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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
Abstract
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
2
to 510
5
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
5
and 410
4
gm
2
per year (Flynn and McKay, 1990), which
corresponds to an annual arrival of organic material at the
martian surface on the order of 10
6
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.
ASTROBIOLOGY
Volume 10, Number 6, 2010
ªMary Ann Liebert, Inc.
DOI: 10.1089/ast.2010.0498
589
atmosphere is primarily composed of CO
2
with small
amounts of CO, O
2
,H
2
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
2
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
2
can
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
Mars.
For this purpose, the Viking landers were equipped with a
biological and a molecular analysis experiment (Soffen,
1977).
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
CO
2
,N
2
,CH
4
,H
2
, and O
2
and the uptake of CO
2
by soil
samples; a pyrolytic release (PR) or carbon assimilation ex-
periment (Horowitz et al., 1977) that measured the incorpo-
ration of carbon-14 (
14
C, radioactive carbon) from
14
CO or
14
CO
2
into organic matter; and a labeled release (LR) ex-
periment (Levin and Straat, 1977) that measured the pro-
duction of
14
C labeled gas upon addition of nutrient containing
14
C labeled organics. Table 1 gives an overview of the three
experiments and their results.
The GEx experiment consisted of an 8.5 cm
3
sample cell that
could be heated and two gas chromatographic columns with
two thermal conductivity detectors, which could measure
H
2
, Ne, N
2
,O
2
, Ar or CO, NO, CH
4
, Kr, CO
2
,N
2
O, and H
2
S
(Oyama et al., 1976; Oyama and Berdahl, 1977). Soil samples
of *1cm
3
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
2
, Kr, and He. This higher
590 TEN KATE
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
3
) 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
3
) 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
2
,N
2
,CH
4
,H
2
, and O
2
and the uptake
of CO
2
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
2
,CO
2
,O
2
, and Ar
was measured. The release of N
2
,CO
2
, and Ar was associ-
ated with surface desorption caused by water, and the O
2
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
2
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
2
evolution in the wet samples was, therefore,
thought to have come from the oxidation of organics present
in the nutrient by g-Fe
2
O
3
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
2
. 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
14
CO or
14
CO
2
into organic matter
Table 1. The Viking Biology Experiment
a
Experiment Measurement Nutrients added
Water
added Illumination Results
Pyrolytic
Release
Incorporation of
14
C from
14
CO or
14
CO
2
into
organic matter
b
None None Light and
dark
Small
14
C yield
None Trace Light and
dark
Heating to 908C has
hardly any effect.
Heating to 1758C
reduced yield by
90%.
c,d,e
Gas Exchange Production of CO
2
,N
2
,
CH
4
,H
2
, and O
2
and
the uptake of CO
2
by soil samples
f
None Moist Dark Release of some CO
2
,
N
2
, and Ar
Rapid release of O
2
upon humidification
f
Concentrated solution
of organic and
inorganic compounds
Wet Dark
Same as moist
Additional CO
2
release
upon recharge
f
Labeled Release Production of
14
C-labeled
gas upon addition of
nutrient containing
14
C-labeled organics
g
Dilute solution
of simple organic
compounds
Moist Dark
14
C-bearing gas produced
Heating to 188C had
no effect.
Heating to 40–508C
slowed production.
Heating to 1608C stopped
production.
Storage at 108C for
4 months stopped
production.
h,i,j
a
Adapted from Klein, 1977.
b
Horowitz et al., 1972.
c
Hubbard, 1976.
d,e
Horowitz et al., 1976, 1977.
f
Oyama and Berdahl, 1977.
g
Levin and Straat, 1976a.
h
Levin and Straat, 1976b.
i
Levin and Straat, 1977.
j
Levin and Straat, 1979.
ORGANICS ON MARS? 591
(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
of
14
CO and
14
CO
2
(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
14
CO and
14
CO
2
. The cell was then
heated to 6358C to pyrolyze potential organic compounds and
release
14
CO and
14
CO
2
that was adsorbed onto the soil. This
gas passed through a column, which trapped potential or-
ganics, but the adsorbed
14
CO and
14
CO
2
, the ‘‘pyrolysis
CO
2
,’’ continued into the radiation detector for analysis. By
heating the trap to 7008C, the ‘‘trapped organics’’ were re-
leased and oxidized into CO
2
, and subsequently analyzed in
the radiation detector as well. If
14
CO and
14
CO
2
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
14
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-
592 TEN KATE
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
1
. 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
1
, 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
2
, 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
3
(Boynton et al., 2009), H
2
O (Smith et al.,
2009), and O
2
(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
ORGANICS ON MARS? 593
Table 2. Laboratory Simulations on Organics on Mars
a
Atmospheric composition (%) Solar radiation
Year Sample
Incubation
method
Temperature (8C) Pressure
(mbar) CO
2
N
2
Ar O
2
He (nm) Light source
Water
addition Oxides Reference
1979 Adenine, glycine,
naphthalene
on quartz
Murchison
Quartz tubes 10 to 25
b
0.001 - 100 - - - 200–300 Mercury-
xenon
- - Oro
´and Holzer,
1979
1979 Olivine Pyroxene Mars
chamber
22 to 0 1000
c
- - - - 100 - - <67.8 mbar (to
be frozen
onto
sample)
Huguenin
et al., 1979
1982 Murchison
TiO
2
Pyrex flasks R.T.
b,d
3.4 - - - 100 - UV-visible
>200
Not
mentioned
- - 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
Eppendorf
tubes
23 to þ10
b
1000
c
------ - - H
2
O
2
McDonald
et al., 1998
2000 Labradorite 30 6 Mars gas
mixture
254 (peak) Mercury - - Yen et al., 2000
0.34; 3.4;
23,700
- - - 100 -
2005 Glycine D-alanine Mars
chamber
R.T. 410
6
- - - - - 190–325 Deuterium - - ten Kate et al.,
2005
120–180 Hydrogen
2006 Glycine D-alanine Mars
chamber
63 and R.T. 10
7
and 7 99.9 - - - - 190–325 Deuterium - - ten Kate et al.,
2006
2006 Salten Skov
JSC Mars-1
Mars
chamber
25 *10
5
- - - - - 190–325 Deuterium - - Garry et al.,
2006
63 7 99.9
2006 Amino acids Pyrex glass
vials
R.T. 1,000
c
- 100 - - - 0.50–
5.42 MGy
e
60
Co - - Kminek and
Bada, 2006
2008 ATP Mars
chamber
80, 10,
and þ20
7.1 Mars gas
mixture
200–280 Xenon-arc - - Schuerger
et al., 2008
594
2009 Brines with amino
acids, with and
without iron
Silica glass
vessels in
Mars
chamber
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
minerals
Suprasil
tubes
R.T. 196 1,000
c
air 355 Nd:YAG
pulsed
laser (6 ns)
In suspension Goethite,
hematite,
anatase
Shkrob and
Chemerisov,
2009
2009 Carboxylic
acids
Glass jar 65 10
2
- - - - - 190–250 Xenon-arc - - Stalport et al.,
2009
2010 Phenanthrene,
octadecane,
octadecanoic
acid,
decanophenone,
and benzoic
acid
Mars
chamber
10 R.T. 6.9 95.5 2.7 1.6 0.13 - 200–280
1,500 V
Xenon-arc
Glow
discharge
0.03% in
atmosphere
- Hintze et al.,
2010
2010 Aqueous
solutions
of organics
on minerals
Suprasil
tubes
196 1000
c
100 355 Nd:YAG
pulsed
laser
(6 ns)
In
suspension
Goethite,
hematite,
anatase
Shkrob
et al., 2010
a
Adapted from Jensen et al., 2008.
b
Constant temperature.
c
Ambient terrestrial atmospheric pressure.
d
R.T. ¼room temperature *208C.
e
Applied doses: 0.50, 1.00, 2.05, 5.42 MGy.
595
Mg(ClO
4
)
2
or Ca(ClO
4
)
2
(Hecht et al., 2009). These findings
were further endorsed by the detection of O
2
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
2
, CO, and H
2
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
H
2
O
2
of biological origin (Houtkooper and Schulze-Makuch,
2007). Viking GCMS-like experiment results (Navarro-
Gonza
´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
2
),
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.
Navarro-Gonza
´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
´lez
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.,
2010).
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
2
O
2
, which is known to
destroy organics, could have caused the evolution of CO
2
in
the LR experiments by reacting with the formate (CHOO
)
in the nutrient solution that was part of the experiment. This
H
2
O
2
could be produced due to photochemical reactions in
the martian atmosphere (Hunten, 1979) but also by frost
weathering (interaction of minerals with H
2
O frost) of olivine
(Huguenin et al., 1979; Huguenin, 1982). Oyama and Berdahl
(1979) determined that g-Fe
2
O
3
, 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
2
O
2
. This could explain the slow CO
2
pro-
duction in the LR and GEx experiments and the CO
2
production in the LR experiment. Ponnamperuma et al.
596 TEN KATE
(1977) had earlier obtained similar results by showing
14
CO
2
production upon adding Viking nutrient mixture to g-Fe
2
O
3
.
On the other hand, there were also arguments against H
2
O
2
.
Levin and Straat (1981) proposed that H
2
O
2
reacted also with
compounds in nutrients other than only formate, which
suggests that the H
2
O
2
hypothesis did not account for the
fact that only one compound in LR was oxidized to CO
2
.
Furthermore, they found that there should by at least 2 wt%
of H
2
O
2
in the soil to reproduce the LR results. This was
considered to be doubtful because there was hardly any
H
2
O
2
detected in the atmosphere (Hanel and Maguire, 1980,
in Levin and Straat, 1981), and H
2
O
2
has a short lifetime
(10
4
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
14
Cto
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
2
OtoO
2
, as seen in the GEx
experiment, are thermally labile or unstable against reduc-
tion by atmospheric CO
2
, and the oxidants most often sug-
gested to explain the LR experiment, including H
2
O
2
, are
expected to decompose rapidly under martian UV (Zent and
McKay, 1994). The interaction between absorbed H
2
O, (ferric
and ferrous) iron, and H
2
O
2
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
2
O
2
. Other oxidizing mechanisms that are not
based on H
2
O
2
have been proposed as well. Oxygen radicals
(O
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
2
and NO
x
pre-
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
2
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
2
O
2
–H
2
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
1
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
1
(Farrell
et al., 2004; Renno et al., 2004). Unsaturated electric fields near
120 kV m
1
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
1
(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
1
. 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
1
), 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,
ORGANICS ON MARS? 597
Delory et al. (2006) found that under near-breakdown con-
ditions the dissociation of H
2
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
2
O
2
(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.
Oro
´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
2
,
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
4
g of carbon m
2
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
12
O
9
). 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
2
compared to
Earth’s atmospheric shield of 1000 g cm
2
, 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
2
, 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
2
), goethite (R-FeOOH), and hematite (R-Fe
2
O
3
)
598 TEN KATE
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
CO
2
and CH
4
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
4
. 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
Navarro-Gonza
´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
4
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)].
Acknowledgments
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
notice.
Abbreviations
ATP, adenosine triphosphate; GCMS, gas chromato-
graph–mass spectrometer; GEx, gas exchange; LR, labeled
release; PR, pyrolytic release; SAM, the Sample Analysis at
Mars; TEGA, Thermal and Evolved Gas Analyzer.
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Address correspondence to:
Inge L. ten Kate
NASA Goddard Space Flight Center
Greenbelt, MD 20771
USA
E-mail: Inge.L.TenKate@nasa.gov
Submitted 7 May 2010
Accepted 29 May 2010
ORGANICS ON MARS? 603
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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.
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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.