Thermally evolved gas analysis (TEGA) of hyperarid soils doped with microorganisms from the Atacama Desert in southern Peru: Implications for the Phoenix mission

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Advances in Space Research 44 (2009) 254–266 www.elsevier.com/locate/asr

Thermally evolved gas analysis (TEGA) of hyperarid soils doped with microorganisms from the Atacama Desert in southern Peru: Implications for the Phoenix mission Julio E. Valdivia-Silva a,*, Rafael Navarro-Gonza´lez a, Christopher McKay b a

Laboratorio de Quı´mica de Plasmas y Estudios Planetarios, Instituto de Ciencias Nucleares, Universidad Nacional Auto´noma de Me´xico, Mexico, Distrito Federal, C.P. 04510, Mexico b Space Science Division, NASA Ames Research Center, Moffett Field, CA, USA Received 11 November 2008; received in revised form 15 February 2009; accepted 16 February 2009

Abstract TEGA, one of several instruments on board of the Phoenix Lander, performed differential scanning calorimetry and evolved gas analysis of soil samples and ice, collected from the surface and subsurface at a northern landing site on Mars. TEGA is a combination of a high temperature furnace and a mass spectrometer (MS) that was used to analyze samples delivered to the instrument via a robotic arm. The samples were heated at a programmed ramp rate up to 1000 °C. The power required for heating can be carefully and continuously monitored (scanning calorimetry). The evolved gases generated during the process can be analyzed with the evolved gas analyzer (a magnetic sector mass spectrometer) in order to determine the composition of gases released as a function of temperature. Our laboratory has developed a sample characterization method using a pyrolyzer integrated to a quadrupole mass spectrometer to support the interpretations of TEGA data. Here we examine the evolved gas properties of six types of hyperarid soils from the Pampas de La Joya in southern Peru (a possible analog to Mars), to which we have added with microorganisms (Salmonella typhimurium, Micrococcus luteus, and Candida albicans) in order to investigate the effect of the soil matrix on the TEGA response. Between 20 and 40 mg of soil, with or without 5 mg of lyophilized microorganism biomass (dry weight), were placed in the pyrolyzer and heated from room temperature to 1200 °C in 1 h at a heating rate of 20 °C/min. The volatiles released were transferred to a MS using helium as a carrier gas. The quadrupole MS was ran in scan mode from 10 to 200 m/z. In addition, 20 mg of each microorganism without a soil matrix were analyzed. As expected, there were significant differences in the gases released from microorganism samples with or without a soil matrix, under similar heating conditions. Furthermore, samples from the most arid environments had significant differences compared with less arid soils. Organic carbon released in the form of CO2 (ion 44 m/z) from microorganisms evolved at temperatures of 326.0 ± 19.5 °C, showing characteristic patterns for each one. Others ions such as 41, 78 and 91 m/z were also found. Interestingly, during the thermal process, the release of CO2 increased and ions previously found disappeared, demonstrating a high-oxidant activity in the soil matrix when it was subjected to high temperature. Finally, samples of soil show CO2 evolved up to 650 °C consistent with thermal decomposition of carbonates. These results indicate that organics mixed with these hyperarid soils are oxidized to CO2. Our results suggest the existence of at least two types of oxidants in these soils, a thermolabile oxidant which is highly oxidative and other thermostable oxidant which has a minor oxidative activity and that survives the heat-treatment. Furthermore, we find that the interaction of biomass added to soil samples gives a different set of breakdown gases than organics resident in the soil. The nature of oxidant(s) present in the soils from Pampas de La Joya is still unknown. Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Thermal analysis; TEGA; Atacama Desert; Pampas de La Joya; Hyperarid soils; Phoenix mission

*

Corresponding author. Tel.: +52 55 56587940. E-mail addresses: [email protected], [email protected] (J.E. Valdivia-Silva), [email protected] (R. NavarroGonza´lez), [email protected] (C. McKay). 0273-1177/$36.00 Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2009.02.008

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1. Introduction Thermal analysis is a convenient processing technique for the transformation of relatively large substances into volatile compounds suitable for analysis by mass spectrometry (MS). High temperatures during rapid heating (pyrolysis) or slow heating (thermolysis) have been used for bond breakage and fractionation of molecules in different types of samples, such as, agricultural soils (Schulten and Leinweber, 1993a,b), microorganisms (Miketova et al., 2003; Snyder et al., 2005), ancient soils (Wang et al., 2000), minerals (Boynton and Ming, 2007; Lauer et al., 2006), soils samples for organic matter quantification (Manning et al., 2005; De la Rosa et al., 2008) and martian regolith by the Viking landers (Biemann et al., 1977). A key goal in the search for evidence of life on Mars is the detection of organic matter. Since thermal analysis does not require any solvents for organic extraction, it has been the method of choice for searching organics on Mars. Thus, one of the primary objectives of the Phoenix scout mission, which landed on Mars on May 25, 2008 was to search for habitable zones by assessing organic or biologically interesting material in icy soils on the surface, using thermal volatilization followed by mass spectrometry (TV–MS) (Smith, 2004; Hoffman et al., 2008). For the Viking landers the detection limit of the GC– MS was ppb for organics (Biemann et al., 1979). However, this does not mean that the overall limit on organics in the soil set by Viking is ppb. The reason is that the method (thermal volatilization) used to release organics from the soil and get them into the GC–MS was not very efficient. In this regard, recent studies have shown that the pyrolysis technique has limitations in transferring intact organic fragments into the GC–MS when the soil contains low levels of organics (Skelley et al., 2005; Navarro-Gonzalez et al., 2006). Navarro-Gonza´lez et al. have shown two important limitations of the pyrolysis technique. First, when organics are present as low level refractory substances, the temperatures reached by Viking (up to 500 °C) may be inadequate to release the organics. Second, the iron present in the soil oxidizes the organics when heated resulting in the formation of CO2 and H2O. The organics have to be present at high levels to overcome this effect. Thus it is probable that the real limit set by Viking on organic content of the soil is more likely to be at ppm even though the instrument detection limit was ppb (Navarro-Gonzalez et al., 2006). It is important to emphasize that the work of Navarro-Gonzalez et al. (2006), only raised limitations in the pyrolysis step but not on the GC–MS instrument, which they concluded it operated flawlessly as the GC–MS was designed and built (Biemann, 2007; Mukhopadhyay, 2007). In addition, other studies using flash TV–GC–MS have shown earlier no reliable detection of organics in agricultural soils if the level of organics were below 50,000 ppm C or in the presence of iron oxides (Schulten and Leinweber, 1993a). Given that martian regolith may have low levels of organic matter

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and contain high levels of metal oxides, including iron oxides (Navarro-Gonzalez et al., 2003), it is useful to investigate the thermal response of certain type of soils considered as ‘‘Mars analogs” to understand the effect of the matrix in the breakdown of organic matter during thermal processing. The Atacama Desert, in northern Chile and southern Peru, is one of the most, if not the most, arid regions on Earth (McKay et al., 2003) and the arid core of the Atacama near Yungay Chile contains Mars-like soil (Navarro-Gonzalez et al., 2003). These soils are Mars-like in that they contain very low levels to organic matter (20–40 ppm of organic C), a non-biological oxidant, the virtual absence of microscopic life, and exotic mineralogical composition including iron oxides, which are common characteristics expected on Mars (Navarro-Gonzalez et al., 2006; Fletcher et al., submitted for publication). However, Pampas de La Joya, the Atacama region located on southern Peru between 15 and 17 °S, has recently had an astrobiological interest because it exhibits hyperarid soils with the lowest levels to organics. This fact suggests that this region may also contain Mars-like soils (Valdivia-Silva et al., 2005). In this paper, we examine the thermal and evolved gas properties of six types of hyperarid soils from the Pampas de La Joya (Atacama Desert in southern Peru). We examine untreated soil samples and samples enriched with three different types of the microorganisms (Salmonella typhimurium, Micrococcus luteus, and Candida albicans), in order to answer two basic questions: what is the TV response of the hyperarid soils from Mars-like soils? and can these soils alter the TV response of the microorganisms present in them? Our experiments simulate the TV approach of the Phoenix Lander by subjecting samples to a slow heating followed by mass spectrometry of the evolved gas analysis (EGA). 2. Methods 2.1. Soil samples and microorganisms Soil samples used in this study were collected from 2004 to 2007 in the Pampas de La Joya, Atacama Desert in southern Peru, located about 50 km from the city of Arequipa between 15 and 17 °S. This extreme hyperarid location is under studied because of geomorphologic, paedologic and climatologic characteristics of astrobiological interest (Navarro-Gonzalez et al., 2006; Valdivia-Silva et al., 2005). The hyperarid zones were derived from an Aridity Index (AI) which is calculated as the ratio P/PET (evotranspiration/precipitation) <0.05 (Thornwaite’s, 1948; UNEP, 1997). The total area of the desert is approximately 150 km2 and it was divided into six types of soils based on texture and mineralogy. Around 200–400 g representing a composite of five individual nearby sites (1.5 m in radius) of each one of these six types of soils were collected from the surface to a depth of 5–10 cm using sterile scoops and stored in sterile polyethylene (Whirlpak ) bags for TM

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Table 1 Characteristics of soil samples. Type of soil

Sample

Latitude

Longitude

Altitude (m)

Organic C (ppm) #

Carbonates (ppm) &

% abund.*

pHs

Granulometry (% abundance)

I II III IV V VI

PE-361 PE-388 PE-386 PE-287 PE-276 PE-001

16°43.985 16°44.563 16°08.742 16°38.386 16°40.586 16°44.419

72°02.067 72°02.579 73°38.080 72°02.679 71°58.279 72°02.064

1033 1140 1022 1150 1183 1147

5.8 3.2 35.0 7.9 22.4 11.4

1272 2110 1338 526 1910 2090

28.73 4.22 4.52 29.01 18.31 15.21

6.70 6.82 5.90 6.78 6.01 6.80

Sa:79.3, S:12.07,Cl:8.63 Sa:65.28, S:23.77,Cl:10.05 Sa:76.58, S:14.41,Cl:9.01 Sa:83.32, S:8.37,Cl:8.31 Sa:92.37, S:3.80,Cl:3.83 Sa:69.19, S:17.20,Cl:13.61

Samples are representative of each one type of soils. (*) Percentage of relative abundance of each type soil on the area evaluated from Pampas de La Joya (150 km2). (#) Organic was evaluated using Permanganate titration and calcination techniques (Fletcher et al., 2009; Perez et al., 2008). Cl, clay; S, silt; Sa, sand. (&) Most abundant carbonates in the samples: CaCO3 , CaMg(CO3)2 (by Scanning Electron Microscopy with X-ray microanalysis).

transport until analysis. The representative sampling sites are shown in Table 1. In addition, some samples were pre-treated to 500 °C for 24 h to remove organic traces. Microorganisms used as soil additions in this study were Gram-positive M. luteus (ATCC 9341), Gram-negative S. typhimurium (ATCC 21102), and fungus C. albicans (ATCC 10231). Bacteria cultures were incubated at 37 °C in nutrient broth (Sigma), while fungus was cultured on dextrosa-sabouraud broth (Sigma) for 3, 4 days until production reached a maximum. Microorganisms were washed three times with 200 ml sterile water, centrifuged to 5000 rpm, resuspended in sterile water, and lyophilized. Organic matter was determined by titration with the oxidation of permanganate and by calcination technique as reported before (Navarro-Gonzalez et al., 2006; Perez et al., 2008; Fletcher et al., submitted for publication). 2.2. TV–MS Similar to the TEGA instrument, our laboratory has developed a sample characterization method using a pyrolyzer integrated to a quadrupole mass spectrometer to support the interpretations of EGA data (TV–MS). In our experiments 20–40 mg of soil, 20–40 mg of microorganism or a mixed 1:4 (5 mg of microorganism mixed with soil) were loaded in a capillary quartz tube previously heated at 500 °C for 8 h to oxidize any organics. Each tube was mounted in the center of platinum coil filament pyrolyzer probe (Pyroprobe 2000 from CDS Analytical, Inc.). Then, the pyrolyzer probe was introduced into a stainless steel chamber. Atmospheric air was removed from the chamber and probe by flushing a stream of helium (20 ml/min, 99.9999%, 60 PSI, 3 min). The sample contained in the quartz tube was subjected to a thermal treatment from 30 to 1200 °C with a heating rate of 20 °C/min. The resulting volatiles evolved from the sample were carried away by helium (3 ml/min, at standard temperature and pressure) from the chamber (maintained at 250 °C) and transferred into a HP quadrupole mass spectrometer (5989B) operating in electron ionization mode at 70 eV with a resolution of 1 m/z. The mass analyzer was scanned from 10 to 200 m/z at a rate 5.3 scans per second. The ionization chamber and the quadrupole were maintained at

250 °C and 100 °C, respectively. The nominal sensitivity of the mass analyzer is 0.02 ppb of hexachlorobenzene. In some cases, mass analyzer was scanned from 40 to 200 m/z in order to eliminate from the analysis ions under 44 m/z (CO2), such as water, nitrogen, oxygen, etc., which caused interference in certain analysis to be performed. Blanks were prepared without soil in the analytical protocols to determine any source of contamination. 3. Results and discussion 3.1. TV response from microorganisms Thermal volatilization coupled to mass spectrometry (TV–MS) provides information from time-temperature thermal fractionation of chemical molecules producing a curve from a mixture of ions which can be monitored tracking their release at a specific temperature (Fig. 1A and B). Thus, different samples have been investigated by Pyr-MS, including microorganisms (mainly bacteria) for individual biochemical constituents (Materazzi and Curini, 2001; Miketova et al., 2003; Vyazovkin, 2004). These compounds are directly related to characteristics which can identify between taxonomic groups and even in many cases, between species and subspecies (Snyder et al., 2005). In this study we found, as expected, patterns of masses containing information which are distinctive microorganism status between S. typhimurium (Gram-negative bacteria), M. luteus (Gram-positive bacteria), and C. albicans (fungus) as demonstrated in other studies (Miketova et al., 2003; Vyazovkin, 2004). Although the literature provides very little thermal analyses for microorganisms; it is known that during flash TV different biochemical compounds, such as proteins, peptides and free aminoacids, yield a series of carboxylic acids, saturated nitriles, and saturated, unsaturated aromatic hydrocarbons (Simmonds et al., 1969). The carbohydrates degrade to a series of aliphatic aldehydes, ketones, carboxylic acids, aromatic compounds and furan derivatives; the fatty acids pyrolyse to alkanes, alkenes, aromatic compounds, and short chain carboxylic acids; the porphyrins degrade to pyrroles; and finally nucleic acid bases release unsaturated nitriles, and substituted furans (Navarro-

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Fig. 1. TV–MS traces for microorganisms. Ions can be monitored according to their release to a specific temperature. (A) Abundant fragments for S. typhimurium, (B) Mass spectra evaluated at 560 °C observing the evolved ions for S. typhimurium. (C) Trace for M. luteus, and (D) C. albicans. Mass analyzer was scanned from 40 to 200 m/z.

Gonzalez et al., 2003). Since the present experiment, like the Phoenix TEGA instrument, is not coupled to a GC for separation of volatiles, TV of microorganisms released a mixture of all the above classes of organic compounds (Fig. 1B). Hence, the objective of this study was not to analyze and characterize each microorganism according to taxonomic groups, but focused instead on detecting characteristic ions which can be monitored during the thermal process and function as possible biomarkers, showing the greatest abundances and release in any of the three different taxonomic groups used (bacteria and fungi). In addition, the yield of fragments during slow heating has not been adequately investigated. Our results showed that the major mass fragments (ions, mass-to charge, m/z, ratios) released by TV–MS were 41, 44, 91 and 107 which were the most representative in the traces for each microorganism (Fig. 1A, C and D). In general, it is well known that derivative mass fragments from linear saturated or unsaturated chemical species yield ions smaller than 78 m/z (to lower masses), and saturated or unsaturated aromatics compounds release mass fragments P78 m/z which represent benzene (to higher masses) (Wieten et al., 2000). Assuming this fact, fragments 41 m/z and 91 m/z (in some cases 107 m/z) were chosen as representative fragments of linear and aromatic molecules, respectively, to follow their response when mixed with soil. Curiously, the intensity of formation of masses 41 and 91 at a low temperature regime (270–

1000 °C) was overwhelmed by that of mass 44 m/z which results to a major extent from carbon dioxide (CO2) and to a minor extent from nitrous oxide (N2O) from the thermal oxidation of nitrogen-containing organics (Hao et al., 1994). The mass 44 forms in the temperature range from 150 to 1200 °C exhibiting maximum located between 350–450 and 800–900 °C. The possible source for the release of CO2 in this case is the thermal oxidation of non-complex organic matter, principally at temperatures <600 °C (Gonzalez-Vila and Almendros, 2003; Philp, 2003); and decomposition of complex molecules such as polyhydroxyalkanoates (PHAs) and their copolymers produced by microorganisms at temperatures >600 °C (Doi and Steinbuchel, 2002; Molto et al., 2005). As the carbonates produce CO2 at temperatures above 600 °C, and they can overlap the CO2 from organic complexes; only CO2 at lower temperatures was seen coming from organic matter. 3.2. TV response in hyperarid soils from Pampas de La Joya Volatile ions released from soils during TV–MS analysis were analyzed searching for organics fragments prior to mixing with microorganisms. As expected, there were significant differences in the evolved gas behavior between hyperarid soils samples under similar heating conditions depending of the soil matrix and contents of organics. Some chemical and textural characteristics of sample soils

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are shown in Table 1. The samples belonging to the most arid environments and containing the lowest levels of organic matter, PE-361 and PE-388 (corresponding to type I and II soils, respectively), had similar responses to thermal processing. These soils contain 3–6 ppm organic C and the TV–MS trace for them shows the release of the following major mass fragments: 18, 44, 48 and 64 m/z (Fig. 2A and B). The chemical nature for these masses was determined in our laboratory using GC–MS after analyzing the stable products arising from a two-step TV process: 100–500 °C, and 500–1000 °C. (Navarro-Gonzalez et al., in press). Indeed, mass 18 originates from water releasing in the course of dehydration processes that is bound to soil minerals, and from the oxidation of organics at different temperatures depending the mineral fraction in

the soil. Mass 44 m/z mostly represents carbon dioxide (CO2) exhibiting a temperature range between 1000 and 1200 °C in these soils. In thermal treatment of soils the sources for CO2 are oxidation of organic matter at temperatures lower than 600 °C (Trofimov and Emelyanenko, 2000; Philp, 2003; Lopez-Capel et al., 2005) and thermal decomposition of carbonates at temperatures higher than 600 °C (Wang et al., 2000; Stalport et al., 2005). Another possible ion with equal mass is nitrous oxide (N2O), but it has less abundance than CO2 and it is released at temperatures lower than 550 °C. It is important to clarify that even though CO2 at lower temperatures could show the presence of organic matter in the soils, its high concentration in the atmosphere and soil could cause problems during detection and quantification of organic compounds.

Fig. 2. TV–MS traces for hyperarid soils (A) type I: PE-361, (B) type II: PE-388, (C) type VI: PE-001; and they mixed with microorganisms, (D) soil type I, (E) soil type II, and (F) soil type VI. Mass analyzer was scanned from 10 to 200 m/z.

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Curiously the evolved CO2 from the carbonates in these soil samples was released at several 100° higher than expected from pure carbonates, which have temperatures of thermal degradation approximately between 500 and 950 °C. The possible cause of this effect will be discussed below. Finally, masses 48 and 64 showed similar thermal behaviors, being released at 550 °C and continuing to rise up to 1200 °C. These masses result from thermal degradation of soil sulfates into SO2+ (64, based peak = 100%), and SO+ (48, peak 50%). In some cases, mass 66 is detected only if the abundance of mass 64 is very high (S34O2+). Curiously, despite the fact that these soils contain low organic levels they did not show any organic fragments detected by TV–MS nor did they show the mass 44 m/z

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release at T < 600 °C which is usually an abundant product from thermal treatment of organics (Gonzalez-Vila and Almendros, 2003; Philp, 2003). Presumably, the absence of ion 44 was due to dilution, given the small amount of organics and heating time (1 h). Another hyperarid soil that contains very low levels of organics (11.5 ppm) is the type VI represented by sample PE-001. Thermal treatment showed similar fragments to type I or II soil (Fig. 2C); however, an important difference was the release of mass 44 at very low temperatures (350 °C) as expected for CO2 released from oxidation of organic matter. Moreover, when soil PE-001 was pre-treated to 500 °C to remove traces of organics ion 44 was released only above 600 °C (Fig. 3C). TV–MS traces for sample of PE-361, 388, and 001 pre-heated to 500 °C showed similar releases to samples that had not been

Fig. 3. TV–MS traces for pre-heated soils (500 °C  24 h), (A) type I (PE-361), (B) type II (PE-388), (C) type VI (PE-001); and mixed with microorganisms. (D) The graphic is representative for soil type I and soil type II, and (E) soil type VI. Mass analyzer was scanned from 40 to 200 m/z.

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treated; both showed fragments 44, 48 and 64 m/z (Fig. 3A–C). Because the mass analyzer was scanned from 40 to 200 m/z for these pre-treated soils, the analysis of ions under 44 m/z, such as water, do not appear in the diagrams. TV–MS trace from soil type V (PE-276), which contains 22.4 ppm of organic C, showed the release of the following ions: 18, 44, 48, and 64 m/z (Fig. 4A). Its trend was similar to previous samples of type I, II, and VI; although in this one, mass 44 presented two maxima points centered at 450 °C and above 1100 °C. As in previous soils, mass 44 originates mostly from CO2 and to a lesser extent (1%) from N2O. The origin of this ion must be the oxidation of organic matter at lower temperatures (Hao et al., 1994) and thermal decomposition of carbonates at higher temperatures (Wang et al., 2000; Stalport et al., 2005), respectively. Again, there were no distinctive organic fragments detected by TV–MS despite the presence of 20– 30 ppm of organic carbon. Moreover, the fragments 18, 48, and 64 m/z were very similar to previous analysis. Thermal process from soil type V (PE-276) pre-heated to 500 °C, did not show the initial release of ion 44, which corroborates that this peak was due to organic matter (Fig. 4B). In contrast, to previous samples, soils type III (PE-386) and IV (PE-287) showed substantial differences when they were analyzed by TV–MS (at peak times, start or maximum points of release) (Fig. 5A and B). The soil sample

PE-386, which contains 35 ppm of organic C, indicated the release of the following major mass fragments: 16, 18, 28, 30, 36, 38, 44, 48 and 64 m/z. Mass 16 was detected by TV–MS between 450 and 750 °C, reaching a maximum at 650 °C. It was likely identified as methane (CH4+), but could be masked by atomic oxygen arising from the electron ionization of molecules that contain oxygen such as CO2, CO, H2O, and/or SO2 during TV–MS. Interestingly, the maximum point coincides with the peaks of CO (28 m/z) and CO2 (44 m/z) (Fig. 5A). Fragment 18 resulted from water released by thermal processing as explained above. A new mass detected in soil type III was 28 m/z between 300 and 800 °C; it results from incomplete oxidation of the organic carbon to carbon monoxide (CO) and/or due to electron dissociation of CO2. Thus, the mass 28 had a similar trend as that of 44. In addition, ion 44 formed in the temperature range from 200 to 1200 °C exhibited two maximum centered at 360 and 640 °C which correspond to the oxidation of organics at low temperature and decomposition of carbonates at higher temperature. The masses 48 and 64 have similar thermal properties, being released at temperatures above 630 °C, reaching a maximum at 800 °C. These masses are due to the decomposition of sulfates into SO+ and SO2+. Other ions released from this sample were 36 and 38 m/z. They had similar thermal properties, being released at temperatures around 470 °C, reaching a maximum at 850 °C, and then decreasing slowly to 1200 °C. Possible assign-

Fig. 4. TV–MS traces for hyperarid soil type V: PE-276. (A) Soil no mixed, (B) soil no mixed and pre-heated (500 °C  24 h), (C) soil mixed with microorganisms, and (D) Soil mixed and pre-treated. Mass analyzer was scanned from 10 to 200 m/z for non-treated soils and from 40 to 200 m/z for pretreated soils.

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Fig. 5. TV–MS traces for hyperarid soils (A) type III: PE-386, (B) type IV: PE-287. For mixed soils (C) type III, and (D) type IV. Mass analyzer was scanned from 10 to 200 m/z.

ments for these masses are the formation of H35Cl+ and H37Cl+, respectively. Hydrogen chloride (HCl) was likely originated from the thermal degradation to chlorides. Finally, mass 30 was released in the temperature range from 180 to 550 °C, and reached a maximum at 320 °C. Probably, this fragment is due to NO which evolves from the thermal oxidation of N-organics at low temperature or degradation of nitrates at high temperatures (Hao 1994; Navarro-Gonzalez et al., in press). The TV–MS trace for soil type IV (PE-287), which contains 8 ppm of organic C, showed the release of the following ions: 16, 18, 28, 36, 44, 48, and 64 m/z (Fig. 5B). In this soil, the mass 44 showed values in the temperature range from 360 to 1200 °C and the highest value at >750 °C, which might suggest that it is due to the thermal decomposition of carbonates at high temperatures; however, this type of soil has the fewest amounts of carbonates in the desert (Table. 1). Alternatively, CO2 could result from the oxidation of refractory organics, which have been detected by pyrolysis-GC–MS in a previous study (Navarro-Gonzalez et al., 2006). The rest of the released ions have already been explained and presented similar paths to the previous sample PE-386. Although this soil contains very low levels of organics similar to PE-001 (8 vs. 11 ppm), it shows greater release of CO2, including CO and atomic oxygen. Again, this fact confirms the presence of refractory organics and/or suggests the importance of the effect of

the mineral matrix of the soil over the organic matter. Indeed, the soil PE-287 is sandier than PE-001, which has a major silty component (Table. 1). Pre-treated soils PE-386 and PE-287 also showed the release of CO2 (44 m/z) above 550 °C suggesting the presence of carbonates and/or refractory organics, respectively (Fig. 6A and B). Nonetheless, both of these soils showed another significant peak of mass 48 between 1000 and 1200 °C attributed to sulfates. Despite the hyperarid conditions of this region, the carbonate inventory is low, similar to the Yungay region located in northern Chile (Michalski et al., 2004). While carbonates are much less soluble than sulfates, their formation is not chemically favored in this area (Ewing et al., 2006). However, carbonates present local deposition in cracks and on prism faces of sulfates where they are protected (Doner and Lynn, 1989), so would require higher temperatures to be released. In some cases, this release was achieved after pre-treating the samples (500 °C during 24 h) and subsequent TV process. Indeed, internal thermal reactions in solids bodies depend on the geometry of the crystal lattice, the nature and direction of the chemical bonds, the anisotropy of the crystal structures and its properties (Stoch, 1991; Smykatz-Kloss, 2002). Mineral and/or organic matrix in complex samples have an effect on the thermal decomposition process (Yarib, 1991; Blazso and Jakab, 1999; Morse et al., 2007; Blazso, 2005). Interestingly, similar to the possible effect of the granulometry of samples (Table. 1) over release of refractory organ-

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Fig. 6. TV–MS traces for pre-treated soils (500 °C  24 h), (A) type III (PE-386), (B) type IV (PE-287); and (C) mixed with microorganisms. The graphic C is representative for both of soil types. Mass analyzer was scanned from 40 to 200 m/z.

ics, the samples with a higher amount of particles size less than 50 lm (silts and clays) showed, in general, release of CO2 from carbonates at higher temperatures (i.e. PE-361, PE-388), although the presence of refractory organic matter seems to increase the process (i.e. PE-276). Other factor to discard is that the sample has not reached the temperature marked by pyrolyzer. Different samples of carbonates (calcite, aragonite, magnesite and double salts) and sulfates were loaded in a capillary quartz and mounted in the center of platinum coil filament pyrolyzer probe as described in materials and methods. The decomposition temperature of these samples coincided with those reported in the literature (Smykatz-Kloss, 2002; Stalport, 2005). In addition, comparison between experiments using thermogravimetry (TGA) versus our TA system showed no significant difference in the decomposition temperatures of the samples (±10.0 °C; data not shown). Irwin (1982) and Wampler (1995) thoroughly reviewed the instrumentation and applications of pyrolysis, demonstrating the advantage of the technique for heating programs. Because the sample, may be placed directly onto the filament during heating, so that there is no thermal gradient as experienced with a furnace design. Although, in practice, the sample is placed into a quartz tube and heated using a coiled filament, the small quartz furnace inside of the resistive element, neither shows a significant temperature gradient (Wampler, 1995). Together, the sum of these factors could directly influence the decomposition of carbonates at higher temperatures.

Recently, Peeters et al. (2008), showed large differences for the physical and chemical properties in samples from Pampas de La Joya, even for samples taken only several metres apart. Desert soil samples were exposed to a simulated Mars environment to test the stability of amino acids. The results show high levels of amino acid destruction. From their results they conclude that the amino acid stability in the desert soil is influenced by the mineralogical composition of the samples. Studies about the mineralogical composition of these soils are in progress and better understand the process of interaction of the matrix with organic matter and carbonates. 3.3. TV response from soils doped with microorganisms In order to demonstrate the effect of the different hyperarid soils over organic matter during the analysis of TEGA, three different microorganisms were mixed with soil samples (Fig. 1A, C, and D), and the TV response was evaluated using the most abundant ions, which are yielded by all microorganisms, as described above (masses 41, 91, and 107 m/z). Soils type I, II and VI doped with microorganisms showed the complete absence of characteristic organic ions 41 and 91 m/z, and the mass 44 (CO2) was the most abundant fragment. In addition, ions 16 and 28 m/z (atomic oxygen and monoxide carbon) appeared coincidentally with the highest peak of CO2 (Fig. 2D–F). Again, ion 16 m/z is released from the electron ionization of CO and

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CO2. These results clearly demonstrate the presence of one or several oxidants that decompose the organic matter in soils into CO and CO2 during thermal volatilization. Curiously, soils pre-treated to 500 °C showed different TV response when they were mixed with microorganisms compared to non-treated soils (Fig. 3D and E). TV–MS traces for soils type I and II (PE-361 and 388, respectively), showed exactly the same graphic (Fig. 3D). Ions 41 and 91 m/z were detected by TV–MS. These fragments started in the low temperature regime (between 320 and 420 °C, respectively), reached the highest point at 500 °C, and then decreased to 1000 and 800 °C, respectively. Nevertheless, despite the high levels of organics added to these samples (1:4), they had low signals during the process and the intensity of these organic masses was overwhelmed by that of CO2 (44 m/z). Pre-treated soil PE-001 presented similar TV response to previous samples (Fig. 3E), showing a marked decreased of characteristics ions of microorganisms (41 and 91) and a sharp increase of ion 44 m/z. These results show that heating the samples to 500 °C partially removed the oxidizing effect on organic matter, although the oxidant effect of the soil matrix is maintained given the high production of CO2 and the decline in the abundance of pattern ions 41 and 91 m/z. While the mass 44 is also observed during the normal process of thermo volatilization by oxidation of organic matter from microorganisms alone (Fig. 1), the production of this ion, if the microorganisms are in the soils, is much more abundant. Then, these data suggest the existence of at least two types of oxidants in these soils, a thermolabile oxidant which is highly oxidative and other thermostable oxidant which has a minor oxidative activity and that survives the heattreatment. Soil type V (PE-276) also showed the same oxidative behavior with total absences of patterns fragments and high values to CO2 and CO. Interestingly, in this soil ion 16 m/z does not appear (Fig. 4C). In the pre-treated soil, nonetheless, ions 41, 91, and 107 m/z evolved during thermal process from microorganisms were detected, but again showed very low levels compared to production of CO2. In addition, masses 48 and 64 m/z (SO+, SO2+) at low temperature regime (350–400 °C) likely to come from thermal oxidation to S-organics such as sulfonamide (Fig. 4D). As in previous non-treated soils as well as pre-treated soils, the presence of oxidative activity by the matrix of the soil is evident, which suggest the presence of at least two types of oxidants. TV–MS response from soil type III (PE-386) mixed with microorganisms did not detect any characteristic organic fragments, but showed the release of the following ions: 16, 18, 28, 30, 36, 38, 44, 48, and 64 m/z (Fig. 5C). These ions were found earlier in the non-mixed soil, but unlike that one, here there is a greater release of CO2 (44 m/z) with a peak at 400 °C and an increase of ions 28 and 16 m/z whose maximum coincide with mass 44 m/z. Significantly, the ion 30 m/z observed in the soil (not mixed), was again detected and at the same levels in the samples with

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and without added microorganisms. This means that although there are organic compounds with nitrogen supplied by microorganisms added to the soil, they are not observed and the ion detected is the product of compounds already present in the native soil. This suggests that the nitrogen in these type of organics oxidizes mostly to the ion N2O masked by the CO2. Indeed, a previous study has shown that the fragment 30 m/z is released from soils but did not show how, during TV process, the ion is released from different organic molecules (Navarro-Gonzalez et al., in press). The mass 30 ion could be supplied by ancient N-organics belongings to recalcitrant pool in the soil. Other masses (18, 36, 38, 48, and 64 m/z) also had no variation. Similar, to the previous sample, soil type IV (PE-287) mixed with microorganisms showed a significant increase in the ion 44 m/z corresponding to CO2, which had a peak at 400 ° C that coincides with thermal oxidation of organic matter and with the maximum release of CO and atomic O (mass 28 and 16 m/z). Curiously mass 16 was the second most abundant ion shown. Ions 41, 91, or 107 were not detected (Fig. 5D). Finally, both of pre-treated soils type III and IV had global similar TV response when they were mixed with microorganisms (Fig. 6C). While the ion 44 m/z had a significant increase during the thermal process, the characteristic ion 41, 91 and 107 m/z did not show a significant decline in their abundance. Our data suggest that thermolabile oxidant(s) of the matrix soil had major oxidative activity over ion patterns on both of these two types of soil, since the pre-treatment of samples with 500 °C eliminated its/their effect. In addition, thermostable oxidant(s) did not show oxidant activity as in previous pre-treated soils. The oxidant activity in these soils during thermal treatment is evident: first, ions which are released from different types of organics at low temperatures (<600 °C), as has been demonstrated in several previous studies (Hao et al., 1994; Manning et al., 2005; Trofimov and Emelyanenko, 2000; Philp, 2003; Lopez-Capel et al., 2005; Wang et al., 2000), were replaced by CO2 (ion 44), CO (ion 28), and methane or atomic oxygen (ion 16) which are produced during complete and incomplete combustion. Second, the dynamic ranges of the temperature and mass spectra used in this work [1200 °C, 200 m/z] were higher than those used in previous works (Wang et al., 2000; Manning et al., 2005; Lopez-Capel et al., 2005; Snyder et al., 2005), including that used by the Phoenix mission (1000 °C, 140 m/z) (Hoffman et al., 2008). So, any organic fragment which could be retained by interacting with the soil matrix or have refractory nature could have been detected. And third, organics from microorganisms added in the soil samples is not comparable with native organics, which have been subjecting to several geophysical and geochemical interactions over time. Therefore, the type of interaction of the organics added with the soil matrix is primarily physical that during the thermal process activates a heterogeneous catalysis

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resulting in the organic matter oxidation (Schulten and Leinweber, 1993a,b; Navarro-Gonzalez et al., 2006). Independently of the surface area, which is low in these soils due to greater amount of sand, the catalytic surface of silt and clay minerals (particles smaller than 50 lm) were able to oxidize organic matter added to soil. 4. Conclusions and implications for the Phoenix mission and beyond In this work, we examine the thermal and evolved gas properties of six types of soils from another hyperarid core region from the Atacama Desert: Pampas de La Joya (southern Peru), in order to investigate the effect of soil matrix and low organics content over TV response as well as the effect of hyperarid soils doped with different microorganisms over TV response. Our results showed interesting ions released from these Mars-like soils by TV analysis, and significant differences in the evolved gas behavior depending of the soil matrix and contents of organic under similar heating conditions. However, samples of these soils did not show organic ions by TV methods. Indeed, soils which contain <30 ppm of organic C show ions 44 and 28 m/z principally; soils which contain >30 ppm of organic C show ions 44, 28, and 30 m/z; and soils which contain very low levels to organics (<10 ppm of organic C) do not show ions presumably because the organic concentration is so dilute. When microorganisms are added to the soils, the results showed the characteristic ions expected from microorganisms (41, 91 and 107 m/z) which could be used to detect them on different soils by TV–MS analysis. However, the oxidative activity of matrix soil eliminates these fragments during thermal processing. TV response in non-treated soils shows total absence of ions previously found during thermal analysis of microorganisms alone, and significant increase of CO2 and CO abundance. In contrast, pre-treated soils show less oxidizing activity over pattern ions and minor but significant production of CO2. Hence, these results demonstrate a high-oxidative process in the soil matrix when it is subjected to temperature. Based on the release of organics with and without preheating of the sample to 500 °C, the presence of both thermolabile and thermostable oxidants are postulated, but the nature of oxidant(s) present in these soils from Pampas de La Joya is still unknown. The evidence for the presence of a strong oxidant in Mars soil has been studied before (Oyama and Berdahl, 1977), and since the return of the Viking data several hypotheses have been presented to explain oxidative activity on its surface (Quinn et al., 2007). Hydrogen peroxide (Encrenaz et al., 2004), superoxides (Yen et al., 2000), UV radiation (McKay et al., 1998), peroxide-modified titanium dioxide (Quinn and Zent, 1999), peroxinitrites (Plumb et al., 1989), and recently perchlorates founded by Phoenix spacecraft on Mars surface, are possible candidates. Other region from the Atacama located in northern Chile, near Yungay, showed the

presence of a non-chirally specific and as-yet-unidentified oxidants (Navarro-Gonzalez et al., 2003). Although this region is characterized by large amounts of deposited salt (Bohlke et al., 1997) and contains highly oxidative species, including iodates (IO3 ), chromates (CrO4 2), perchlorates (ClO4 ) and probably persulfates (S2O8 2) (Ericksen, 1981, 1983); these candidates do not completely explain our results, nor experiments similar to those made by the Viking Lander on Mars (Navarro-Gonzalez et al., 2003; Quinn et al., 2007). Thus, we conclude that if the concentrations of organics in the soils and ice on Mars at the Phoenix landing site were lower than 30 ppm and there are oxidants as expected on the Martian surface, the TEGA instrument would not detect them as organic fragments. Currently, other studies are in progress to evaluate mineral composition in these hyperarid soils, and to better understand and identify the possible oxidizing species. Preliminary results seem to show that it is also a non-chirally specific oxidant similar to what was proposed for Mars and Yungay area. Finally, our results show the utility of the studying Mars-like soils in deserts on Earth as a way to test instruments and experiments for the search for organics on Mars. Acknowledgements Funding for this research comes from Grants from the Universidad Nacional Auto´noma de Me´xico (IN 107107), Concejo Nacional de Ciencia y Tecnologı´a de Me´xico (CONACyT 45810-F), beca doctoral del Posgrado de Ciencias Biologicas de la Universidad Nacional Auto´noma de Me´xico, and by the National Aeronautics and Space Administration Astrobiology Science and Technology for Exploring Planets Program. References Biemann, K., Oro, J., Toulmin, P., et al. The search for organic substances and inorganic volatile compounds on the surface of Mars. J. Geophys. Res. 30, 4641–4658, 1977. Biemann, K. The implications and limitations of the findings of Viking organic analysis experiments. J. Mol. Evol. 14, 65–70, 1979. Biemann, K. On the ability of the Viking gas chromatography–mass spectrometer to detect organic matter. Proc. Natl. Acad. Sci. USA. 104, 10310–10313, 2007. Blazso, M., Jakab, E. Effect of metals, metal oxides, and carboxylates on the thermal decomposition processes of poly (vinyl chloride). J. Anal. Appl. Pyrolysis 49, 125–143, 1999. Blazso, M. In situ modification of pyrolysis products of macromolecules in an analytical pyrolyzer. J. Anal. Appl. Pyrolysis 74, 344–352, 2005. Bohlke, J.K., Ericksen, G.E., Revesz, K. Stable isotopic evidence for an atmospheric origin of desert nitrate deposits in northern Chile and southern California. USA Chem. Geol. 136, 135–152, 1997. Boynton, W.V., Ming, D.W. Use of thermal and evolved-gas analyzer (TEGA) on the Phoenix lander to detected sulfates on Mars. Workshop on Martian sulfates as recorders at atmospheric-fluid–rock interactions. 131, 18, 2007, (LPI contribution #1331).

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