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Alteration of five organic compounds by glow discharge plasma and UV light under simulated Mars conditions
Icarus 208 (2010) 749–757
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Alteration of five organic compounds by glow discharge plasma and UV light under simulated Mars conditions Paul E. Hintze a,*, Charles R. Buhler b, Andrew C. Schuerger c, Luz M. Calle a, Carlos I. Calle d a
Corrosion Technology Laboratory, NASA Kennedy Space Center, FL 32899, United States ASRC Aerospace, NASA Kennedy Space Center, FL 32899, United States c Dept. of Plant Pathology, University of Florida, Space Life Sciences Lab, Kennedy Space Center, FL 32899, United States d Electrostatics and Surface Physics Laboratory, NASA Kennedy Space Center, FL 32899, United States b
a r t i c l e
i n f o
Article history: Received 9 October 2009 Revised 22 February 2010 Accepted 9 March 2010 Available online 19 March 2010 Keywords: Mars, Surface Organic chemistry Mars, Atmosphere Spectroscopy
a b s t r a c t The Viking missions to Mars failed to detect any organic material in regolith samples. Since then, several removal mechanisms of organic material have been proposed. Two of these proposed methods are removal due to exposure to plasmas created in dust devils and exposure to UV irradiation. The experiments presented here were performed to identify similarities between the two potential removal mechanisms and to identify any compounds produced from these mechanisms that would have been difficult for the Viking instruments to detect. Five organic compounds, phenanthrene, octadecane, octadecanoic acid, decanophenone and benzoic acid, were exposed to a glow discharge plasma created in simulated martian atmospheres as might be present in dust devils, and to UV irradiation similar to that found at the surface of Mars. Glow discharge exposure was carried out in a chamber with 6.9 mbar pressure of a Mars like gas composed mostly of carbon dioxide. The plasma was characterized using emission spectroscopy and found to contain cations and excited neutral species including carbon dioxide, carbon monoxide, and nitrogen. UV irradiation experiments were performed in a Mars chamber which simulates the temperature, pressure, atmospheric composition, and UV fluence rates of equatorial Mars. The non-volatile residues left after each exposure were characterized by mass loss, infrared spectroscopy and high resolution mass spectrometry. Oxidized, higher molecular weight versions of the parent compounds containing carbonyl, hydroxyl and alkenyl functional groups were identified. The presence of these oxidized compounds suggests that searches for organic material in soils on Mars use instrumentation suitable for detection of compounds which contain the above functional groups. Discussions of possible reaction mechanisms are given. Published by Elsevier Inc.
1. Introduction In 1976, the Viking landers set down on Mars and performed an array of experiments to search for life. Although some of the experiments gave responses consistent with the presence of life, the consensus remains that the experimental results, including oxygen evolution when the regolith was exposed to water and the lack of organic material, were due to an oxidant present in the regolith. This conclusion is based on generally accepted negative results for life in the Pyrolitic-Release (PR), Gas-Exchange (GeX), and Gas-Chromatography/Mass-Spec. (GC/MS) experiments, and the equivocal results for life in the Labeled-Release (LR) experiment (Biemann et al., 1977; Schuerger and Clark, 2008). In particular, the GC/MS experiment failed to identify any organic compounds * Corresponding author at: Mail Stop NE-L2, Kennedy Space Center, FL 32899, United States. E-mail address: [email protected] (P.E. Hintze). 0019-1035/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.icarus.2010.03.015
in the martian regolith down to the parts per billion level (Biemann, 2007). Not finding organics in martian regolith was unanticipated because theoretical work suggested that a steady influx of carbonaceous meteorites arrives annually on Mars from interplanetary space (Flynn and Mckay, 1990; Flynn, 1996). The organic material found in meteorites consists mostly of organic polymers, with smaller amounts of free extractable molecules. Many types of organic molecules and functional groups including polyaromatic hydrocarbons, alkanes and amino acids are present in the polymer and free molecules found in carbonaceous meteorites (Hayatsu and Anders, 1981; Mullie and Reisse, 1987; Botta and Bada, 2002; Sephton, 2002). There have been a few studies that looked at the degradation of organic compounds under simulated Martian conditions (Oro and Holzer, 1979; Stoker and Bullock, 1997; ten Kate et al., 2005, 2006). In a series of experiments, ten Kate et al. (2005, 2006) investigated the stability of glycine and D-alanine under UV irradiation, in the presence of either a 7 mbar CO2 atmosphere or a
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4 106 mbar vacuum. They determined that the degradation rates were dependent on the UV flux, but found that a CO2 atmosphere did not significantly affect the rates. Stoker and Bullock (1997) measured the degradation rate of the amino acid glycine under simulated martian UV irradiation. They measured the amount of gas phase degradation products, small hydrocarbons, found in the head space of the reaction vessel over time to calculate the degradation rate. From their degradation rate, they concluded that organics were being destroyed faster than they were being brought in from meteoric input. However, the rate of destruction of glycine is not necessarily representative of all meteoric organic matter, and amino acids are only a small fraction of the total organic material from carbonaceous meteorites (Mullie and Reisse, 1987). Oro and Holzer (1979) measured the decomposition of adenine, glycine, naphthalene and the Murchison meteorite during exposure to UV with varying amounts of oxygen present. They found that adenine and glycine were more stable than naphthalene and that degradation rates increased with the amount of oxygen present. None of these studies attempted to measure any nonvolatile residue left over after exposure. There have been suggestions that the Viking results can be explained by the inability of the GC/MS instrument to detect certain organics. Benner et al. (2000), using known oxidation reactions, suggested that meteoric organic matter would form metastable products, including a variety of carboxylic acids or their salts. These acids and salts are difficult to pass through a GC, and may not have been detected if in fact they were present. Recently, Navarro-Gonzalez et al. (2006) attempted to detect organic material in a series of Mars analog soils using a similar method to that on the Viking missions. They compared the total organic matter found in the sample with the amount detected using a thermal volatilization GC/MS. In many cases, thermal volatilization GC/MS detected less organics than were actually present. The inability to fully detect organics could have been due to the nature of the organics (carboxylic acids and salts) or to the fact that the soil was oxidizing the organic matter during the heating procedure. It has been postulated that electrostatic generated glow discharge plasmas would form in martian dust storms and that these plasmas could contribute to the removal of organic compounds from the surface of Mars, and possibly generate the oxidants in regolith (Mills, 1977; Oyama and Berdahl, 1979). Electrical discharges formed from the frictional interaction of dust particles have been observed or modeled (Eden and Vonnegut, 1973; Mills, 1977; Fabian et al., 2001; Krauss et al., 2006) under a variety of Mars-like conditions. The production of reactive species in these discharges, such as hydrogen peroxide, have been modeled (Atreya et al., 2006; Delory et al., 2006). In the current work, we present results comparing the degradation products of five organic compounds exposed to a glow discharge plasma or UV light. The glow discharge was produced by applying a potential between two parallel plate electrodes. The emission spectrum of the plasma was measured to identify the reactive species produced in the plasma. The UV irradiation exposure studies simulated the atmospheric and UV conditions on the surface of equatorial Mars. After each exposure, the organic compounds were analyzed using infrared (IR) spectroscopy and high resolution mass spectrometry. These experiments were performed to determine if compounds similar to those proposed by Benner et al. (2000), such as carboxylic acids, carbonyls or alcohols, would be produced under these sets of conditions.
decanophenone (C16H24O), and benzoic acid (C7H6O). All compounds were at least 98% pure and used as obtained from Sigma–Aldrich, St. Louis, MO. Organic compounds were selected to represent alkanes, polyaromatic hydrocarbons, ketones and carboxylic acids typically found in carbonaceous meteorites. UV spectra of the compounds were measured with a Jasco V670 spectrophotometer (Easton, MD). The compounds were dissolved in hexane and the spectra were obtained in a 1 cm path length quartz cuvette. Glow discharge (GD) experiments were performed in a vacuum chamber using a Mars gas mixture composed of 95.5% CO2, 2.7% N2, 1.6% Ar, 0.13% O2 and 0.07% CO at a total pressure of 6.9 mbar. The GD apparatus consisted of two parallel plate electrodes separated by 6 mm. Fig. 1 shows the apparatus, with an inlay showing the emission of a plasma. The GD plasma was formed by applying a 1500 V potential, with a random waveform, to one of the electrodes. The organic compounds were placed on small microscope coverslips and placed between the two electrodes. The compounds were exposed to the plasma for 30 min. Control samples were placed in the vacuum chamber, away from the electrodes. A fiber optic cable was placed near the electrodes to measure the emission spectrum of the plasma. The emission spectrum of the plasma was obtained with a SpectraPro 2300i spectrometer and a Spec10 CCD Detector (Princeton Instruments, Trenton, NJ). UV irradiation experiments were performed in a Mars Simulation Chamber (MSC), shown in Fig. 2. The MSC system creates conditions similar to equatorial Mars (see Schuerger et al. (2008) for full description). In brief, the MSC is a stainless steel low-pressure cylindrical chamber with internal dimensions measuring 70 cm long by 50 cm in diameter. A Mars gas mix, consisting of 95.3%
2. Materials and methods
Fig. 2. Mars Simulation Chamber (MSC). The liquid nitrogen cold-plate (LN2) was used as the primary temperature control system for the MSC. The UV light was produced with a xenon-arc lamp (Xe) and is calibrated to yield a Mars-normal UV– VIS–NIR flux at the upper surface of the LN2 system, as shown. Coupons with organic compounds were placed directly onto the upper surface of the LN2 system.
The organic compounds used in the study were phenanthrene (C14H10), octadecane (C18H38), octadecanoic acid (C18H36O2),
Fig. 1. The parallel plate apparatus used to generate the plasma. Inlay shows the emission from the plasma.
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CO2, 2.7% N2, 1.7% Ar, 0.2% O2 and 0.03% H2O, was delivered to the MSC through a mass-flow controller from commercially obtained tank mixes (Boggs Gasses, Titusville, FL). A liquid-nitrogen (LN2) thermal control system (model TP1265, Sigma Systems Corp., San Diego, CA) served as the primary temperature control system for the MSC, and was maintained at 10 °C for all UV experiments. This temperature was selected to simulate the average summer daytime highs at the two Viking lander sites (Schuerger et al., 2003, 2006) Mars-normal UV–VIS–NIR irradiation was supplied to the inside of the MSC through fused silica glass ports. UV irradiation was produced by one, 1000 W xenon-arc lamp (model 6269; Newport Corp., Stratford, CT), calibrated to deliver Mars UV fluence rates, as described previously (Schuerger et al., 2008). The mean UVC (200–280 nm) fluence rate within the Mars chamber was calibrated to 4.1 W m2 (±0.2 W m2), which delivered approximately 14.8 kJ m2 h1 of UVC irradiation to the organic samples. The mean UVB (280–320) and UVA (320–400) fluence rates on the materials were approximately 5.6 and 15.8 W m2, respectively. This corresponds to an approximate relationship of one Earth day being equal to three martian sols. Organic compounds were placed on glass slides and placed within the MSC. Control samples were wrapped in aluminum foil and placed outside of the direct UV irradiation beam. UV exposure occurred for 21 days. The post-GD or UV exposed organic products were analyzed using IR spectroscopy and mass spectrometry. The IR spectra were obtained with a Pike Technologies lMax IR microscope coupled to a Nicolet 760 Fourier transform IR bench. All samples were measured using the microscope in transmission mode by placing small amounts of individual organic samples between two IR transparent crystals of zinc selenide, with no other sample preparation. All samples were kept in a desiccator until analyzed and were com-
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pared to controls that had undergone the same treatments to eliminate the possibility of water or other contaminants interfering with the measurements. Mass spectra were acquired using a JEOL DART-ACCUTOF Mass Spectrometer (Peabody, MA), operated by the Department of Chemistry at Florida Institute of Technology, Melbourne, FL. The JEOL mass-spec. provides the exact mass of compounds present in the sample, and is able to sample items without any previous separation or preparation. The mass spectrometer technique employs a soft ionization method that keeps the molecule whole, with only the addition or loss of a hydrogen ion to give the parent molecule a charge. The mass to charge ratio is accurate to 1 part per 1000 and can therefore be used to calculate the molecular formula using the exact masses of the atoms. The molecular formulas were calculated using Molecular Fragment Calculator 1.0 (Smith and Deline, 1994). 3. Results 3.1. Plasma exposure The emission spectrum of the glow discharge plasma, Fig. 3, was measured at wavelengths between 190 and 800 nm without any organic material present. All emission lines are due to the plasma, as the measurements were made in the dark and the background spectrum was a flat line. The species produced in the plasma were identified by comparing the experimental emission spectrum to previously measured emission spectra or to energies calculated from known spectroscopic constants (Barth et al., 1971; Fox et al., 1977; Bertaux et al., 2005; Huber et al., Retrieved 2009). 2 2 The emission is dominated by the COþ 2 A P ? X P system, with
Fig. 3. Emission spectra of the GD plasma between 190–250, 275–550, and 740–800 nm.
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a number of lines seen at 314, 325, 338, 351, 367 and 385 nm. Lines 2 2 + were also identified from the COþ 2 B P ? X P (287, 289 nm), CO B1R+ ? X1R+ (211, 219 and 230 nm), CO C1R+ ? A1P (341, 360 and 368 nm), N2 C3Pu ? B3Pg (316, 337, 353 and 380 nm) and 2 2 Nþ 2 B Ru ? X Rg (391 and 427 nm) systems. In addition, lines identified as atomic carbon (193 and 247 nm), atomic oxygen (777 nm) and argon (750 and 764 nm) were detected. NO C2P ? X2P emission was seen at 195, 199 and 201 nm. Thus, a wide range of reactive species were formed in the plasma. No changes in the emission spectrum were detected when organic material was present. Upon removal of the samples from the plasma, most samples had undergone a visual change. Phenanthrene, benzoic acid and decanophenone had changed from their initial white color to a distinct brown or yellow color. Octadecane showed a phase separation, where small drops of liquid had separated from the waxy starting material. The mass loss was measured after exposure and compared to controls and is shown in Table 1. The mass loss results for the controls follow the same general trend as the vapor pressures of the compounds; the lower the vapor pressures of the compounds tested, the greater the losses. The vapor pressures for the compounds (Lide, 1992) are as follows: benzoic acid (1 mm Hg at 96.0 °C) > phenanthrene (1 mm Hg at 118.2 °C) > octadecane (1 mm Hg at 119.6 °C) > octadecanoic acid (1 mm Hg at 173.7 °C). No data was found for decanophenone. The mass loss was less for the plasma exposed samples as compared to the control for all compounds except benzoic acid, and did not follow the same trends. This indicates that there are other mass loss or gain factors in addition to volatilization. After exposure to GD plasma, the IR spectra of the materials were obtained. Table 2 lists the new peaks and corresponding functional groups identified in each material after exposure. The new features indicate the addition of oxygen to the base molecule as evidenced by the presence of hydroxyl (OH) and carbonyl (C@O) functional groups. Alkenyl (C@C) functional groups were also iden-
tified after exposure. The alkenyl group is likely the result of hydrogen abstraction and is not indicative of any change to the number of carbons in the molecule. The mass spectra of the exposed samples indicated new compounds with molecular weights different from the parent compound. Table 3 lists the mass to charge ratio (m/z) and molecular formula of new compounds found after exposure to the plasma. The corresponding formulas of the identified molecular ions assume the ionization mechanism for the original molecule is consistent with ionization mechanism for the products. For example, the predominant ion of phenanthrene (C14H10) contains an additional proton, H+, and is identified as C14 Hþ 11 . Therefore, the ion identified as C14H11O+ is expected to be formed from the ionization of C14H10O. All new compounds identified after plasma exposure showed the addition of oxygen or increase in the number of unsaturated carbon atoms, but no loss in the total number of carbons as compared to the original molecule. The exact mass identified with the mass spectrometer can be used to identify the molecular formula, but not the structure, of any new compounds formed during exposure. Although the structure cannot be uniquely identified, the amount of saturation of the hydrocarbon combined with the IR spectrum can help identify possible structures. A hydrocarbon is completely saturated if it has only single bonds and does not contain a ring. Each unsaturated carbon indicates the presence of one double bond or one ring. Octadecane is a fully saturated hydrocarbon while phenanthrene has 10 unsaturated carbons corresponding to the three rings and seven double bonds in its structure. Figs. 4 and 5 show possible products, based on the identified molecular formulas and amount of unsaturated carbons, for phenanthrene and octadecane that had been exposed to the GD plasma. Note that other possible structures and isomers may exist for each molecular formula. However, the proposed functional groups were identified in the IR spectrum and are present in the compound. In Fig. 4, the location of the two oxygen atoms could be anywhere
Table 1 Percentage mass loss of GD and UV-exposed samples compared to controls.
Table 3 Original compounds, in italics, and identified mass to charge ratios (m/z) found after GD plasma exposure.
Benzoic acid Phenanthrene Octadecane Decanophenone Octadecanoic acid
GD exposure (%)
UV exposure (%)
Control
Exposed
Control
Exposed
59.7 ± 4.1 14.1 ± 1.5 4.9 ± 0.6 4.5 ± 1.6 1.5 ± 0.5
32.3 ± 4.5 1.9 ± 0.6 2.5 ± 1.3 3.0 ± 0.5 0.4 ± 0.8
12.4 ± 2.8 7.1 ± 5.3 0.3 ± 10.1 15.0 ± 5.6 0.6 ± 7.5
102.4 ± 12.4 59.0 ± 15.4 35.5 ± 8.1 39.4 ± 4.2 15.7 ± 5.6
Original compound, molecular formula
m/z of molecular ion
Corresponding formula of molecular ion
Unsaturated carbonsa
Phenanthrene, C14H10
179.084 194.075 209.061
C14H11+ C14H11O+ C14 H9 Oþ 2
10 10 11
225.058
C14 H9 Oþ 3
11
285.278
C 18 H37 Oþ 2 C18 H33 Oþ 3
Octadecanoic acid, C18H36O2
Octadecane, C18H38
Table 2 New infrared peaks found after exposure to the Mars gas plasma. Compound
New peak after exposure, cm1
Assignment
Phenanthrene
3400 1750–1570 1275 1050
OH stretch C@O and C@C stretch Aromatic alcohol? Alcohol?
3400 1712 1625
OH stretch C@O stretch C@C stretch
Octadecanoic acid
1680–1600a
C@O or C@C stretch?
Benzoic acid
No changes
Decanophenone
3400 1750–1570
Octadecane
a
The octadecanoic acid carbonyl peak is broadened after exposure.
253.293 267.269 281.253 283.274 285.283 297.246 299.265
Decanophenone, C16H24O
233.189 247.169 261.150 263.166 279.163
Benzoic acid, C7H6O2 OH stretch C@O and C@C stretch
297.250 299.264
a
123.042
C18 H35 Oþ 3 C 18 Hþ 37 C18H35O+ C18 H33 Oþ 2 C18 H35 Oþ 2 C18 H37 Oþ 2 C18 H33 Oþ 3 C18 H35 Oþ 3 C16H25O+ C16 H23 Oþ 2 C16 H21 Oþ 3 C16 H23 Oþ 3 C16 H23 Oþ 4 C 7 H7 Oþ 2
1 3 2 0 1 2 1 0 2 1 5 6 7 6 6 5 +
It is assumed that the detected molecular ion has an additional H in the case of phenanthrene, octadecanoic acid and decanophenone and is missing an H in the case of phenanthrene, octadecanoic acid and 1-phenyl-decan-1-one.
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Fig. 6. Molar absorptivity of the UV exposed compounds between 200 and 400 nm.
Fig. 4. Structure of phenanthrene and possible structures for compounds found after plasma exposure of phenanthrene.
around the ring, but based on other work the most likely possibility is 9,10-phenanthrenequinone (Helmig and Harger, 1994; Cvrckova and Ciganek, 2005; Wang et al., 2007). Possible structures for the other compounds could be identified with a similar strategy. 3.2. UV exposure UV exposure could cause degradation through two possible mechanisms: Direct photolysis by the absorption of UV photons and reaction with an oxidizing species in the atmosphere. UV/visible absorbance spectra, shown in Fig. 6, were measured for each organic compound between 200 and 400 nm to assess the likelihood of direct photolysis. Phenanthrene, benzoic acid and decanophenone all begin absorbing light at wavelengths less than 300 nm. They have large absorbance at 250 nm and shorter wave-
lengths and are transparent in the longer UV range and the visible. Octadecane and octadecanoic acid only weakly absorb near 200 nm and are transparent everywhere else. Therefore, the degradation of octadecane and octadecanoic acid would be unlikely to depend on direct absorption of UV photons. After exposure, all of the samples, except octadecane, had changed to very light yellow colors from generally white opaque coloration prior to UV exposures. The yellow color change had been observed previously for naphthalene (Oro and Holzer, 1979), but the responsible compound was not identified. Mass loss measurements during UV exposure showed more loss as compared to plasma experiments, probably due to the longer time the samples were exposed to martian pressures in the UV experiments: 21 days, as compared to the 30 min GD exposures. The weight loss again follows the trends of the vapor pressure and, in fact, the benzoic acid was almost completely lost. New IR absorbance features found after UV exposure are given in Table 4. Phenanthrene, octadecane and octadecanoic acid did not show any differences in the IR spectrum between the control sample and the samples exposed to UV. After exposure, the decanophenone sample showed signs of a new OH group as well as a broad absorbance feature around 1700, possibly indicating a
Fig. 5. Structure of octadecane and possible structures for compounds found after plasma exposure of octadecane.
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Table 4 New infrared peaks found after exposure in the Mars chamber for 7 days. Compound
New peak after exposure, cm1 Assignment
Phenanthrene
No detectable changes
Octadecane
No detectable changes
Octadecanoic acid
No detectable changes
Benzoic acid
800–1850
Changes in fingerprint region Distinct CH stretch region
2800–3150 Decanophenone
3400
OH stretch
mixture of carbon–carbon double bonds, which overlaps with the carbonyl feature in the parent compound. Benzoic acid showed the most dramatic change. The control showed the broad absorbance centered at 2800 cm1 typical of carboxylic acids. The sample that had been exposed to UV irradiation had a clear feature for OH (3500 cm1) not seen in the control, and the carbonyl as well as other features in the fingerprint region, all had small but distinct shifts in frequency. Nearly all of the benzoic acid was lost during exposure, and it is possible that this newly identified product was purified during the exposure. This might have enhanced the ability to detect this product under these conditions as compared to the GD plasma exposed samples since not as much mass was lost in that experiment. Results of MS analysis for the UV-exposed samples are shown in Table 5. No degradation products were found for phenanthrene, octadecane or octadecanoic acid after UV exposure. Decanophenone showed the addition of oxygen much like, but to a lesser extent, than, the GD plasma exposed samples. The residue from the benzoic acid exposure showed a product with the addition of oxygen that was not identified after exposure to plasma.
4. Discussion 4.1. Chemistry Chemistry in plasma is driven by reactions due to four key factors: free ions, UV radiation, free radicals, and chemically reactive neutral species. The emission spectrum of the plasma can be used to identify some of these reactive species. Cationic species of CO2, N2 and CO, as well as excited neutral species, with enough energy
Table 5 Original compounds, in italics, and identified mass to charge ratios (m/z) found after Mars chamber exposure. Original compound, molecular formula
m/z of molecular ion
Corresponding formula of molecular ion
Unsaturated carbonsa
Phenanthrene, C14H10 Octadecanoic acid, C18H36O2 Octadecane, C18H38 Decanophenone, C16H24O
179.086 285.278
C 14 Hþ 11 C 18 H37 Oþ 2
10 1
253.293 233.189 247.171
C 18 Hþ 37 C16H25O+ C16 H23 Oþ 2
0 5 6
Benzoic acid, C7H6O2
261.153 123.047
C16 H21 Oþ 3 C 7 H7 Oþ 2
7 5
139.041 159.066
C7 H7 Oþ 3 C7 H11 Oþ 4
5 3
a It is assumed that the detected molecular ion has an additional H+ in the case of phenanthrene, octadecanoic acid and decanophenone and is missing an H in the case of octadecane, due to the MS ionization process.
to initiate chemistry were identified in the plasma. The high energy species produced in the plasma may lose their energy through reactions with other gas phase species, reactions with the larger organic molecules present in the experiment, radiative decay, or þ collisional deactivation. For example, the Nþ 2 and CO2 ions can react with small hydrocarbons, both breaking a CH bond and still leaving reactive species (Anicich, 1993). Presumably, similar reactions could occur with the larger hydrocarbons used in these experiments. The excited neutral species of CO and N2 present in the plasma can decay into states that still have considerable energy compared to the ground states. For example, the excited A state of CO and B state of N2, both identified in the emission spectrum, have about 780 kJ/mol and 600 kJ/mol of energy, respectively, relative to their ground states. The A state of CO would be expected to radiative decay to its ground state, giving off a photon at a wavelength, 154 nm, which is below the short wavelength limit of this experiment, 190 nm. However, the A and B excited states of N2 should be long lasting, as radiative decay to the ground state is unlikely because it is spin forbidden. The excited states of N2 would likely undergo collisional deactivation, liberating between 600 and 700 kJ/mol energy. This is enough energy to break typical carbon–hydrogen bonds, which have strengths of about 400 kJ/mol. These reactions, in addition to the high energy UV light given off in the plasma, cause the chemistry that occurs in the plasma. After plasma exposure, new products were found for each compound except benzoic acid. The new products were similar among phenanthrene, octadecane, octadecanoic acid, and decanophenone, in that they all had either the addition of one or more oxygen atoms or the formation of unsaturated carbons. Even though there were many reactive species in the GD plasma, the oxygen-enriched products were the only types found in these experiments. The identification methods used in this study only measured the nonvolatile compounds remaining after exposure and would have missed compounds that were volatilized during the exposure, such as those seen by Stoker and Bullock (1997). The vast number of reactive species available in the plasma, makes predicting the reactions that formed the new compounds difficult. However, the new compounds have been found to occur in known reactions. The reaction of phenanthrene with hydroxyl radical has been studied for the Earth’s atmosphere. The production of 9,10-phenanthrenequinone has been reported (Helmig and Harger, 1994; Cvrckova and Ciganek, 2005; Wang et al., 2007), and may be one of the possible compounds identified in this study, suggesting that the oxidation process in the GD plasma is similar to the oxidation processes occurring in Earth’s atmosphere. UV initiated chemistry proceeds through two mechanisms: direct photolysis caused by the absorbance of a UV photon or reaction with a UV created reactive radical, such as the hydroxyl radical, OH. The hydroxyl radical is thought to carry out the oxidation of organic compounds on Mars (Barth et al., 1992). The likelihood of direct photolysis can be determined from the UV absorbance spectra of the compounds. The reactions with OH are known for many organic compounds and can be used to evaluate potential reaction paths in these experiments. Benzoic acid and decanophenone were the only compounds with identified products after UV exposure. The lack of any identified reaction products for octadecane and octadecanoic acid after UV exposure can be explained by their weak absorption of UV light, as shown in Fig. 6. Direct absorbance of UV by these two molecules is considerably less than the others tested. Therefore, the only mechanism of degradation would be via OH radical reactions. Although the hydroxyl radicals are presumably produced under high UV fluence rates in the presence of water, no direct measurements of OH radicals were made. The removal of hydrogen by the hydroxyl radical depends on the carbon– hydrogen bond strength in the compound. The weaker the bond,
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the easier it is to remove the hydrogen. The bond strengths found in saturated hydrocarbons such as octadecane are stronger than those found in other compounds in the study. Octadecanoic acid, has similar bond strengths for its long hydrocarbon chain, and would only have weaker carbon–hydrogen bonds near the acid group. These strong bonds may account for slower reactions as compared to the other compounds. Both decanophenone and benzoic acid have properties lending to reactions under the simulated martian conditions. Both have significant UV absorbance that could lead to direct photolysis. Decanophenone has hydrogen moieties that are more susceptible to abstraction by hydroxyl radicals than some of the other compounds evaluated here. The relative bond strengths for these compounds are assumed here based on similar compounds. The strength of carbon–hydrogen bonds found close to aromatic rings and carbonyls can be much lower than in unsubstituted alkanes. For example, the carbon–hydrogen bond strengths in similar compounds are 85.4 kcal/mol for ethyl benzene (H–CH(CH3)C6H5), 86.9 for benzaldehyde (H-COC6H5) and 92 for 2-butanone (H–CH (CH3)COCH3) as compared to 96–100 found for typical alkanes (Lide, 1992). This indicates that decanophenone would react more aggressively with hydroxyl radicals in addition to direct UV photolysis that might occur. The reaction of poly aromatic hydrocarbons (PAH’s) in the Earth’s atmosphere, including phenanthrene, proceeds via an initial reaction where a hydroxyl radical is added to the PAH forming an intermediate. This is followed by either a further reaction, often with molecular oxygen (O2) (Helmig and Harger, 1994), resulting in an oxidized product, or the decomposition of the intermediate to the original reactants, preserving the phenanthrene molecule (Atkinson and Arey, 2007). During UV irradiation under martian conditions, it may be possible that a lack of molecular oxygen might explain the paucity of identified products for phenanthrene. The intermediate hydroxyl adduct may be formed, but it may not proceed any further and return to phenanthrene. There were clear differences between the behavior of benzoic acid in the plasma and when exposed to UV. In both cases, there was considerable mass loss, but in the UV exposed sample there were detectable products. Nearly all of the benzoic acid was gone after UV exposure, while only 30% was lost after GD plasma exposure. It is possible that the identified product was concentrated by evaporation of benzoic acid or volatilization of other products. 4.2. Comparison between GD plasma and UV-exposed samples results More degradation products were found for all other compounds in the GD plasma exposed samples as compared to the UV-exposed samples. The plasma was a more aggressive environment than the martian UV conditions. Unfortunately, there was no way to quantify the aggressiveness of the plasma in any way other than exposure time. In both types of exposure, only compounds that had the same number of carbons as the initial compound were identified. The only reactions that could be identified were the addition of oxygen or the formation of carbon double bonds from an alkane. No products smaller than the initial compounds were identified, although the presence of other compounds is not precluded. For example, there was a change in color in phenanthrene and octadecanoic acid after UV exposure, indicating the formation of a new compound. However, the analytical methods used in this study were unable to detect the color inducing compounds. The differences in identified products are probably due not only to the aggressiveness, but to the presence of additional reactants in the plasma. For example, the reaction of phenanthrene occurs through a stepwise process were a hydroxylated phenanthrene intermediate adduct reacts with an oxygenated species to form a stable oxidized species. The presence of more reactive oxygen spe-
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cies in the plasma as compared to the UV conditions probably led to more reactions for phenanthrene. Previous experiments (Oro and Holzer, 1979; Stoker and Bullock, 1997; ten Kate et al., 2005, 2006) on the decomposition of organic compounds under simulated martian conditions focused on the rate of decomposition, and not on identifying the products of the decomposition reactions. In both the GD and UV exposure studies described here, the residual organic material was characterized in hopes that the results could be used to design instruments for identifying organic material on Mars. Similar compounds containing additional oxygen were identified after both GD and UV exposure. These compounds are similar to, or are intermediates leading to, the refractory organic compounds that have been previously postulated to not having been identified in the Viking instruments (Benner et al., 2000; Cabane et al., 2004; NavarroGonzalez et al., 2006). No compounds with smaller numbers of carbon atoms were identified. There are a few factors that could have led to this result: (1) the majority of the mass loss could have been due to evaporation, and there was in fact very little conversion of the initial compounds to smaller compounds; (2) the smaller compounds were more volatile and therefore were not in the residue collected after the experiments; (3) any smaller compounds that were collected were not identified with the techniques used in this study. The importance of each factor would be highly dependent on the compounds formed. For example, octadecane might lose two carbon atoms to form a series of compounds with 16 carbon atoms and similar functional groups to what was found for octadecane: hexadecane, hexadecyl alcohol, hexadecene and hexadecanone. Hexadecane, hexadecanone and hexadecene are more volatile than octadecane, while hexadecyl alcohol is less volatile (Lide, 1992). This would lead to a concentration of hexadecyl alcohol, while the other compounds would evaporate over time. In situ measurements of volatile compounds would be needed to determine if smaller compounds are formed. Infrared spectroscopy, as was performed in these experiments, is not sensitive enough to determine if hexadecane was present as a trace compound in octadecane. Although the mass spectrometer used is very sensitive, it is possible that some compounds were missed due to preferential ionization of some compounds. Identification of all trace compounds, such as those causing the color change in phenanthrene and octadecanoic acid after UV exposure, would probably require extraction and separation methods (e.g. gas or liquid chromatography). However, the experiments reported here do show that oxidized compounds are formed during GD or UV exposure under Mars-like conditions.
5. Conclusions A GD plasma induced by electrostatic discharges in a dust storm on Mars is likely to generate highly reactive species, capable of reacting with all types of suspended organic compounds. Reactive species produced in the plasma and high energy UV emissions were identified. The GD plasma produced a more oxidative environment than the UV irradiation created within the MSC system, but a direct comparison of the reactivity of the two cannot be made since the reactivity of the plasma cannot be quantified. In both plasma and UV exposures, the degradation reactions produced oxidized, higher molecular weight compounds as compared to the starting materials. Although the compounds exposed in this study are only found at trace levels in meteorites, it is thought that they might act as models for the functional groups such as alkanes, polyaromatic hydrocarbons, ketones and carboxylic acids found in the organic polymers prevalent in meteorites. The non-volatile compounds with polar functional groups produced in these experiments would be difficult to analyze with
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gas chromatography. No compounds with smaller molecular weight than the original materials were identified. Future missions searching for organics need to be capable of identifying organics with varying degrees of oxidation. Detection of oxidized products can be aided by finding different ways to extract the organics from the soil and separating the compounds in the analysis instrument. Thermal volatilization was used during the Viking missions, and may cause compounds to be catalytically oxidized to carbon dioxide during heating, causing a loss of information about the organics (Navarro-Gonzalez et al., 2006). Furthermore, polar, high boiling, organic compounds such as organic acids, are not ideally separated by gas chromatography. Solvent extractions (Buch et al., 2003) and chemical derivatization (Cabane et al., 2001, 2004; Rodier et al., 2001) have been proposed as methods to aid in extraction of organics. Solvent extraction is a common method for removing organic compounds from environmental samples. Chemical derivatization involves converting the polar groups in a compound to non-polar groups making the original compound more volatile. Chemical derivatization has the advantage of making the separation with GC much easier. The Sample Analysis at Mars instrument suite will use chemical derivatization to search for organic material as part of the Mars Science Laboratory mission (Mahaffy, 2008). Another approach could be to use supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC). These methods use a supercritical fluid, most commonly carbon dioxide, for extraction of compounds from a matrix and as the mobile phase in chromatography. SFE and SFC have been used in a variety of systems (Bowadt and Hawthorne, 1995; Smith, 1999). Supercritical carbon dioxide extraction is currently an approved EPA method for recovering various organic compounds, as described in Methods 3560, 3561 and 3562 of ‘‘Test Methods for Evaluating Solid Waste, Physical/ Chemical Methods” (SW-846) (2008a,b). Supercritical carbon dioxide needed for the extractions and separations could be made in situ from the martian atmosphere. SFE with pure carbon dioxide as solvent is not efficient at removing polar species; however, the use of small amounts of co-solvents improves performance greatly (Smith, 1999) and has been used to extract amino acids from biological samples (Bernal et al., 2008). SFC is similar to liquid chromatography, has been used for polar solvents without derivitization, and has been successfully used to separate chiral compounds (Berger, 1997; Smith, 1999). Using this technique, the chirality of amino acids and other potentially biological materials could be determined. Acknowledgments The authors would like to acknowledge NASA Science Mission Directorate for funding (Grant #MFRP04-0028-0090). The authors thank Dr. Nasri Nesnas at Florida Institute of Technology for help with the DART mass spectral technique. P.E.H. would like to thank the National Research Council for a Research Associateship Award at NASA Kennedy Space Center. References Anicich, V.G., 1993. Evaluated bimolecular ion-molecule gas-phase kinetics of positive-ions for use in modeling planetary-atmospheres, cometary comae, and interstellar clouds. J. Phys. Chem. Ref. Data 22, 1469–1569. Atkinson, R., Arey, J., 2007. Mechanisms of the gas-phase reactions of aromatic hydrocarbons and PAHs with OH and NO3 radicals. Polycyclic Aromat. Compd. 27, 15–40. Atreya, S.K., Wong, A.-S., Renno, N.O., Farrell, W.M., Delory, G.T., Sentman, D.D., Cummer, S.A., Marshall, J.R., Rafkin, S.C.R., Catling, D.C., 2006. Oxidant enhancement in martian dust devils and storms: Implications for life and habitability. Astrobiology 6, 439–450. doi:10.1089/ast.2006.6.439. Barth, C.A., Hord, C.W., Pearce, J.B., Kelly, K.K., Anderson, G.P., Stewart, A.I., 1971. Mariner 6 and 7 ultraviolet spectrometer experiment: Upper atmosphere data. J. Geophys. Res. 76, 2213–2227.
Barth, C.A., Stewart, A.I.F., Bougher, S.W., Hunten, D.M., Bauer, S.J., Nagy, A.F., 1992. Aeronomy of the current martian atmosphere. In: Kieffer, H.H. (Ed.), Mars. University of Arizona Press, Tucson, pp. 1054–1089. Benner, S.A., Devine, K.G., Matveeva, L.N., Powell, D.H., 2000. The missing organic molecules on Mars. Proc. Natl. Acad. Sci. USA 97, 2425–2430. Berger, T.A., 1997. Separation of polar solutes by packed column supercritical fluid chromatography. J. Chromatogr., A 785, 3–33. Bernal, J.L., Nozal, M.J., Toribio, L., Diego, C., Mayo, R., Maestre, R., 2008. Use of supercritical fluid extraction and gas chromatography–mass spectrometry to obtain amino acid profiles from several genetically modified varieties of maize and soybean. J. Chromatogr., A 1192, 266–272. Bertaux, J.L., and 10 colleagues, 2005. Nightglow in the upper atmosphere of Mars and implications for atmospheric transport. Science 307, 566–569. Biemann, K., 2007. On the ability of the Viking gas chromatograph–mass spectrometer to detect organic matter. Proc. Natl. Acad. Sci. USA 104, 10310– 10313. Biemann, K., and 11 colleagues, 1977. The search for organic substances and inorganic volatile compounds in the surface of Mars. J. Geophys. Res. 82, 4641– 4657. Botta, O., Bada, J.L., 2002. Extraterrestrial organic compounds in meteorites. Surv. Geophys. 23, 411–467. Bowadt, S., Hawthorne, S.B., 1995. Supercritical-fluid extraction in environmentalanalysis. J. Chromatogr., A 703, 549–571. Buch, A., Sternberg, R., Meunier, D., Rodier, C., Laurent, C., Raulin, F., Vidal-Madjar, C., 2003. Solvent extraction of organic molecules of exobiological interest for in situ analysis of the martian soil. J. Chromatogr., A 999, 165–174. Cabane, M., Coll, P., Rodier, C., Israel, G., Raulin, F., Sternberg, R., Niemann, H., Mahaffy, P., Jambon, A., Rannou, P., 2001. In situ inorganic and organic analysis (Pyr/CD-GC/MS) of the martian soil, on the Mars 2005 mission. 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Alteration of five organic compounds by glow discharge plasma and UV light under simulated Mars conditions Paul E. Hintze a,*, Charles R. Buhler b, Andrew C. Schuerger c, Luz M. Calle a, Carlos I. Calle d a
Corrosion Technology Laboratory, NASA Kennedy Space Center, FL 32899, United States ASRC Aerospace, NASA Kennedy Space Center, FL 32899, United States c Dept. of Plant Pathology, University of Florida, Space Life Sciences Lab, Kennedy Space Center, FL 32899, United States d Electrostatics and Surface Physics Laboratory, NASA Kennedy Space Center, FL 32899, United States b
a r t i c l e
i n f o
Article history: Received 9 October 2009 Revised 22 February 2010 Accepted 9 March 2010 Available online 19 March 2010 Keywords: Mars, Surface Organic chemistry Mars, Atmosphere Spectroscopy
a b s t r a c t The Viking missions to Mars failed to detect any organic material in regolith samples. Since then, several removal mechanisms of organic material have been proposed. Two of these proposed methods are removal due to exposure to plasmas created in dust devils and exposure to UV irradiation. The experiments presented here were performed to identify similarities between the two potential removal mechanisms and to identify any compounds produced from these mechanisms that would have been difficult for the Viking instruments to detect. Five organic compounds, phenanthrene, octadecane, octadecanoic acid, decanophenone and benzoic acid, were exposed to a glow discharge plasma created in simulated martian atmospheres as might be present in dust devils, and to UV irradiation similar to that found at the surface of Mars. Glow discharge exposure was carried out in a chamber with 6.9 mbar pressure of a Mars like gas composed mostly of carbon dioxide. The plasma was characterized using emission spectroscopy and found to contain cations and excited neutral species including carbon dioxide, carbon monoxide, and nitrogen. UV irradiation experiments were performed in a Mars chamber which simulates the temperature, pressure, atmospheric composition, and UV fluence rates of equatorial Mars. The non-volatile residues left after each exposure were characterized by mass loss, infrared spectroscopy and high resolution mass spectrometry. Oxidized, higher molecular weight versions of the parent compounds containing carbonyl, hydroxyl and alkenyl functional groups were identified. The presence of these oxidized compounds suggests that searches for organic material in soils on Mars use instrumentation suitable for detection of compounds which contain the above functional groups. Discussions of possible reaction mechanisms are given. Published by Elsevier Inc.
1. Introduction In 1976, the Viking landers set down on Mars and performed an array of experiments to search for life. Although some of the experiments gave responses consistent with the presence of life, the consensus remains that the experimental results, including oxygen evolution when the regolith was exposed to water and the lack of organic material, were due to an oxidant present in the regolith. This conclusion is based on generally accepted negative results for life in the Pyrolitic-Release (PR), Gas-Exchange (GeX), and Gas-Chromatography/Mass-Spec. (GC/MS) experiments, and the equivocal results for life in the Labeled-Release (LR) experiment (Biemann et al., 1977; Schuerger and Clark, 2008). In particular, the GC/MS experiment failed to identify any organic compounds * Corresponding author at: Mail Stop NE-L2, Kennedy Space Center, FL 32899, United States. E-mail address: [email protected] (P.E. Hintze). 0019-1035/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.icarus.2010.03.015
in the martian regolith down to the parts per billion level (Biemann, 2007). Not finding organics in martian regolith was unanticipated because theoretical work suggested that a steady influx of carbonaceous meteorites arrives annually on Mars from interplanetary space (Flynn and Mckay, 1990; Flynn, 1996). The organic material found in meteorites consists mostly of organic polymers, with smaller amounts of free extractable molecules. Many types of organic molecules and functional groups including polyaromatic hydrocarbons, alkanes and amino acids are present in the polymer and free molecules found in carbonaceous meteorites (Hayatsu and Anders, 1981; Mullie and Reisse, 1987; Botta and Bada, 2002; Sephton, 2002). There have been a few studies that looked at the degradation of organic compounds under simulated Martian conditions (Oro and Holzer, 1979; Stoker and Bullock, 1997; ten Kate et al., 2005, 2006). In a series of experiments, ten Kate et al. (2005, 2006) investigated the stability of glycine and D-alanine under UV irradiation, in the presence of either a 7 mbar CO2 atmosphere or a
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4 106 mbar vacuum. They determined that the degradation rates were dependent on the UV flux, but found that a CO2 atmosphere did not significantly affect the rates. Stoker and Bullock (1997) measured the degradation rate of the amino acid glycine under simulated martian UV irradiation. They measured the amount of gas phase degradation products, small hydrocarbons, found in the head space of the reaction vessel over time to calculate the degradation rate. From their degradation rate, they concluded that organics were being destroyed faster than they were being brought in from meteoric input. However, the rate of destruction of glycine is not necessarily representative of all meteoric organic matter, and amino acids are only a small fraction of the total organic material from carbonaceous meteorites (Mullie and Reisse, 1987). Oro and Holzer (1979) measured the decomposition of adenine, glycine, naphthalene and the Murchison meteorite during exposure to UV with varying amounts of oxygen present. They found that adenine and glycine were more stable than naphthalene and that degradation rates increased with the amount of oxygen present. None of these studies attempted to measure any nonvolatile residue left over after exposure. There have been suggestions that the Viking results can be explained by the inability of the GC/MS instrument to detect certain organics. Benner et al. (2000), using known oxidation reactions, suggested that meteoric organic matter would form metastable products, including a variety of carboxylic acids or their salts. These acids and salts are difficult to pass through a GC, and may not have been detected if in fact they were present. Recently, Navarro-Gonzalez et al. (2006) attempted to detect organic material in a series of Mars analog soils using a similar method to that on the Viking missions. They compared the total organic matter found in the sample with the amount detected using a thermal volatilization GC/MS. In many cases, thermal volatilization GC/MS detected less organics than were actually present. The inability to fully detect organics could have been due to the nature of the organics (carboxylic acids and salts) or to the fact that the soil was oxidizing the organic matter during the heating procedure. It has been postulated that electrostatic generated glow discharge plasmas would form in martian dust storms and that these plasmas could contribute to the removal of organic compounds from the surface of Mars, and possibly generate the oxidants in regolith (Mills, 1977; Oyama and Berdahl, 1979). Electrical discharges formed from the frictional interaction of dust particles have been observed or modeled (Eden and Vonnegut, 1973; Mills, 1977; Fabian et al., 2001; Krauss et al., 2006) under a variety of Mars-like conditions. The production of reactive species in these discharges, such as hydrogen peroxide, have been modeled (Atreya et al., 2006; Delory et al., 2006). In the current work, we present results comparing the degradation products of five organic compounds exposed to a glow discharge plasma or UV light. The glow discharge was produced by applying a potential between two parallel plate electrodes. The emission spectrum of the plasma was measured to identify the reactive species produced in the plasma. The UV irradiation exposure studies simulated the atmospheric and UV conditions on the surface of equatorial Mars. After each exposure, the organic compounds were analyzed using infrared (IR) spectroscopy and high resolution mass spectrometry. These experiments were performed to determine if compounds similar to those proposed by Benner et al. (2000), such as carboxylic acids, carbonyls or alcohols, would be produced under these sets of conditions.
decanophenone (C16H24O), and benzoic acid (C7H6O). All compounds were at least 98% pure and used as obtained from Sigma–Aldrich, St. Louis, MO. Organic compounds were selected to represent alkanes, polyaromatic hydrocarbons, ketones and carboxylic acids typically found in carbonaceous meteorites. UV spectra of the compounds were measured with a Jasco V670 spectrophotometer (Easton, MD). The compounds were dissolved in hexane and the spectra were obtained in a 1 cm path length quartz cuvette. Glow discharge (GD) experiments were performed in a vacuum chamber using a Mars gas mixture composed of 95.5% CO2, 2.7% N2, 1.6% Ar, 0.13% O2 and 0.07% CO at a total pressure of 6.9 mbar. The GD apparatus consisted of two parallel plate electrodes separated by 6 mm. Fig. 1 shows the apparatus, with an inlay showing the emission of a plasma. The GD plasma was formed by applying a 1500 V potential, with a random waveform, to one of the electrodes. The organic compounds were placed on small microscope coverslips and placed between the two electrodes. The compounds were exposed to the plasma for 30 min. Control samples were placed in the vacuum chamber, away from the electrodes. A fiber optic cable was placed near the electrodes to measure the emission spectrum of the plasma. The emission spectrum of the plasma was obtained with a SpectraPro 2300i spectrometer and a Spec10 CCD Detector (Princeton Instruments, Trenton, NJ). UV irradiation experiments were performed in a Mars Simulation Chamber (MSC), shown in Fig. 2. The MSC system creates conditions similar to equatorial Mars (see Schuerger et al. (2008) for full description). In brief, the MSC is a stainless steel low-pressure cylindrical chamber with internal dimensions measuring 70 cm long by 50 cm in diameter. A Mars gas mix, consisting of 95.3%
2. Materials and methods
Fig. 2. Mars Simulation Chamber (MSC). The liquid nitrogen cold-plate (LN2) was used as the primary temperature control system for the MSC. The UV light was produced with a xenon-arc lamp (Xe) and is calibrated to yield a Mars-normal UV– VIS–NIR flux at the upper surface of the LN2 system, as shown. Coupons with organic compounds were placed directly onto the upper surface of the LN2 system.
The organic compounds used in the study were phenanthrene (C14H10), octadecane (C18H38), octadecanoic acid (C18H36O2),
Fig. 1. The parallel plate apparatus used to generate the plasma. Inlay shows the emission from the plasma.
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CO2, 2.7% N2, 1.7% Ar, 0.2% O2 and 0.03% H2O, was delivered to the MSC through a mass-flow controller from commercially obtained tank mixes (Boggs Gasses, Titusville, FL). A liquid-nitrogen (LN2) thermal control system (model TP1265, Sigma Systems Corp., San Diego, CA) served as the primary temperature control system for the MSC, and was maintained at 10 °C for all UV experiments. This temperature was selected to simulate the average summer daytime highs at the two Viking lander sites (Schuerger et al., 2003, 2006) Mars-normal UV–VIS–NIR irradiation was supplied to the inside of the MSC through fused silica glass ports. UV irradiation was produced by one, 1000 W xenon-arc lamp (model 6269; Newport Corp., Stratford, CT), calibrated to deliver Mars UV fluence rates, as described previously (Schuerger et al., 2008). The mean UVC (200–280 nm) fluence rate within the Mars chamber was calibrated to 4.1 W m2 (±0.2 W m2), which delivered approximately 14.8 kJ m2 h1 of UVC irradiation to the organic samples. The mean UVB (280–320) and UVA (320–400) fluence rates on the materials were approximately 5.6 and 15.8 W m2, respectively. This corresponds to an approximate relationship of one Earth day being equal to three martian sols. Organic compounds were placed on glass slides and placed within the MSC. Control samples were wrapped in aluminum foil and placed outside of the direct UV irradiation beam. UV exposure occurred for 21 days. The post-GD or UV exposed organic products were analyzed using IR spectroscopy and mass spectrometry. The IR spectra were obtained with a Pike Technologies lMax IR microscope coupled to a Nicolet 760 Fourier transform IR bench. All samples were measured using the microscope in transmission mode by placing small amounts of individual organic samples between two IR transparent crystals of zinc selenide, with no other sample preparation. All samples were kept in a desiccator until analyzed and were com-
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pared to controls that had undergone the same treatments to eliminate the possibility of water or other contaminants interfering with the measurements. Mass spectra were acquired using a JEOL DART-ACCUTOF Mass Spectrometer (Peabody, MA), operated by the Department of Chemistry at Florida Institute of Technology, Melbourne, FL. The JEOL mass-spec. provides the exact mass of compounds present in the sample, and is able to sample items without any previous separation or preparation. The mass spectrometer technique employs a soft ionization method that keeps the molecule whole, with only the addition or loss of a hydrogen ion to give the parent molecule a charge. The mass to charge ratio is accurate to 1 part per 1000 and can therefore be used to calculate the molecular formula using the exact masses of the atoms. The molecular formulas were calculated using Molecular Fragment Calculator 1.0 (Smith and Deline, 1994). 3. Results 3.1. Plasma exposure The emission spectrum of the glow discharge plasma, Fig. 3, was measured at wavelengths between 190 and 800 nm without any organic material present. All emission lines are due to the plasma, as the measurements were made in the dark and the background spectrum was a flat line. The species produced in the plasma were identified by comparing the experimental emission spectrum to previously measured emission spectra or to energies calculated from known spectroscopic constants (Barth et al., 1971; Fox et al., 1977; Bertaux et al., 2005; Huber et al., Retrieved 2009). 2 2 The emission is dominated by the COþ 2 A P ? X P system, with
Fig. 3. Emission spectra of the GD plasma between 190–250, 275–550, and 740–800 nm.
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a number of lines seen at 314, 325, 338, 351, 367 and 385 nm. Lines 2 2 + were also identified from the COþ 2 B P ? X P (287, 289 nm), CO B1R+ ? X1R+ (211, 219 and 230 nm), CO C1R+ ? A1P (341, 360 and 368 nm), N2 C3Pu ? B3Pg (316, 337, 353 and 380 nm) and 2 2 Nþ 2 B Ru ? X Rg (391 and 427 nm) systems. In addition, lines identified as atomic carbon (193 and 247 nm), atomic oxygen (777 nm) and argon (750 and 764 nm) were detected. NO C2P ? X2P emission was seen at 195, 199 and 201 nm. Thus, a wide range of reactive species were formed in the plasma. No changes in the emission spectrum were detected when organic material was present. Upon removal of the samples from the plasma, most samples had undergone a visual change. Phenanthrene, benzoic acid and decanophenone had changed from their initial white color to a distinct brown or yellow color. Octadecane showed a phase separation, where small drops of liquid had separated from the waxy starting material. The mass loss was measured after exposure and compared to controls and is shown in Table 1. The mass loss results for the controls follow the same general trend as the vapor pressures of the compounds; the lower the vapor pressures of the compounds tested, the greater the losses. The vapor pressures for the compounds (Lide, 1992) are as follows: benzoic acid (1 mm Hg at 96.0 °C) > phenanthrene (1 mm Hg at 118.2 °C) > octadecane (1 mm Hg at 119.6 °C) > octadecanoic acid (1 mm Hg at 173.7 °C). No data was found for decanophenone. The mass loss was less for the plasma exposed samples as compared to the control for all compounds except benzoic acid, and did not follow the same trends. This indicates that there are other mass loss or gain factors in addition to volatilization. After exposure to GD plasma, the IR spectra of the materials were obtained. Table 2 lists the new peaks and corresponding functional groups identified in each material after exposure. The new features indicate the addition of oxygen to the base molecule as evidenced by the presence of hydroxyl (OH) and carbonyl (C@O) functional groups. Alkenyl (C@C) functional groups were also iden-
tified after exposure. The alkenyl group is likely the result of hydrogen abstraction and is not indicative of any change to the number of carbons in the molecule. The mass spectra of the exposed samples indicated new compounds with molecular weights different from the parent compound. Table 3 lists the mass to charge ratio (m/z) and molecular formula of new compounds found after exposure to the plasma. The corresponding formulas of the identified molecular ions assume the ionization mechanism for the original molecule is consistent with ionization mechanism for the products. For example, the predominant ion of phenanthrene (C14H10) contains an additional proton, H+, and is identified as C14 Hþ 11 . Therefore, the ion identified as C14H11O+ is expected to be formed from the ionization of C14H10O. All new compounds identified after plasma exposure showed the addition of oxygen or increase in the number of unsaturated carbon atoms, but no loss in the total number of carbons as compared to the original molecule. The exact mass identified with the mass spectrometer can be used to identify the molecular formula, but not the structure, of any new compounds formed during exposure. Although the structure cannot be uniquely identified, the amount of saturation of the hydrocarbon combined with the IR spectrum can help identify possible structures. A hydrocarbon is completely saturated if it has only single bonds and does not contain a ring. Each unsaturated carbon indicates the presence of one double bond or one ring. Octadecane is a fully saturated hydrocarbon while phenanthrene has 10 unsaturated carbons corresponding to the three rings and seven double bonds in its structure. Figs. 4 and 5 show possible products, based on the identified molecular formulas and amount of unsaturated carbons, for phenanthrene and octadecane that had been exposed to the GD plasma. Note that other possible structures and isomers may exist for each molecular formula. However, the proposed functional groups were identified in the IR spectrum and are present in the compound. In Fig. 4, the location of the two oxygen atoms could be anywhere
Table 1 Percentage mass loss of GD and UV-exposed samples compared to controls.
Table 3 Original compounds, in italics, and identified mass to charge ratios (m/z) found after GD plasma exposure.
Benzoic acid Phenanthrene Octadecane Decanophenone Octadecanoic acid
GD exposure (%)
UV exposure (%)
Control
Exposed
Control
Exposed
59.7 ± 4.1 14.1 ± 1.5 4.9 ± 0.6 4.5 ± 1.6 1.5 ± 0.5
32.3 ± 4.5 1.9 ± 0.6 2.5 ± 1.3 3.0 ± 0.5 0.4 ± 0.8
12.4 ± 2.8 7.1 ± 5.3 0.3 ± 10.1 15.0 ± 5.6 0.6 ± 7.5
102.4 ± 12.4 59.0 ± 15.4 35.5 ± 8.1 39.4 ± 4.2 15.7 ± 5.6
Original compound, molecular formula
m/z of molecular ion
Corresponding formula of molecular ion
Unsaturated carbonsa
Phenanthrene, C14H10
179.084 194.075 209.061
C14H11+ C14H11O+ C14 H9 Oþ 2
10 10 11
225.058
C14 H9 Oþ 3
11
285.278
C 18 H37 Oþ 2 C18 H33 Oþ 3
Octadecanoic acid, C18H36O2
Octadecane, C18H38
Table 2 New infrared peaks found after exposure to the Mars gas plasma. Compound
New peak after exposure, cm1
Assignment
Phenanthrene
3400 1750–1570 1275 1050
OH stretch C@O and C@C stretch Aromatic alcohol? Alcohol?
3400 1712 1625
OH stretch C@O stretch C@C stretch
Octadecanoic acid
1680–1600a
C@O or C@C stretch?
Benzoic acid
No changes
Decanophenone
3400 1750–1570
Octadecane
a
The octadecanoic acid carbonyl peak is broadened after exposure.
253.293 267.269 281.253 283.274 285.283 297.246 299.265
Decanophenone, C16H24O
233.189 247.169 261.150 263.166 279.163
Benzoic acid, C7H6O2 OH stretch C@O and C@C stretch
297.250 299.264
a
123.042
C18 H35 Oþ 3 C 18 Hþ 37 C18H35O+ C18 H33 Oþ 2 C18 H35 Oþ 2 C18 H37 Oþ 2 C18 H33 Oþ 3 C18 H35 Oþ 3 C16H25O+ C16 H23 Oþ 2 C16 H21 Oþ 3 C16 H23 Oþ 3 C16 H23 Oþ 4 C 7 H7 Oþ 2
1 3 2 0 1 2 1 0 2 1 5 6 7 6 6 5 +
It is assumed that the detected molecular ion has an additional H in the case of phenanthrene, octadecanoic acid and decanophenone and is missing an H in the case of phenanthrene, octadecanoic acid and 1-phenyl-decan-1-one.
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Fig. 6. Molar absorptivity of the UV exposed compounds between 200 and 400 nm.
Fig. 4. Structure of phenanthrene and possible structures for compounds found after plasma exposure of phenanthrene.
around the ring, but based on other work the most likely possibility is 9,10-phenanthrenequinone (Helmig and Harger, 1994; Cvrckova and Ciganek, 2005; Wang et al., 2007). Possible structures for the other compounds could be identified with a similar strategy. 3.2. UV exposure UV exposure could cause degradation through two possible mechanisms: Direct photolysis by the absorption of UV photons and reaction with an oxidizing species in the atmosphere. UV/visible absorbance spectra, shown in Fig. 6, were measured for each organic compound between 200 and 400 nm to assess the likelihood of direct photolysis. Phenanthrene, benzoic acid and decanophenone all begin absorbing light at wavelengths less than 300 nm. They have large absorbance at 250 nm and shorter wave-
lengths and are transparent in the longer UV range and the visible. Octadecane and octadecanoic acid only weakly absorb near 200 nm and are transparent everywhere else. Therefore, the degradation of octadecane and octadecanoic acid would be unlikely to depend on direct absorption of UV photons. After exposure, all of the samples, except octadecane, had changed to very light yellow colors from generally white opaque coloration prior to UV exposures. The yellow color change had been observed previously for naphthalene (Oro and Holzer, 1979), but the responsible compound was not identified. Mass loss measurements during UV exposure showed more loss as compared to plasma experiments, probably due to the longer time the samples were exposed to martian pressures in the UV experiments: 21 days, as compared to the 30 min GD exposures. The weight loss again follows the trends of the vapor pressure and, in fact, the benzoic acid was almost completely lost. New IR absorbance features found after UV exposure are given in Table 4. Phenanthrene, octadecane and octadecanoic acid did not show any differences in the IR spectrum between the control sample and the samples exposed to UV. After exposure, the decanophenone sample showed signs of a new OH group as well as a broad absorbance feature around 1700, possibly indicating a
Fig. 5. Structure of octadecane and possible structures for compounds found after plasma exposure of octadecane.
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Table 4 New infrared peaks found after exposure in the Mars chamber for 7 days. Compound
New peak after exposure, cm1 Assignment
Phenanthrene
No detectable changes
Octadecane
No detectable changes
Octadecanoic acid
No detectable changes
Benzoic acid
800–1850
Changes in fingerprint region Distinct CH stretch region
2800–3150 Decanophenone
3400
OH stretch
mixture of carbon–carbon double bonds, which overlaps with the carbonyl feature in the parent compound. Benzoic acid showed the most dramatic change. The control showed the broad absorbance centered at 2800 cm1 typical of carboxylic acids. The sample that had been exposed to UV irradiation had a clear feature for OH (3500 cm1) not seen in the control, and the carbonyl as well as other features in the fingerprint region, all had small but distinct shifts in frequency. Nearly all of the benzoic acid was lost during exposure, and it is possible that this newly identified product was purified during the exposure. This might have enhanced the ability to detect this product under these conditions as compared to the GD plasma exposed samples since not as much mass was lost in that experiment. Results of MS analysis for the UV-exposed samples are shown in Table 5. No degradation products were found for phenanthrene, octadecane or octadecanoic acid after UV exposure. Decanophenone showed the addition of oxygen much like, but to a lesser extent, than, the GD plasma exposed samples. The residue from the benzoic acid exposure showed a product with the addition of oxygen that was not identified after exposure to plasma.
4. Discussion 4.1. Chemistry Chemistry in plasma is driven by reactions due to four key factors: free ions, UV radiation, free radicals, and chemically reactive neutral species. The emission spectrum of the plasma can be used to identify some of these reactive species. Cationic species of CO2, N2 and CO, as well as excited neutral species, with enough energy
Table 5 Original compounds, in italics, and identified mass to charge ratios (m/z) found after Mars chamber exposure. Original compound, molecular formula
m/z of molecular ion
Corresponding formula of molecular ion
Unsaturated carbonsa
Phenanthrene, C14H10 Octadecanoic acid, C18H36O2 Octadecane, C18H38 Decanophenone, C16H24O
179.086 285.278
C 14 Hþ 11 C 18 H37 Oþ 2
10 1
253.293 233.189 247.171
C 18 Hþ 37 C16H25O+ C16 H23 Oþ 2
0 5 6
Benzoic acid, C7H6O2
261.153 123.047
C16 H21 Oþ 3 C 7 H7 Oþ 2
7 5
139.041 159.066
C7 H7 Oþ 3 C7 H11 Oþ 4
5 3
a It is assumed that the detected molecular ion has an additional H+ in the case of phenanthrene, octadecanoic acid and decanophenone and is missing an H in the case of octadecane, due to the MS ionization process.
to initiate chemistry were identified in the plasma. The high energy species produced in the plasma may lose their energy through reactions with other gas phase species, reactions with the larger organic molecules present in the experiment, radiative decay, or þ collisional deactivation. For example, the Nþ 2 and CO2 ions can react with small hydrocarbons, both breaking a CH bond and still leaving reactive species (Anicich, 1993). Presumably, similar reactions could occur with the larger hydrocarbons used in these experiments. The excited neutral species of CO and N2 present in the plasma can decay into states that still have considerable energy compared to the ground states. For example, the excited A state of CO and B state of N2, both identified in the emission spectrum, have about 780 kJ/mol and 600 kJ/mol of energy, respectively, relative to their ground states. The A state of CO would be expected to radiative decay to its ground state, giving off a photon at a wavelength, 154 nm, which is below the short wavelength limit of this experiment, 190 nm. However, the A and B excited states of N2 should be long lasting, as radiative decay to the ground state is unlikely because it is spin forbidden. The excited states of N2 would likely undergo collisional deactivation, liberating between 600 and 700 kJ/mol energy. This is enough energy to break typical carbon–hydrogen bonds, which have strengths of about 400 kJ/mol. These reactions, in addition to the high energy UV light given off in the plasma, cause the chemistry that occurs in the plasma. After plasma exposure, new products were found for each compound except benzoic acid. The new products were similar among phenanthrene, octadecane, octadecanoic acid, and decanophenone, in that they all had either the addition of one or more oxygen atoms or the formation of unsaturated carbons. Even though there were many reactive species in the GD plasma, the oxygen-enriched products were the only types found in these experiments. The identification methods used in this study only measured the nonvolatile compounds remaining after exposure and would have missed compounds that were volatilized during the exposure, such as those seen by Stoker and Bullock (1997). The vast number of reactive species available in the plasma, makes predicting the reactions that formed the new compounds difficult. However, the new compounds have been found to occur in known reactions. The reaction of phenanthrene with hydroxyl radical has been studied for the Earth’s atmosphere. The production of 9,10-phenanthrenequinone has been reported (Helmig and Harger, 1994; Cvrckova and Ciganek, 2005; Wang et al., 2007), and may be one of the possible compounds identified in this study, suggesting that the oxidation process in the GD plasma is similar to the oxidation processes occurring in Earth’s atmosphere. UV initiated chemistry proceeds through two mechanisms: direct photolysis caused by the absorbance of a UV photon or reaction with a UV created reactive radical, such as the hydroxyl radical, OH. The hydroxyl radical is thought to carry out the oxidation of organic compounds on Mars (Barth et al., 1992). The likelihood of direct photolysis can be determined from the UV absorbance spectra of the compounds. The reactions with OH are known for many organic compounds and can be used to evaluate potential reaction paths in these experiments. Benzoic acid and decanophenone were the only compounds with identified products after UV exposure. The lack of any identified reaction products for octadecane and octadecanoic acid after UV exposure can be explained by their weak absorption of UV light, as shown in Fig. 6. Direct absorbance of UV by these two molecules is considerably less than the others tested. Therefore, the only mechanism of degradation would be via OH radical reactions. Although the hydroxyl radicals are presumably produced under high UV fluence rates in the presence of water, no direct measurements of OH radicals were made. The removal of hydrogen by the hydroxyl radical depends on the carbon– hydrogen bond strength in the compound. The weaker the bond,
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the easier it is to remove the hydrogen. The bond strengths found in saturated hydrocarbons such as octadecane are stronger than those found in other compounds in the study. Octadecanoic acid, has similar bond strengths for its long hydrocarbon chain, and would only have weaker carbon–hydrogen bonds near the acid group. These strong bonds may account for slower reactions as compared to the other compounds. Both decanophenone and benzoic acid have properties lending to reactions under the simulated martian conditions. Both have significant UV absorbance that could lead to direct photolysis. Decanophenone has hydrogen moieties that are more susceptible to abstraction by hydroxyl radicals than some of the other compounds evaluated here. The relative bond strengths for these compounds are assumed here based on similar compounds. The strength of carbon–hydrogen bonds found close to aromatic rings and carbonyls can be much lower than in unsubstituted alkanes. For example, the carbon–hydrogen bond strengths in similar compounds are 85.4 kcal/mol for ethyl benzene (H–CH(CH3)C6H5), 86.9 for benzaldehyde (H-COC6H5) and 92 for 2-butanone (H–CH (CH3)COCH3) as compared to 96–100 found for typical alkanes (Lide, 1992). This indicates that decanophenone would react more aggressively with hydroxyl radicals in addition to direct UV photolysis that might occur. The reaction of poly aromatic hydrocarbons (PAH’s) in the Earth’s atmosphere, including phenanthrene, proceeds via an initial reaction where a hydroxyl radical is added to the PAH forming an intermediate. This is followed by either a further reaction, often with molecular oxygen (O2) (Helmig and Harger, 1994), resulting in an oxidized product, or the decomposition of the intermediate to the original reactants, preserving the phenanthrene molecule (Atkinson and Arey, 2007). During UV irradiation under martian conditions, it may be possible that a lack of molecular oxygen might explain the paucity of identified products for phenanthrene. The intermediate hydroxyl adduct may be formed, but it may not proceed any further and return to phenanthrene. There were clear differences between the behavior of benzoic acid in the plasma and when exposed to UV. In both cases, there was considerable mass loss, but in the UV exposed sample there were detectable products. Nearly all of the benzoic acid was gone after UV exposure, while only 30% was lost after GD plasma exposure. It is possible that the identified product was concentrated by evaporation of benzoic acid or volatilization of other products. 4.2. Comparison between GD plasma and UV-exposed samples results More degradation products were found for all other compounds in the GD plasma exposed samples as compared to the UV-exposed samples. The plasma was a more aggressive environment than the martian UV conditions. Unfortunately, there was no way to quantify the aggressiveness of the plasma in any way other than exposure time. In both types of exposure, only compounds that had the same number of carbons as the initial compound were identified. The only reactions that could be identified were the addition of oxygen or the formation of carbon double bonds from an alkane. No products smaller than the initial compounds were identified, although the presence of other compounds is not precluded. For example, there was a change in color in phenanthrene and octadecanoic acid after UV exposure, indicating the formation of a new compound. However, the analytical methods used in this study were unable to detect the color inducing compounds. The differences in identified products are probably due not only to the aggressiveness, but to the presence of additional reactants in the plasma. For example, the reaction of phenanthrene occurs through a stepwise process were a hydroxylated phenanthrene intermediate adduct reacts with an oxygenated species to form a stable oxidized species. The presence of more reactive oxygen spe-
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cies in the plasma as compared to the UV conditions probably led to more reactions for phenanthrene. Previous experiments (Oro and Holzer, 1979; Stoker and Bullock, 1997; ten Kate et al., 2005, 2006) on the decomposition of organic compounds under simulated martian conditions focused on the rate of decomposition, and not on identifying the products of the decomposition reactions. In both the GD and UV exposure studies described here, the residual organic material was characterized in hopes that the results could be used to design instruments for identifying organic material on Mars. Similar compounds containing additional oxygen were identified after both GD and UV exposure. These compounds are similar to, or are intermediates leading to, the refractory organic compounds that have been previously postulated to not having been identified in the Viking instruments (Benner et al., 2000; Cabane et al., 2004; NavarroGonzalez et al., 2006). No compounds with smaller numbers of carbon atoms were identified. There are a few factors that could have led to this result: (1) the majority of the mass loss could have been due to evaporation, and there was in fact very little conversion of the initial compounds to smaller compounds; (2) the smaller compounds were more volatile and therefore were not in the residue collected after the experiments; (3) any smaller compounds that were collected were not identified with the techniques used in this study. The importance of each factor would be highly dependent on the compounds formed. For example, octadecane might lose two carbon atoms to form a series of compounds with 16 carbon atoms and similar functional groups to what was found for octadecane: hexadecane, hexadecyl alcohol, hexadecene and hexadecanone. Hexadecane, hexadecanone and hexadecene are more volatile than octadecane, while hexadecyl alcohol is less volatile (Lide, 1992). This would lead to a concentration of hexadecyl alcohol, while the other compounds would evaporate over time. In situ measurements of volatile compounds would be needed to determine if smaller compounds are formed. Infrared spectroscopy, as was performed in these experiments, is not sensitive enough to determine if hexadecane was present as a trace compound in octadecane. Although the mass spectrometer used is very sensitive, it is possible that some compounds were missed due to preferential ionization of some compounds. Identification of all trace compounds, such as those causing the color change in phenanthrene and octadecanoic acid after UV exposure, would probably require extraction and separation methods (e.g. gas or liquid chromatography). However, the experiments reported here do show that oxidized compounds are formed during GD or UV exposure under Mars-like conditions.
5. Conclusions A GD plasma induced by electrostatic discharges in a dust storm on Mars is likely to generate highly reactive species, capable of reacting with all types of suspended organic compounds. Reactive species produced in the plasma and high energy UV emissions were identified. The GD plasma produced a more oxidative environment than the UV irradiation created within the MSC system, but a direct comparison of the reactivity of the two cannot be made since the reactivity of the plasma cannot be quantified. In both plasma and UV exposures, the degradation reactions produced oxidized, higher molecular weight compounds as compared to the starting materials. Although the compounds exposed in this study are only found at trace levels in meteorites, it is thought that they might act as models for the functional groups such as alkanes, polyaromatic hydrocarbons, ketones and carboxylic acids found in the organic polymers prevalent in meteorites. The non-volatile compounds with polar functional groups produced in these experiments would be difficult to analyze with
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gas chromatography. No compounds with smaller molecular weight than the original materials were identified. Future missions searching for organics need to be capable of identifying organics with varying degrees of oxidation. Detection of oxidized products can be aided by finding different ways to extract the organics from the soil and separating the compounds in the analysis instrument. Thermal volatilization was used during the Viking missions, and may cause compounds to be catalytically oxidized to carbon dioxide during heating, causing a loss of information about the organics (Navarro-Gonzalez et al., 2006). Furthermore, polar, high boiling, organic compounds such as organic acids, are not ideally separated by gas chromatography. Solvent extractions (Buch et al., 2003) and chemical derivatization (Cabane et al., 2001, 2004; Rodier et al., 2001) have been proposed as methods to aid in extraction of organics. Solvent extraction is a common method for removing organic compounds from environmental samples. Chemical derivatization involves converting the polar groups in a compound to non-polar groups making the original compound more volatile. Chemical derivatization has the advantage of making the separation with GC much easier. The Sample Analysis at Mars instrument suite will use chemical derivatization to search for organic material as part of the Mars Science Laboratory mission (Mahaffy, 2008). Another approach could be to use supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC). These methods use a supercritical fluid, most commonly carbon dioxide, for extraction of compounds from a matrix and as the mobile phase in chromatography. SFE and SFC have been used in a variety of systems (Bowadt and Hawthorne, 1995; Smith, 1999). Supercritical carbon dioxide extraction is currently an approved EPA method for recovering various organic compounds, as described in Methods 3560, 3561 and 3562 of ‘‘Test Methods for Evaluating Solid Waste, Physical/ Chemical Methods” (SW-846) (2008a,b). Supercritical carbon dioxide needed for the extractions and separations could be made in situ from the martian atmosphere. SFE with pure carbon dioxide as solvent is not efficient at removing polar species; however, the use of small amounts of co-solvents improves performance greatly (Smith, 1999) and has been used to extract amino acids from biological samples (Bernal et al., 2008). SFC is similar to liquid chromatography, has been used for polar solvents without derivitization, and has been successfully used to separate chiral compounds (Berger, 1997; Smith, 1999). Using this technique, the chirality of amino acids and other potentially biological materials could be determined. Acknowledgments The authors would like to acknowledge NASA Science Mission Directorate for funding (Grant #MFRP04-0028-0090). The authors thank Dr. Nasri Nesnas at Florida Institute of Technology for help with the DART mass spectral technique. P.E.H. would like to thank the National Research Council for a Research Associateship Award at NASA Kennedy Space Center. References Anicich, V.G., 1993. Evaluated bimolecular ion-molecule gas-phase kinetics of positive-ions for use in modeling planetary-atmospheres, cometary comae, and interstellar clouds. J. Phys. Chem. Ref. Data 22, 1469–1569. Atkinson, R., Arey, J., 2007. Mechanisms of the gas-phase reactions of aromatic hydrocarbons and PAHs with OH and NO3 radicals. Polycyclic Aromat. Compd. 27, 15–40. Atreya, S.K., Wong, A.-S., Renno, N.O., Farrell, W.M., Delory, G.T., Sentman, D.D., Cummer, S.A., Marshall, J.R., Rafkin, S.C.R., Catling, D.C., 2006. Oxidant enhancement in martian dust devils and storms: Implications for life and habitability. Astrobiology 6, 439–450. doi:10.1089/ast.2006.6.439. Barth, C.A., Hord, C.W., Pearce, J.B., Kelly, K.K., Anderson, G.P., Stewart, A.I., 1971. Mariner 6 and 7 ultraviolet spectrometer experiment: Upper atmosphere data. J. Geophys. Res. 76, 2213–2227.
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