shown that an extract of terrestrial soil organics as
well as organic macromolecules, such as tholins and humic
acid, subjected to a H
O solution do not decrease
measurably, either at 208C or at low temperatures (588C
and 1238C), and might be stable against oxidation on the
martian surface, at least in the polar regions, over the entire
history of Mars.
7.2. Glow discharge
A second mechanism for organic degradation could be the
formation of electrostatically generated glow discharge plasmas
in martian dust storms, which could alter the local atmo-
spheric chemistry on Mars to produce reactive species, such
as hydrogen peroxide, and breakdown species, such as or-
ganic compounds (Mills, 1977; Oyama and Berdahl, 1979). In
Earth’s atmosphere, anhydrous electrical currents are gen-
erated in dust devils and dust storms (Freier, 1960; Crozier,
1964; Ette, 1971). In terrestrial dust devils, dust particles
generate and transfer electric charge through collisions with
each other and with the surface (Ette, 1971; Eden and Von-
negut, 1973; Farrell et al., 2003). In this process, smaller
particles get charged negatively and are eddied up in dust
storms, whereas larger grains become positively charged and
stay close to the surface (Ette, 1971; Gierasch and Goody,
1973; Melnik and Parrot, 1998; Farrell et al., 2003; Krauss
et al., 2003). This displacement in grain charge types creates a
dust storm–sized electric dipole moment, which results in
the development of coherently varying electric fields that
extend well outside the dust storm. Field strengths near
500 V m
at a distance of many 10s of meters from the
features have been measured (Freier, 1960; Crozier, 1964). In
desert tests that combined electrical, magnetic, and meteo-
rological measurements, electric fields were found to be co-
herent, and large scale, and to exceed 20 kV m
et al., 2004; Renno et al., 2004). Unsaturated electric fields near
120 kV m
were measured from dust devils in the Mojave
Desert ( Jackson and Farrell, 2006), and simple saltating
grains were found to generate electric fields exceeding
160 kV m
(Schmidt et al., 1998). These coherent electric
fields from dust devils are not impulsive ‘‘discharge fields’’
or lightning, but long-lasting electrostatic fields associated
with the buildup of vertical, well-separated charge centers in
the feature. Discharges occur when these electrostatic fields
become anomalously large, which creates ‘‘breakdown’
conditions and leads to increased impulsive electron flow.
It is anticipated, with Earth as an analogy, that martian
dust storms are also electrical in nature, as long as (1) vertical
winds exist to mass stratify grains and (2) the lifted grains
vary in size and composition [required for efficient grain-
grain charge generation (Desch and Cuzzi, 2000)]. Both re-
quired conditions exist with the easily lifted iron mineral/
basalt grain mix on Mars. Melnik and Parrot (1998) modeled
these electrical processes in dust storms and found that the
macroscopic electrostatic fields within a martian dust cloud
could reach breakdown levels of *20 kV m
. The analytical
model of Farrell et al. (2003) compared the development of a
terrestrial and a martian dust storm of similar sizes and
found that both would ultimately develop comparable elec-
tric field strengths (>20 kV m
), with the martian storm’s
temporal development about 15% slower due to current
leakage into the more conductive martian atmosphere.
Modeling has suggested that this dust-induced electro-
chemistry can significantly increase the destruction rate of
methane, both via direct dissociation and the enhanced
production of OH- and H- (Farrell et al., 2006). Furthermore,
Delory et al. (2006) found that under near-breakdown con-
ditions the dissociation of H
O via electron collisions pro-
duces negative ions (OH- and H-) at much higher rates than
photochemical processes, which will lead to a *200 times
larger subsequent production of H
(Atreya et al., 2006).
Within this context, Hintze et al. (2010) subjected five organic
compounds—phenanthrene, octadecane, octadecanoic acid,
decanophenone, and benzoic acid—to a glow discharge
plasma in a simulated martian atmosphere. The plasma
contained cations and excited neutral species, including
carbon dioxide, carbon monoxide, and nitrogen. After ex-
posure to the plasma, oxidized, higher-molecular-weight
versions of the parent compounds containing carbonyl, hy-
droxyl, and alkenyl functional groups were identified.
7.3. UV degradation
The third proposed organic destruction mechanism fo-
cuses on the relatively high UV flux into short wavelength
ranges (190–400 nm, compared to 290–400 nm on the Earth’s
surface) that may efficiently destroy organic compounds.
´and Holzer (1979) exposed adenine, glycine, naphtha-
lene, and the Murchison meteorite to UV in the presence of
varying amounts of oxygen and measured their decompo-
sition. Adenine and glycine were found to be more stable
than naphthalene, and the degradation rates increased with
the amount of oxygen present. Another mechanism could
be titanium oxide–catalyzed photooxidation, where the or-
ganics are oxidized through interaction with a combination
of UV radiation, gaseous oxygen (present at 0.13% in the
martian atmosphere), and a catalyzing surface, such as TiO
which is also present on Mars (Chun et al., 1978; Pang et al.,
1982). Stoker and Bullock (1997) UV irradiated samples in
which glycine was mixed with a Mars analogue (palagonite
from the Mauna Kea Volcano on Hawaii). They measured
the headspace gases over the samples during irradiation for
traces of methane, assuming that the destruction of one
glycine molecule produced one methane molecule. They
found destruction rates of 2.24 1.210
g of carbon m
per year when scaled to martian surface conditions and
concluded that the surface of Mars should be depleted of
organics, based on the annual influx of organic material
(Flynn, 1996). Ten Kate et al. (2005, 2006) also studied the
effects of UV radiation on glycine as well as on alanine, in the
form of thin layers of the pure amino acids. They found as
well that glycine and alanine have a short lifetime on the
martian surface, with half-lives of 22 5 h for glycine and
31 h for alanine (*500 and *81 h, respectively, in the lab),
when extrapolated to martian surface conditions. Further-
more, they found that these rates slowed down by a factor of
*10 when the samples were cooled to an average Mars’
surface temperature of 608C. A similar approach was used
by Stalport et al. (2009), who irradiated cold (558C) thin
films of mellitic, benzoic, and oxalic acids in vacuum. The
half-lives of benzoic and oxalic acid as observed in the lab are
0.8 0.2 and 1.8 0.5 h, respectively, which is considerably
shorter than that of the amino acids studied by ten Kate et al.
(2005, 2006). Mellitic acid, on the other hand, produces a UV
radiotolerant compound identified as benzenehexacarboxylic
acid–trianhydride (C
). This result led the authors to
conclude that, in spite of the eventual presence of oxidation
processes and UV radiation on the martian surface, com-
pounds produced by photolysis of benzenecarboxylic acids
(such as mellitic acid) may have accumulated into the mar-
tian regolith (Stalport et al., 2009). This conclusion is in line
with predictions by Benner et al. (2000). Schuerger et al.
(2008) studied the response of thin films of adenosine tri-
phosphate (ATP), a molecule essential for life, to UV irradi-
ation in a Mars-like atmosphere. They found a half-life for
ATP of *4 days in the lab, which corresponds to *22 sols
(&22.6 Earth days) when extrapolated to martian surface
conditions. In the same facility, Hintze et al. (2010) exposed
the same five organic compounds as used in their glow
discharge experiments—phenanthrene, octadecane, octade-
canoic acid, decanophenone, and benzoic acid—to Mars-like
UV radiation in a simulated martian environment. The
UV degradation reactions also produced oxidized, higher-
molecular-weight compounds as compared to the starting
materials; however, this process seemed to be less efficient
than glow discharge.
Besides UV, other ranges of the solar spectrum could af-
fect the martian surface as well. UV radiation will penetrate
only a few microns into the martian surface, which implies
that organics are safe from UV radiation when buried under
even a thin layer of soil (e.g., ten Kate et al., 2005). However,
cosmic rays will produce a higher dose-rate than they pro-
duce on Earth, since the martian atmospheric column density
at normal incidence is only 16–27 g cm
compared to
Earth’s atmospheric shield of 1000 g cm
, which is equiva-
lent to a depth of 9 m below ground on Mars (Clark, 1979).
Other studies have suggested that influence of galactic cos-
mic rays and solar energetic particles reaches much deeper
(several meters), compared to UV (Pavlov et al., 2002; Dart-
nell et al., 2007a, 2007b). Laboratory experiments in which
amino acids were irradiated with grays indicated that amino
acids buried in the first meter of the martian subsurface
would be destroyed through radiolysis. Below a radiation
shielding depth of 400–500 g cm
, amino acids would not be
degraded substantially (Kminek and Bada, 2006).
7.4. The interaction of UV with organics absorbed
onto minerals
Recent studies have not only focused on the effect of UV
by itself but also on the catalytic effects mineral surfaces
could have on the interaction of UV with organic matter, a
process already suggested by Chun et al. (1978) and Pang
et al. (1982). Apart from radiolytic oxidation, racemization
[the process in which one enantiomer of an amino acid
converts to the other, e.g., L-alanine to D-alanine, until a 1:1
mixture is reached (Johnson and Pratt, 2010)] has been pro-
posed. Experiments involving five amino acids indicated that
the rates of racemization in the presence of metals in solution
are an order of magnitude slower than degradation through
radiolytic oxidation, and both are several orders of magni-
tude faster than previously reported by Bada and Schroeder
(1975), Snider and Wolfenden (2000), and Li and Brill
(2003). Shkrob and Chemerisov (2009) studied UV (355 nm)–
induced reactions of carboxylic, hydroxycarboxylic, and
aminocarboxylic acids, carboxylated aromatics, and R-amino
acids and peptides adsorbed on the hydrated metal oxides
anatase (TiO
), goethite (R-FeOOH), and hematite (R-Fe
at low (196 to 738C) and room (228C) temperature. The
UV wavelength of 355 nm was chosen because at this
wavelength the light is completely absorbed by the oxides
and no direct photolysis of the organics takes place. They
showed that the main photodegradation path in these ex-
periments is decarboxylation catalyzed by the metal oxides.
This photocatalytic decarboxylation is inconsistent with the
Benner et al. (2000) hypothesis that chemical evolution of the
organic component of the soil results in the accumulation of
stable, nonvolatile carboxylated and polycarboxylated mol-
ecules in martian soil. This led the authors to suggest that
there may be no ‘‘safe haven’’ for organics on Mars. Further
analysis of these experiments, however, indicated that
during the process of photocatalytic decarboxylation both
and CH
form via the photo-Kolbe reaction (Shkrob et
al., 2010). In the sequence of processes suggested in this
work, the ultimate end product of organics delivered by
meteorites on the martian surface is CH
. Therefore, these
reactions may account for seasonally variable methane
production (Shkrob et al., 2010).
8. Conclusion
After 45 years of research on organic matter on the surface
of Mars, which included two missions that could have de-
tected organics, Viking and Phoenix, it is still unclear as
to whether organic matter is present on Mars. Work by
´lez et al. (2006, 2009, 2010) suggested that
thermal volatilization gas chromatography–mass spectrom-
etry, the method used on both missions, may not be the best
method to look for trace amounts of organics, because or-
ganics can be destroyed upon heating in a pyrolysis unit.
Organic matter should be detectable by thermal volatiliza-
tion mass spectrometry (e.g., ten Kate et al., 2010). Very recent
work by Shkrob and Chemerisov (2009), and Shkrob et al.
(2010), suggested that active removal processes take place
that could explain the observation that the martian surface is
depleted of organics, as well as the production of the CH
detected in the martian atmosphere. The best way to deter-
mine whether there are organics on Mars would be to look
for them in situ. The upcoming Mars Science Laboratory
scheduled for launch in the fall of 2011 is equipped with the
Sample Analysis at Mars (SAM) instrument suite (Mahaffy,
2008). A primary focus of SAM will be the detection and
identification of organic molecules by way of thermal vola-
tilization gas chromatography–mass spectrometry. To en-
hance organic detectability by the GCMS, some of SAM’s
sample cups are filled with derivatization agents that will
dissolve organics from the soil before pyrolysis and make
them more volatile (Mahaffy, 2008). Two derivatization
methods to be used both react with amines, acids, and al-
cohols. Moreover, one of the methods is more suitable for
free organics, such as amino acids (Buch et al., 2006), the
other for bonded macromolecules. The effects of perchlorates
on these derivatization steps are currently under investiga-
tion ( J.L. Eigenbrode, D.P. Glavin, personal communication).
Even after a successful Mars Science Laboratory, future
missions should incorporate instruments that enable organic
detection. For these future missions, other organic detection
techniques should therefore be considered as well [e.g.,
capillary electrophoresis (Skelley et al., 2005)].
The author would like to thank Chris McKay and Sherry
Cady for the invitation to write this review paper and for
their helpful reviewer comments. Furthermore, many thanks
to Michael Chesnes from the Goddard Library for his tre-
mendous help in providing essential articles on very short
ATP, adenosine triphosphate; GCMS, gas chromato-
graph–mass spectrometer; GEx, gas exchange; LR, labeled
release; PR, pyrolytic release; SAM, the Sample Analysis at
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Address correspondence to:
Inge L. ten Kate
NASA Goddard Space Flight Center
Greenbelt, MD 20771
Submitted 7 May 2010
Accepted 29 May 2010
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The iconic Viking Landers that landed on Mars in 1976 demonstrated that the Martian surface is an extreme place, dominated by high UV fluxes and regolith chemistry capable of oxidizing organic molecules. From follow-on missions , we have learned that Mars was much warmer and wetter in its early history, and even some areas of Mars (such as crater lakes, possibly with sustained hydrothermal activity) were habitable places (e.g. Grotzinger et al. (2014). Science (New York, N.Y.) 343; Mangold et al. (2021). Science (New York, N.Y.). However, based on the Viking results we have learnt that the search for life and its remains is challenged by abiotic breakdown and alteration of organic material. In particular, the harsh radiation climate at the Martian surface that directly and indirectly could degrade organics has been held accountable for the lack of organics in the Martian regolith. Recent work simulating wind-driven erosion of basalts under Mars-like conditions has shown that this process, comparable to UV-and ionizing radiation, produces reactive compounds, kills microbes and removes methane from the atmosphere. and thereby could equally jeopardize the success of life-seeking missions to Mars. In this review, we summarize and discuss previous work on the role of physical and chemical mechanisms that affect the persistence of organics, and their consequences for the detection of life and/or its signatures in the Martian regolith and in the atmosphere.
The degradation of glycine (Gly), proline (Pro), and tryptophan (Trp) was studied under simulated Mars conditions during UV-driven production of oxychlorines and compared under Mars ambient and humid conditions, as films, and with addition of sodium chloride (NaCl), sodium chlorate (NaClO3), and sodium perchlorate (NaClO4) salts. It was shown that glycine sustained no significant destruction in either of the non-salt samples under Mars ambient or humid conditions. However, its degradation increased in the presence of any of the three salts and under both conditions though more under humid conditions. Proline degradation followed the order No Salt > NaCl > NaClO3 > NaClO4 under Mars ambient conditions but the reverse order under Mars humid conditions. A mechanism is proposed to explain how water and silica participate in these degradation reactions and how it is strongly influenced by the identity of the salt and its ability to promote deliquescence. No difference was observed for tryptophan between Mars ambient and humid conditions, or for the different salts, suggesting its degradation mechanism is different compared to glycine and proline. The results reported here will help to better understand the survival of amino acids in the presence of oxychlorines and UV on Mars and thus provide new insights for the detection of organic compounds on future Mars missions.
Traces of life may have been preserved in ancient martian rocks in the form of molecular fossils. Yet the surface of Mars is continuously exposed to intense UV radiation detrimental to the preservation of organics. Because the payload of the next rovers going to Mars to seek traces of life will comprise Raman spectroscopy tools, laboratory simulations that document the effect of UV radiation on the Raman signal of organics appear critically needed. The experiments conducted here evidence that UV radiation is directly responsible for the increase of disorder and for the creation of electronic defects and radicals within the molecular structure of S-rich organics such as cystine, enhancing the contribution of light diffusion processes to the Raman signal. The present results suggest that long exposure to UV radiation would ultimately be responsible for the total degradation of the Raman signal of cystine. Yet because the degradation induced by UV is not instantaneous, it should be possible to detect freshly excavated S-rich organics with the Raman instruments on board the rovers. Alternatively, given the very short lifetime of organic fluorescence (nanoseconds) compared to most mineral luminescence (micro- to milliseconds), exploiting fluorescence signals might allow the detection of S-rich organics on Mars. In any case, as illustrated here, we should not expect to detect pristine S-rich organic compounds on Mars, but rather by-products of their degradation.
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The search for, and characterisation of, organic matter on Mars is central to efforts in identifying habitable environments and detecting evidence of life in the martian surface and near surface. Iron oxides are ubiquitous in the martian regolith and are known to be associated with the deposition and preservation of organic matter in certain terrestrial environments, thus iron oxide-rich sediments are potential targets for life detection missions. The most frequently used protocol for martian organic matter characterisation (also planned for use on ExoMars) has been thermal extraction for the transfer organic matter to gas chromatography-mass spectrometry detectors. For the effective use of thermal extraction for martian samples it is necessary to explore how potential biomarker organic molecules evolve during this process in the presence of iron oxides. We have thermally decomposed iron oxides simultaneously with (z)-octadec-9-enoic and n-octadecanoic acids and analysed the products via pyrolysis-gas chromatography – mass spectrometry. We found that the thermally driven dehydration, reduction and recrystallization of iron oxides transformed fatty acids. Overall detectability of products greatly reduced, molecular diversity decreased, unsaturated products decreased and aromatisation increased. The severity of this effect increased as reduction potential of the iron oxide, and inferred free radical formation, increased. Of the iron oxides tested haematite showed the least transformative effects, followed by magnetite, goethite, then ferrihydrite. It was possible to identify the saturation state of the parent carboxylic acid at high (0.5 wt. %) concentrations by the distribution of n-alkylbenzenes in the pyrolysis products. When selecting life detection targets on Mars, localities where haematite is the dominant iron oxide could be targeted preferentially, otherwise thermal analysis of carboxylic acids, or similar biomarker molecules, will lead to enhanced polymerisation, aromatization and breakdown that will reduce the fidelity of the original biomarker, similar to changes normally observed during thermal maturation.
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The search for molecular biosignatures at the surface of Mars is complicated by an intense irradiation in the mid- and near-ultraviolet (UV) spectral range for several reasons: (i) many astrobiologically relevant molecules are electronically excited by efficient absorption of UV radiation and rapidly undergo photochemical reactions; (ii) even though the penetration depth of UV radiation is limited, aeolian erosion continually exposes fresh material to radiation; and (iii) UV irradiation generates strong oxidants such as perchlorates that can penetrate deep into soils and cause subsurface oxidative degradation of organics. As a consequence, it is crucial to investigate the effects of UV radiation on organic molecules embedded in mineral matrices mimicking the martian soil, in order to validate hypotheses about the nature of the organic compounds detected so far at the surface of Mars by the NASA Mars Science Laboratory’s (MSL) Curiosity rover, as well as organics that will be possibly found by the next rover missions Mars 2020 (NASA) and ExoMars 2022 (ESA-Roscosmos). In addition, studying the alteration of possible molecular biosignatures in the martian environment will help to redefine the molecular targets for life detection missions and devise suitable detection methods. Here we report the results of mid- and near-UV irradiation experiments of Mars soil analog samples obtained adsorbing relevant organic molecules on a clay mineral that is quite common on Mars, i.e. montmorillonite, doped with 1 wt% of magnesium perchlorate. Specifically, we chose to investigate the photostability of a plausible precursor of the chlorohydrocarbons detected on Mars by the Curiosity rover, namely phthalic acid, along with the biomarkers of extant life L-phenylalanine and L-glutamic acid, which are proteomic amino acids, and adenosine 5’-monophosphate, which is a nucleic acid component. We monitored the degradation of these molecules adsorbed on montmorillonite through in situ spectroscopic analysis, investigating the reflectance properties of the samples in the Near InfraRed (NIR) spectral region. Such spectroscopic characterization of molecular alteration products provides support for two upcoming robotic missions to Mars that will employ NIR spectroscopy to look for molecular biosignatures, through the instruments SuperCam on board Mars 2020, ISEM, Ma_Miss and MicrOmega on board ExoMars 2022.
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The discovery and subsequent investigations of atmospheric oxygen on Mars are reviewed. Free oxygen is a biomarker produced by photosynthesizing organisms. Oxygen is reactive and on Mars may be destroyed in 10 years and is continually replenished. Diurnal and spring/summer increases in oxygen have been documented, and these variations parallel biologically induced fluctuations on Earth. Data from the Viking biological experiments also support active biology, though these results have been disputed. Although there is no conclusive proof of current or past life on Mars, organic matter has been detected and specimens resembling green algae / cyanobacteria, lichens, stromatolites, and open apertures and fenestrae for the venting of oxygen produced via photosynthesis have been observed. These life-like specimens include thousands of lichen-mushroom-shaped structures with thin stems, attached to rocks, topped by bulbous caps, and oriented skyward similar to photosynthesizing organisms. If some of these specimens are fossilized is unknown. However the evidence of so many different types of life-like specimens make it almost indisputable that there is life on Mars. The overall body of evidence indicates are likely producing and replenishing atmospheric oxygen. Abiogenic processes might also contribute to oxygenation via sublimation and seasonal melting of subglacial water-ice deposits coupled with UV splitting of water molecules; a process of abiogenic photosynthesis that could have significantly depleted oceans of water and subsurface ice over the last 4.5 billion years; and, which would have provided moisture to these Martian organisms and their ancestors.
Anticipated human missions to Mars require a methodical understanding of the unconsolidated bulk sediment that mantles its surface, given its role as an accessible resource for water and as a probable substrate for food production. However, classifying martian sediment as soil has been pursued in an ad-hoc fashion, despite emerging evidence from in situ missions for current and paleo-pedological processes. Here we find that in situ sediment at Gusev, Meridiani and Gale are consistent with pedogenesis related to comminuted basalts mixing with older phyllosilicates – perhaps of pluvial origin -- and sulfates. Furthermore, a notable presence of hydrated amorphous phases indicates significant chemical weathering that mirrors pedogenesis at extreme environments on Earth. Effects of radiation and reactive oxygen species are also reminiscent of such soils at Atacama and Mojave. Some related phases, like perchlorates and Fe-sulfates, may sustain brine-driven weathering in modern martian soils. Meanwhile, chemical diversity across in situ and regional soils suggests many different soil types and processes. But the two main soil classification systems –the World Reference Base for Soil Resources (WRB) and the U.S. Soil Taxonomy – only inadequately account for such variability. While WRB provides more process insight, it needs refinement to represent variability of martian soils even at the first level of categorical detail. That will provide a necessary reference for future missions when identifying optimal pedological protocols to systematically survey martian soil. Updating Earth-based soil classification systems for this purpose will also advance soil taxonomy as a research field.
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Perchlorate (ClO₄⁻) was discovered in Martian soil by the Phoenix lander, with important implications for potential Martian biology, photochemistry, aqueous chemistry, and the chlorine cycle on Mars. Perchlorate was subsequently reported in both loose sediment and bedrock samples analyzed by the Sample Analysis at Mars instrument onboard the Curiosity rover in Gale crater based on a release of O₂ at 200–500°C. However, the continually wet paleoenvironment recorded by the sedimentary rocks in Gale crater was not conducive to the deposition of highly soluble salts. Furthermore, the preservation of ancient perchlorate to the modern day is unexpected due to its low thermodynamic stability and radiolytic decomposition associated with its long exposure to radioactivity and cosmic radiation. We therefore investigate alternative sources of O₂ in Sample Analysis at Mars analyses including superoxides, sulfates, nitrate, and nanophase iron and manganese oxides. Geochemical evidence and oxygen release patterns observed by Curiosity are inconsistent with each of these alternatives. We conclude that perchlorate is indeed the most likely source of the detected O2 release at 200–500°C, but contend that it is unlikely to be ancient. Rather than being associated with the lacustrine or early diagenetic environment, the most likely origin of perchlorate in the bedrock is late stage addition by downward percolation of water through rock pore space during transient wetting events in the Amazonian. The conclusion that the observed perchlorate in Gale crater is most likely Amazonian suggests the presence of recent liquid water at the modern surface.
The Quest for a Universal Theory of Life - by Carol E. Cleland September 2019
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The surface of Mars, unshielded by thick atmosphere or global magnetic field, is exposed to high levels of cosmic radiation. This ionising radiation field is deleterious to the survival of dormant cells or spores and the persistence of molecular biomarkers in the subsurface, and so its characterisation is of prime astrobiological interest. Here, we present modelling results of the absorbed radiation dose as a function of depth through the Martian subsurface, suitable for calculation of biomarker persistence. A second major implementation of this dose accumulation rate data is in application of the optically stimulated luminescence technique for dating Martian sediments. We present calculations of the dose-depth profile in the Martian subsurface for various scenarios: variations of surface composition (dry regolith, ice, layered permafrost), solar minimum and maximum conditions, locations of different elevation (Olympus Mons, Hellas basin, datum altitude), and increasing atmospheric thickness over geological history. We also model the changing composition of the subsurface radiation field with depth compared between Martian locations with different shielding material, determine the relative dose contributions from primaries of different energies, and discuss particle deflection by the crustal magnetic fields.
Carbon delivered to the Earth by interplanetary dnst particles may have been an important source of pre-biotic organic matter (Anders, 1989). Interplanetary dust is shown to deliver an order-of-magnitude higher surface concent rat on of carbon onto Mars than onto Earth, suggesting interplanetary dust may be an important source of carbon on Mars as well.
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.
Carbonaceous meteorites, whose falls have been witnessed during the past century and a half, have not been heated in their interiors during their passage through the atmosphere. They may contain twenty per cent water and seven per cent of organic matter. Modern analytical techniques have shown the latter to consist of hydrocarbons, fatty and aromatic acids, porphyrins, and an aromatic polymer-like substance. Interest in carbonaceous meteorites has been further stimulated by the discovery that they possess clearly defined microstructures. The author discusses the theories that have been advanced to account for the origin of these structures and for their organic content.
The Labeled Release extraterrestrial life detection experiment onboard the Viking spacecraft is described as it will be implemented on the surface of Mars in 1976. This experiment is designed to detect heterotrophic life by supplying a dilute solution of radioactive organic substrates to a sample of Martian soil and monitoring for evolution of radioactive gas. A significantly attenuated response by a heat-sterilized control sample of the same soil would confirm a positive metabolic response. Experimental assumptions as well as criteria for the selection of organic substrates are presented. The Labeled Release nutrient has been widely tested, is versatile in eliciting terrestrial metabolic responses, and is stable to heat sterilization and to the long-term storage required before its use on Mars. A testing program has been conducted with flight-like instruments to acquire science data relevant to the interpretation of the Mars experiment. Factors involved in the delineation of a positive result are presented and the significance of the possible results discussed.
We observed Mars near the peak of the strongest SO2 band at 1364-1373 cm-1 with resolving power of 7×104 using the Texas Echelon Cross Echelle Spectrograph on the NASA Infrared Telescope Facility. The spectrum shows absorption lines of three CO2 isotopomers and three H2O isotopomers. The water vapor abundance derived from the HDO lines assuming D/H = 5.5 times the terrestrial value is 13±1.0 pr. μm, in agreement with the simultaneous MGS/TES observations of 14 pr. μm at the latitudes (50°S to 10°N) of our observation. Summing of spectral intervals at the expected positions of eleven SO2 lines puts a 2σ upper limit on SO2 of 0.8 ppb. SO2 may be emitted into the martian atmosphere by seepage and is removed by three-body reactions with OH and O. The SO2 lifetime, 2 years, is longer than the global mixing time 0.5 year, so SO2 should be rather uniformly distributed across Mars. Seepage of SO2 is less than 14,000 tons per year on Mars which is smaller than the volcanic production of SO2 on the Earth by a factor of 700. CH4/SO2 is typically 10-4 - 10-3 in volcanic gases on the Earth, and this does not support seepage as a possible source of the recently discovered methane on Mars and makes even more plausible its biogenic origin. Possible productions of ethane and propane are weaker than that of methane, and these gases should be additionally depleted photochemically by factors of 25 and 250 relative to methane on Mars.
The Thermal and Evolved Gas Analyzer (TEGA) on the Mars Polar Lander spacecraft is composed of two separate components which are closely coupled: a Differential Scanning Calorimeter (DSC) and an Evolved Gas Analyzer (EGA). TEGA has the capability of performing differential scanning calorimetry on eight small (0.038 mL) soil samples selected in the vicinity of the lander. The samples will be heated in ovens to temperatures up to 950°C, and the volatile compounds water and carbon dioxide, which are released during the heating, will be analyzed in the EGA. The power required by the sample oven is continuously monitored during the heating and compared to that required to heat simultaneously a similar, but empty, oven. The power difference is the output of the DSC. Both endothermic and exothermic phase transitions can be detected, and the data can be used in the identification of the phases present. By correlating the gas release with the calorimetry, the abundance of the volatile compounds associated with the different phases can be determined. The EGA may also be able to detect the release of oxygen associated with any superoxide that may be on the surface of the soil grains. The instrument can detect the melting of ice in the DSC down to abundances on the order of 0.2% of the sample, and it can detect the decomposition of calcite, CaCO3, down to abundances of 0.5%. Using the EGA, TEGA can detect small amounts of water, down to 8 ppm in the sample, and it can detect the associated release of CO2 down to the equivalent abundances of 0.03%. The EGA also has the ability to determine the 13C/12C ratio in the evolved CO2, but it is not clear if the accuracy of this ratio will be sufficient to address the scientific issues.
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 oxidative destruction of phenol, cyclohexanol (CyOH), and 4-nitroaniline (4-NA) in aqueous solution by the photo-Fenton reaction and TiO2/UV-A is described and compared. The decomposition mechanisms were investigated by studying the intermediate products by HPLC and a photodiode array. The degradation of 4-NA was monitored by GC/MS to trace the decomposition products (nitrobenzene, p-benzoquinone, hydroquinone and resorcine). During the photochemical degradation of 4-NA nitrate ions were detected by an ion selective electrode. The degradation rates with the photo-Fenton reaction had the following order: Phenol > 4-NA >> CyOH, in contrast to the order CyOH > Phenol > 4-NA obtained with TiO2/UV-A. The same intermediate decomposition products were obtained for each substrate by both methods.