Daniel P Glavin
email: daniel.p.glavin@nasa.gov

D Archer

A E Brunner

A Buch

Michel Cabane

Patrice Coll

P Conrad

David Coscia

J P Dworkin

Jennifer L Eigenbrode

Rocknest D Glavin

C Freissinet

P Mahaffy

M Martin

C Mckay

K Miller

D Ming

R Navarro-González

A Steele

R E Summons

B Sutter

C Szopa

S Teinturier

Investigating the Origin of Chlorohydrocarbons Detected by the Sample Analysis at Mars (SAM) Instrument at Rocknest

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction:

The search for organic compounds on Mars, including molecules of either abiotic or biological origin is one of the key goals of the Mars Science Laboratory (MSL) mission. Previously the Viking and Phoenix Lander missions searched for organic compounds, but did not find any definitive evidence of martian organic material in the soils. The Viking pyrolysis gas chromatography mass spectrometry (GCMS) instruments did not detect any organic compounds of martian or exogenous origin above a level of a few parts-per-billion (ppb) in the near surface regolith at either landing site [1]. Viking did detect chloromethane and dichloromethane at pmol levels (up to 40 ppb) after heating the soil samples up to 500°C (Table 1), although it was originally argued that the chlorohydrocarbons were derived from cleaning solvents used on the instrument hardware, and not from the soil samples themselves [1]. More recently, it was suggested that the chlorohydrocarbons detected by Viking may have been formed by oxidation of indigenous organic matter during pyrolysis of the soil in the presence of perchlorates [2]. Although it is unknown if the Viking soils contained perchlorates, Phoenix did reveal relatively high concentrations (~ 0.6 wt%) of perchlorate salt in the icy regolith [3], therefore, it is possible that the chlorohydrocarbons detected by Viking were produced, at least partially, during the experiments [2,4].

The Sample Analysis at Mars (SAM) instrument suite on MSL analyzed the organic composition of the soil at Rocknest in Gale Crater using a combination of pyrolysis evolved gas analysis (EGA) and GCMS. One empty cup procedural blank followed by multiple EGA-GCMS analyses of the Rocknest soil were carried out. Here we will discuss the results from these SAM measurements at Rocknest and the steps taken to determine the source of the chlorohydrocarbons.

SAM Measurement Protocol: Prior to the analysis of the soil, a procedural blank run using an empty quartz cup was carried out to characterize the background of the SAM instrument. A quartz cup was sealed inside the pyrolysis oven and heated to ~835°C at a rate of 35°C/min under He carrier gas flow. A small fraction of the gas released from the cup was measured directly by electron impact quadrupole mass spectrometry (QMS mass range 2-535 Da). A larger fraction of the gas was sent to a hydrocarbon trap (Silica beads/Tenax TA/Carbosieve G) at an initial temperature of 5°C. The trap was subsequently heated to a temperature of ~300°C for 4 min under He flow and the volatiles released were sent to the Tenax TA injection trap on the GC column. Trapped compounds were separated on the MXT-CLP column (30 m x 0.25 mm I.D.) set at an initial temperature of 50°C followed by heating to 220°C at 10°C/min. Eluting compounds were detected by thermal conductivity [5] and by the QMS using a smart scanning mass algorithm and identified by their retention time and comparison of the mass fragmentation pattern to the NIST11 library.

A sample of the Rocknest soil (scoop #5) collected by Curiosity's solid sample acquisition and handling system called CHIMRA was sieved (< 150 m fraction), portioned (< 76 mm 3 ), and a single portion delivered to a preconditioned SAM quartz cup via the SAM solid sample inlet tube. The mass estimates of the Rocknest portions delivered to SAM remain uncertain [6]. EGA-GCMS analyses of the individual Rocknest soil portions were performed using conditions similar to those that were used for the blank.

SAM Pyrolysis GCMS Results: Several chloromethanes including chloromethane (CH 3 Cl), dichloromethane (CH 2 Cl 2 ), and trichloromethane (CHCl 3 ) were detected by SAM GCMS above blank levels (Fig. 1). Individual m/z values in counts per second comprising each peak were Gaussian fitted in IGOR [7] and each compound was identified by the mass fragmentation pattern generated from the peak fits (Fig. 1, inset). The total abundance of each compound was estimated by calculating the sum of the area of each m/z value and comparing the total area to known concentrations of hexane that were analyzed by GCMS on the SAM flight instrument during pre-flight calibration. Differences in the relative molar response of hexane and the chloromethanes were accounted for.

1080.pdf 44th Lunar and Planetary Science Conference (2013)

The abundances of the chloromethanes detected by SAM in the Rocknest soil analyses #1 and #2 ranged from ~0.02 to 2.4 nmol and were all well above blank levels (Table 1). These abundances are much higher than the pmol values for chloromethane and dichloromethane previously measured by Viking (Table 1). The low levels of chloromethanes measured in the Rocknest soil#3 sample is due to the fact that the GCMS measured the high sample temperature cut (533-822°C) during a time when no chloromethanes were released from the sample based on EGA. 3) reaction of martian soil Cl with martian organics during pyrolysis and/or martian chlorohydrocarbons in the soil. The possible detection of perchlorate [6,8] and the correlation between the evolution of O 2 and the production of chloromethanes at cup temperatures above 200°C measured in SAM EGA mode [9], strongly suggest that these chlorohydrocarbons are being produced during combustion. The lack of any chlorohydrocarbons in the SAM procedural blank run indicates that the chlorohydrocarbons themselves are not contaminants from the SAM instrument. It is possible that the chlorohydrocarbons were derived from contamination in the sample handling chain, although it is important to note that CHIMRA was scrubbed with the soil multiple times prior to the first SAM soil analysis and the surfaces were found to be organically clean prior to launch [9,10]. Analysis of the MSL organic check material is needed to further assess this possibility. Terrestrial N-methyl-N-(t-butyldimethylsilyl)trifluoroacetamide (MTBSTFA) and associated reaction products from the SAM wet chemistry experiment were identified in both the blank and soil EGA-GCMS analyses [11,[START_REF] Freissinet | ROSES MSL Participating Scientist Program. 1080.pdf 44th Lunar and Planetary Science Conference[END_REF]. Based on laboratory pyrolysis GCMS experiments, combustion of MTBSTFA during SAM pyrolysis and reaction with martian Cl can explain the presence of the chloromethanes and a chloromethylpropene also detected by SAM [9]. Furthermore, these are the two chlorohydrocarbon classes expected by MTBSTFA combustion. Thus, at this time, while MTBSTFA contamination can explain all of the chlorohydrocarbons observed, we cannot exclude the possibility that traces of martian organics contributed to the chloromethanes measured by SAM.

Figure 1 .

 1 Figure 1. SAM GCMS analysis #2 of the Rocknest soil showing the peaks corresponding to CH 3 Cl, CH 2 Cl 2 , and CHCl 3 . The compounds were identified by retention time and comparison of the mass spectra (inset) to the NIST library and were present well above blank levels.Discussion: There are three possible explanations for the source of chlorohydrocarbons detected by SAM acting either individually or together in unknown proportions including: (1) reaction of martian soil Cl with terrestrial organics from the sample handling chain during pyrolysis and/or chlorohydrocarbons in the sample handling chain, (2) reaction of martian soil Cl with terrestrial organics in SAM during pyrolysis and/or chlorohydrocarbons in SAM, and (3) reaction of martian soil Cl with martian organics during pyrolysis and/or martian chlorohydrocarbons in the soil. The possible detection of perchlorate[6,8] and the correlation between the evolution of O 2 and the production of chloromethanes at cup temperatures above 200°C measured in SAM EGA mode[9], strongly suggest that these chlorohydrocarbons are being produced during combustion. The lack of any chlorohydrocarbons in the SAM procedural blank run indicates that the chlorohydrocarbons themselves are not contaminants from the SAM instrument. It is possible that the chlorohydrocarbons were derived from contamination in the sample handling chain, although it is important to note that CHIMRA was scrubbed with the soil multiple times prior to the first SAM soil analysis and the

  pmol calculated from[1] assuming 100 mg samples; ND = not determined; b SAM cup temperature range indicates temperature cut where volatiles were collected on the hydrocarbon trap for GCMS analysis. SAM Soil#1 and Soil#2 abundances in nmol were corrected for the gas fraction sent to the hydrocarbon trap using EGA data. Errors determined by the standard deviation of the average of five separate hexane calibration runs on SAM.

Table 1 .

 1 Comparison of the abundances of chloromethanes detected by the Viking and SAM GCMS instruments.

	Sample

Temp CH 3 Cl CH 2 Cl 2 CHCl 3 Viking (VL1) a


C Freissinet
email: caroline.freissinet@nasa.gov

A Buch

D P Glavin

M Cabane

P Coll

J L Eigenbrode

A Steele

C Szopa

P R Mahaffy

FROM BACKGROUND TO SIGNAL: CHALLENGES OF A SOLID SAMPLE ANALYSIS USING SAM GC-MS

Introduction: amongst the Sample Analysis at Mars (SAM) experiment capabilities, the GCMS mode (coupling of the Gas-Chromatograph and the quadrupole Mass-Spectrometer instruments) was designed for the separation and identification of the chemical components of the gases evolved from a solid sample, either processed by heat in pyrolysis or by chemical reactant in wet chemistry [1]. Prior to the three portioned samples already analyzed in pyrolysis GCMS, an internal SAM blank run was carried out with an empty quartz cup. This blank analysis was required to understand the background signal intrinsic to the GCMS response of the SAM experiment. With this aim, it was run using analytical conditions similar to those used for the Rocknest samples. The identification of the compounds present in the background and the understanding of their origin are necessary to perform quantitative analysis and to aid the interpretation of the solid sample results.

SAM blank analysis: the GCMS blank was generated as the first Martian GCMS analysis, prior to the delivery of any solid sample into SAM (Fig. 1). The quartz cup underwent the same treatment than for the subsequent solid sample experiments, with a first preconditioning followed by an exposition towards SAM inlets, and was then sealed into the pyrolysis oven and heated to ~840ºC at a rate of 35ºC/min in 1 sccm He flow. A fraction of the evolved gases corresponding to a temperature cut of 146-532ºC (cup temperature) was sent to the hydrocarbon trap (containing glass beads, Tenax TA and Carbosieve G) for the subsequent analysis in GCMS. Compounds were separated on the MXT-CLP column (30 m x 0.25 mm I.D.) set at an initial temperature of 50°C followed by heating to 220°C at a 10°C/min rate.

Discussion: Origin of background: the chromatogram displays whatever is present and volatilized in the sample manipulation system (SMS), transfer lines (heated to 135ºC), valves, hydrocarbon trap, injection trap and capillary column. GCMS signal can result from the volatilization of any molecule present in the path, or be a result of a reaction between internal components in SAM. Thus, the blank corresponds to the background level of the internal SAM gas flow path at a given time. It has to be taken as a reference, and a signal will be defined as any molecule above the background.

To track the origin of the background, it is necessary to determine first the nature of the compounds present on the chromatogram. An Igor tool [2] has been specifically developed to obtain the mass spectrum of an integrated peak. This mass spectrum can then be matched against the NIST library to identify potential candidates and determine the degree of confidence in the identification. The identification of the background is made easier on the chromatogram, as by its intrinsic purpose, the GC separates the complex mix of molecules into discrete ones. However, looking at the background signal in the Evolved Gas Analysis (EGA) mode (i.e. pyrolysis-QMS analysis) also helps assigning the origin of few molecules. Thus, aromatic compounds such as benzene and toluene, which are present in the GCMS blank signal but do not display a peak in the EGA, are likely to come from the traps and are suspected to be released from the Tenax TA. Aromatics such as ethylbenzene and xylene are suspected to be present in the blank signal, with no clear evidence about their origin. Other molecules present in the blank are H 2 O, CO 2 and SO 2 , internal to the system. Also, column bleeding products are observed, resulting from the thermal degradation of the internal polysiloxane phase of the column.

Other major contributors to the background signal are the reaction products of one of the chemicals used . This molecule was sealed inside each of the seven derivatization cups present in the SMS. Although none of them have been punctured yet for the actual wet chemistry experiment, it is suspected that one or more may be releasing some of its reactant. Any MTBSTFA present inside the SMS would readily react with the water present to form monosilylated water, bisilylated water and a trifluoro-N-acetamide byproduct (Fig. 2). Those 3 compounds were identified in the GCMS blank by their mass spectra. Major peaks have been identified, however, work is still ongoing to characterize all the ~30 peaks observed in the GCMS background and understand their origin. This is an important process for the subsequent investigation of the organic molecules detected in the pyrolysis of the solid samples, to attest if they are from Martian origin or come from terrestrial carbon present in the system.

Background quantification and signal limit of detection: the presence of such a background has numerous implications for the solid sample analysis:

(1) It can make uncertain any detection of a compound also present in the background and make difficult to confirm a detection of a molecule present at a concentration lower than the background level. The quantification of these molecules is thus necessary to determine the limit of detection of organics in the solid samples above background. (2) It interferes with the analysis of trace levels of molecules, by overlapping peaks. Thus, lowering the effect of background in general, and MTBSTFA in particular, would significantly improve the signal to noise ratio, and strategies are under investigation in this purpose (cold sample drop off, He flush of the SMS, low temperature sample heating prior to higher temperature EGA analysis). (3) The presence of reactive species in the background can create newly formed products by interaction with suspected Martian molecules, for instance perchlorates. Before claiming detection of Martian molecules, it has to be ruled out that these compounds can not come from reaction with internal molecules. For this reason, laboratory experiments are performed under SAM-like conditions to understand the reaction products of molecules in the background with suspected molecules on Mars soil. The chlorinated hydrocarbons [4,5] and nitrates [6,7] detected on the portioned samples from Rocknest scoop#5 are thus under deep investigation to identify any possible pathway of formation from MTBSTFA and perchlorates. (4) Residual molecules in SAM can also contribute to combustion into CO 2 and have a role in the isotope ratio changes [8,9,10]. For all these reasons, the initial abundance of MTBSTFA has been extrapolated and its contribution to the total background is determined to be from 30 to 55 nmoles.

Moreover, another contributor to the background signal could be the scoop to CHIMRA sampling chain, which the Rocknest sample went through before being delivered to SAM (not used in the blank run). However, intensive cleaning and monitoring of sampling chain makes this possible source of organics to SAM for the first delivery unlikely [5,11].

Conclusion: SAM background signal has been intensively studied from the first internal SAM blank. It led to the detection of ~30 peaks in the GCMS. The compounds with the highest GCMS response come from SAM internal sources, such as thermal degradation from the traps and reaction products from MTBSTFA. This latter molecule might interact with molecules released from solid samples, which could complicate the interpretation of their origin. By quantifying the background and determining its precise origin, it helps finding ways to lower it to better extract the signal and identify the peaks of interest. A complete understanding of the GCMS background is thus an essential first step to investigate an endemic signal in a Martian solid samples.

References: [1] Mahaffy, P. et al. (2012) Space Sci Rev, 170, 401-478. [2] Brunner, A. et al. (2013), this meeting. [3] Buch, A. et al. (2013) this meeting.

Figure 1 :

 1 Figure 1: blank GCMS analysis. The chromatogram displays bands of masses (sum of specific range of m/z): the top ones correspond to low molecular masses bands (m/z 45-86) and the bottom ones high molecular masses bands (m/z 87-535). The red line plots the GC column temperature. Major peaks, 1: CO2 H2O, 2: SO2, 3: benzene, 4: toluene, 5: trifluoro-N-methylacetamide, 6: monosilylated water, 7: branched aromatics, 9: bisilylated water, 9: methylsilylborate.

Figure 2 :

 2 Figure 2: reactive pathways of the MTBSTFA with water to form products of monosilylated water (1), bisilylated water (2) and a trifluoro-N-methylacetamide byproduct (3), potentially at the origin of signals observed in the background.

  [4] Glavin, D. et al. (2013), this meeting. [5] Eigenbrode, J. et al. (2013), this meeting. [6] Wray, J. et al. (2013), this meeting. [7] Navarro-Gonzales, R. et al. (2013), this meeting. [8] Franz, H. et al. (2013), this meeting. [9] Stern, J. et al. (2013), this meeting. [10] Sutter, B. et al. (2013), this meeting. [11] Anderson, M. et al. (2012) Rev. Sci. Instrum. 83, 105-109.


Maeva Millan
email: maeva.millan@latmos.ipsl.fr

Cyril Szopa

A Buch

Imène Belmahdi

Patrice Coll

Daniel P Glavin

Caroline Freissinet

P D Archer

B Sutter

Roger E Summons

Rover M Millan

R Navarro-González

M Cabane

P Mahaffy

Effect of the Presence of Chlorates and Perchlorates on the Pyrolysis of Organic Compounds: Implications for Measurements Done with the Sam Experiment Onboard the Curiosity Rover

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction:

The Mars Science Laboratory (MSL) Curiosity Rover carries a suite of instruments, one of which is the Sample Analysis at Mars (SAM) experiment. SAM is devoted to the in situ molecular analysis of gases evolving from solid samples collected by Curiosity on Mars surface/sub-surface. Among its three analytical devices, SAM has a gaschromatograph coupled to a quadrupole mass spectrometer (GC-QMS) [1]. The GC-QMS is devoted to the separation and identification of organic and inorganic material. Before proceeding to the GC-QMS analysis, the solid sample collected by Curiosity is subjected to a thermal treatment thanks to the pyrolysis oven to release the volatiles into the gas processing system. Depending on the sample, a derivatization method by wet chemistry: MTBSTFA of TMAH can also be applied to analyze the most refractory compounds. The GC is able to separate the organic molecules which are then detected and identified by the QMS (Figure 1). For the second time after the Viking landers in 1976 [2], SAM detected chlorinated organic compounds with the pyrolysis GC-QMS experiment [3,4]. The detection of perchlorates salts (ClO 4 -) in soil at the Phoenix Landing site [6] suggests that the chlorohydrocarbons detected could come from the reaction of organics with oxychlorines. Indeed, laboratory pyrolysis experiments have demonstrated that oxychlorines decomposed into molecular oxygen and volatile chlorine (HCl and/or Cl 2 ) when heated which then react with the organic matter in the solid samples by oxidation and/or chlorination processes. [3,5,7,[START_REF] Steninger | Planet[END_REF].

Objectives: During the SAM pyrolysis, samples are heated to 850°C which favors chemical reactions between oxychlorine phases, probably homogeneously distributed at Mars's surface [6], and potential organic material in martian sediments. The first chlorohydrocarbons (chloromethane and di-and trichloromethane) detected by SAM were entirely attributed to reaction products occurring during the pyrolysis experiment between these oxychlorines and organic carbon from SAM instrument background [3], leading to chlorination or oxychlorination.

But SAM discovered for the first time in the Sheepbed mudstone of Gale crater, chlorobenzene and C2 to C4 dichloroalkanes produced by reaction between Mars endogenous organic compounds with oxychlorines [4]. The potential organic precursors of these chlorinated compounds are currently under investigation in the laboratory under SAM-like operating conditions [START_REF] Miller | French Space Agency (CNES) support for SAM-GC development and exploitation. 1418.pdf 47th Lunar and Planetary Science Conference[END_REF]. To help understanding of the influence of perchlorate and chlorate salts on organic matter during SAM pyrolysis, we systemically study the reaction products formed during pyrolysis of various organic compounds from different chemical families mixed with various perchlorates and chlorates.

Selected samples and preparation: We selected various organic compounds from simple molecule

1418.pdf 47th Lunar and Planetary Science Conference (2016)

forms as for instance hydrocarbons, PAHs and amino acids to more complex material (> 30 carbon atoms) such as kerogen.

The perchlorates and chlorates and the organics are first prepared in silica to simulate the remaining soil. The perchlorate and chlorate salts are prepared at 1 wt % concentration, which is slightly higher than SAM abundances (e.g., 0.2 to 0.5 wt % at Rocknest [3]). The organics are then mixed with the oxychlorines at various concentrations to study the potential qualitative and/or quantitative effects.

Pyrolysis-GC-QMS experiments:

The experiments are performed on a laboratory GC-QMS with a Restek Rxi-5 column (30m x 0.25mm x 0.25µm) and an Intersciences pyrolyser mounted on the injector upstream the column. The mixture is pyrolyzed at different temperatures up to 900°C to cover the SAM temperature range. Different experiments are done to discriminate the pyrolysis products directly coming from the organic, and those produced from the reaction with oxychlorine. This series of experiments is under progress and should bring key information on the potential to identify Mars organic molecules when pyrolyzing solid samples.

Conclusion:

The products obtained during the pyrolysis of pure organic matter and mixed with various perchlorates and/ or chlorates is expected to improve the understanding of the behavior of organic matter during SAM pyrolysis experiments on Mars subject to oxidation or oxychlorination. Depending on the organic families studied, we may find reccuring molecules which are potentially present in Mars' surface samples. This work could thus hightlight some organic precursors of the chlorinated compounds found on Mars, and support the interpretation of the SAM measurements.

Figure 1 :

 1 Figure 1: simplified analytical channel from sample collection to the detection and identification of the organic molecules.


Sam Helps A Buch

C Freissinet

C Szopa

D Glavin

P Coll

M Cabane

J Eigenbrode

R Navarro-Gonzalez

J C Stern

D Coscia

S Teinturier

J P Dworkin

P Mahaffy

DETECTION OF ORGANICS AT MARS: HOW WET CHEMISTRY ONBOARD

Introduction:

For the first time in the history of space exploration, a mission of interest to astrobiology could be able to analyze refractory organic compounds in the soil of Mars. Wet chemistry experiment allow organic components to be altered in such a way that improves there detection either by releasing the compounds from sample matricies or by changing the chemical structure to be amenable to analytical conditions. The latter is particular important when polar compounds are present. Sample Analysis at Mars (SAM), on the Curiosity rover of the Mars Science Laboratory mission, has onboard two wet chemistry experiments: derivatization [1-2] and thermochemolysis [3][4]. Here we report on the nature of the MTBSTFA derivatization experiment on SAM, the detection of MTBSTFA in initial SAM results, and the implications of this detection.

MTBSTFA derivatization:

Chemical derivatization of polar molecular compounds is achieved by the MTBSTFA (N-Methyl-Ntert-butyldimethylsilyltrifluoroacetamide) / DMF (Dimethylformamide) silylation reaction in order to transform refractory polar compounds into a more volatile form that can be analyzed and detected by GCMS (Figure 1). The advantage of using MTBSTFA as the derivatization reagent is that it is a single-step reaction; it is less susceptible to hydrolysis compared with other reagents and it does not require separation of the derivatives prior to GC separation analysis. In addition, MTBSTFA is sensitive to a wide range of organic compounds with acidic hydrogen atoms. The reaction yields obtained using this derivatization reagent are typically in the 90-100% range.

Finally, the MTBSTFA has also the major advantage to react in the gaseous phase, generally present in pyrolyzing system like the one used in SAM to treat the solid samples.

First SAM Solid Sample Experiment Results:

The first scoops of the Martian soil (Rocknest, Gale crater) have been analyzed by evolved gas analysis (EGA) and via GC using thermal conductivity (TCD) and MS detection [5][6]. The samples have been heated up to approximately 840 °C with a heating rate of 35 °C/min under He flow. The evolved gas was analyzed directly by the QMS in EGA mode. For GC analyses, the majority of the gas released was trapped on a hydrocarbon trap (silica beads, Tenax TA, Carbosieve G) over a specific temperature range. Trapped volatiles were then released by heating to ~300 °C and sent to the GC under He flow. Then the content of the GC injection trap (IT) is released in the GC column (CLP-MXT 30 m x 0.25 mm x 0.25 µm) by radidly heating the IT at 300 °C. Helium is used (0.9 bar inlet column pressure) as the carrier gas. The column temperature program started at 50 °C, and increased up to 220 °C with a ramp rate of 10 °C/min.

The first results obtained when running an analysis with an empty cup (no solid sample) showed the presence of MTBSTFA in the system. MTBSTFA was first detected in the EGA-QMS analysis blank then by GC-TCD-QMS analysis. Meaning that MTBSTFA is part of the background signal under its gaseous phase and is derived from at least one of the seven MTBSTFA/DMF derivatization cups in SAM [7]. Since MTBSTFA is able to react in the gaseous phase, its detection implies the possibility to have some MTBSTFA reactions with all the labile compounds possibly present in the Martian soil, in the sampling system and/or inside the SAM instrument.

The analysis of the different scooped samples confirmed the presence of MTBSTFA in the background. Indeed, on the EGA and GC-TCD-MS run we have detected the by-product acetamide-2,2,2-trifluoro-Nmethyl that is characteristic of the presence of MTBSTFA. In addition, we also have observed the presence of compounds resulting from the derivatization reaction between MTBSTFA and water. Indeed, water has been detected by two ways: the EGA experiment and the GC-TCD-MS run. Due to the presence of mono-and bi-silylated water derivatives, several characteristic ions can be detected in the EGA mode (e.g. m/z = 147, 73), and two characteristic peaks in the GC-TCD-MS analysis are also observed (Figure 2). In addition to water, a sylilated chloride compound has been detected after pyrolysis of the Rocknest soil. This compound is the simplest chloride derivative compound: chloro(1,1-dimethylethyl)dimethyl-silane (Figure 3), and it co-elutes with the mono-sylilated water derivative.

1512.pdf 44th Lunar and Planetary Science Conference (2013)

Discussion:

The presence of MTBSTFA in the EGA and GC-TCD-MS background allows us to confirm the first results obtained in EGA and TLS experiments meaning the presence of water in the Martian atmosphere and/or soil. In contrast to the TLS measurement, the detection of water after derivatization is dependent on the concentration of MTBSTFA in the background. Indeed if the MTBSTFA is the limiting species then the water could not be totally derivatized and detected as well. Moreover, the presence of two chromatographic peaks associated with water derivatives increases the difficulty of quantification of water. Indeed the presence of the mono-silylated water can come from: (1) an incomplete MTBSTFA reaction, (2) from the degradation of the bi-silylated water under high temperature, (3) or a combination of (1) and ( 2). For all of these cases, the sum of the two peak will allow us to estimate the water quantity.We have compared the GC-MS peaks area related with water (mono and bi-sylilated) of the different Rocknest portions as a function of the temperature cut. If we assume that the same quantity of sample is injected in the GC then, the main conclusion could be that for the same temperature cut there is a higher water concentration in the first portion compared to the blank. That could mean that like the Martian atmosphere, the Rocknest sample contains water as well. This hypothesis is confirmed by the GC-MS analysis of the second portion where the temperature cut is lower and the water concentration higher. The opposite is true when the temperature cut is between 533 to 822°C where water concentration drastically decreases because it has been eliminated at lower temperature. Table 1: Peak area calculated after Gaussian fitting by using the "SAM" Igor tool. For the mono and bi sylilated water we have used the base peak, respectively m/z: 75 and m/z: 147.

The chloride compounds detected in the GC run could come from three different sources: (1) HCl compounds which is able to react with MTBSTFA to lead silylated chloride Since HCl derivative has not been detected in the blank, it could be originated from soil or it could be generated as a secondary reaction occured during the pyrolysis. (2) the reaction of MTBSTFA with the perchlorate, if it is present in the soil sample [8]. Indeed the MTBSTFA is a highly reactive compound able to react with labile compounds but it also could be oxidized at high temperature by oxidizing compounds such as perchlorate. (3) At high temperature, perchlorate degradation could lead to HCl formation. varro-Gonzalez, .(2010), JGR, 115.

Figure 1 .

 1 Figure 1.Scheme of the derivatization reaction of MTBSTFA/DMF with an amino acid.

Figure 2 .

 2 Figure 2. GC-TCD (top) and GC-MS (bottom) analysis of the first portion at Rocknest. Only The MTBSTFA charateristic m/z values have been plotted on this figure.

Figure 3 :

 3 Figure 3: Mass spectrum of the sylilated chloride compound identified in the Rocknest portion #3 GC-MS data by using NIST library.

  : [1] Buch, A. et al. (2009) J chrom. A, 43, 143-151. [2] Stalport, F. et al. (2012) Planet. Space Sci. 67: 1-13 [3] Geffroy-Rodier, C. et al. (2009) JAAP, 85, 454-459. [4] Eigenbrode et al. (2011) LPS LXXIII, abstract 1460. [5] Glavin, D., A. et al. (2013) this meeting. [6] Eigenbrode, J. et al. (2013), this meeting. [7]Freissinet, C. et al. (2013) this meeting.[8] Na-

Acknowledgements: SAM-GC team acknowledg-

es support from the French


Sam Instrument M Millan

A J Wil- Liams

A Buch

A Bai

C Freissinet

C Szopa

J L Eigenbrode

D P Glavin

R Navarro-González

P Mahaffy

S S Johnson

PRESERVATION OF ORGANIC MOLECULES IN MARS-ANALOG SAMPLES USING PYROLYSIS AND DERIVATIZATION GCMS EXPERIMENTS FROM THE

Introduction:

The Curiosity rover is currently at the base of Mount Sharp, analyzing its stratigraphic rocks layers to find hints of Mars' habitability [1]. The Sample Analysis at Mars (SAM) instrument suite aboard Curiosity is a pyrolyzer coupled to a gas chromatograph-mass spectrometer (pyro-GCMS) essentially dedicated to the search for organic molecules on Mars [2]. SAM is able to perform in situ molecular analysis of gases evolved from the heat (up to 850°C) of the solid samples collected by Curiosity. SAM can then detect, separate and identify volatiles inorganic and organic compounds released from the samples. SAM also carries two derivatization reagents to perform wet chemistry experiments in order to extract non-volatile compounds so they can be analyzed by GCMS. The Nmethyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide (MTBSTFA) is one of the two chemistry reagents carried by SAM and allows the separation, detection, and identification of complex, polar and/or refractory molecules. The method, called derivatization, is a silylation method where the labile H of the targeted molecule is replaced by the MTBSTFA silyl group transforming molecules into volatile derivatives easily amenable to GCMS analysis (Figure 1). The MTBSTFA is then able to react with many organics of astrobiological interest such nucleobases, amino acids, carboxylic acids, sugars, etc while preserving their chemical structure from degradation. For the first time on Mars, a full derivatization experiment was successfully performed on the Ogonguit Beach (OG) sand sample before being analyzed by SAM [3]. This OG analysis is challenging because it led to the production of numerous MTBSTFA by-products coming from reactions between the organics, inorganics, and minerals present in the sample and high mass derivative species yet non identified.

Objectives:

To evaluate the efficiency of the MTBSTFA derivatization on Martian samples and better understand the precursors and origins of the organic compounds detected by SAM to date [4,5], we are conducting laboratory experiments on a set of fifteen martian analogs. The nature and content in organics present in those analogs are evaluated to highlight the best preservation environments and types of minerals. It will help target the most fruitful solid samples to be analyzed by SAM, as well as future instruments such the Mars Organic Molecular Analyzer (MOMA), the next pyro-GCMS instrument that will be sent to Mars onboard the ESA Exomars rover. To characterize the potential limits and biases of the flight-like pyrolysis/derivatization experiments on Mars, experiments were performed in differents steps. Steps include the characterization of the samples by laboratory procedures and flight-like analyses.

Sample targeted:

The rock samples have been collected in various Earth environment and represent similar mineralogies to those detected on Mars by remote and in-situ measurements. The fifteen samples studied include: an Antartic Paleolake containing C-bryophytes, used as the organic-rich control sample (OM2); iron oxides from a modern mineral precipitate and older oxidized surface gossan at Iron Mountain, CA; ancient acid saline lake sediments from Australia; siliceous sinter hot springs from a modern near-vent site and an inactive deposit from Iceland; a modern carbonate ooids from Pigeon Cay, The Bahamas; and an organic-rich shale from Messel, Germany [6].

Pyrolysis and derivatization GCMS experiments:

Samples were analyzed in three steps. First, they were analyzed in flash and SAM-like pyrolysis to highlight the most promising samples in terms of organic content (step 0). We then performed solid-liquid extractions with water:isopropanol, followed by derivatization with MTBSTFA and direct injection in the GCMS (step 1). The last step is a "one-pot/one step" MTBSTFA derivatization procedure where the organics from the solid samples are extracted and derivatized at the same time to then be pyrolyzed using a SAM-like ramp temperature (step 2). Laboratory experiments were performed on a Agilent (?) GCMS with an MXT-1 column and a Frontier Lab pyrolyser mounted upstream of the GC column, in the injector. A cold-trap was used to trap the volatiles compounds during the SAM-like pyrolysis ramp temperature experiments.

Results: First results from the OM2 sample (Figure 2) analysed in SAM-like pyrolysis (step 0) indicated the presence of more than 300 organic compounds including alkanes, alkenes, alkynes, ketones, aromatic hydrocarbons, alcohols, nitriles and N-, O-and S-heteocycles. They could come from a variety of organic molecules of interest including amino acids, carboxylic acids, polycyclic aromatic hydrocarbons, etc. OM2 is then a good organic control sample and a very promising sample in terms of organic content. The step 1 and step 2 analysis allowed to discriminate the nature of organic present. It led to the detection of mainly carboxylic and dicarboxylic acids from 1 to 17 carbon atoms. Laboratory experiments and data treatment of the fourteen other samples are ongoing to highlight the differences between the mineralogies selected and the step analyses.

Conclusions:

This ongoing work will help us identify the MTBSTFA by-products present in the SAM chromatograms and the high molecular derivatives detected in the last OG run. The investigation will also enable a better minerals target selection for the preservation of organic molecules on Mars and help the identification of the precursors of the organics detected on Mars. These experiments will also be helpful to better understand the limits and biases of the flight-like experiments to prepare th efuture analyses and the best pyrolysis and derivatization procedures to be done with the SAM and MOMA pyro-GCMS instruments. Step 0: SAM-like pyrolysis

Step 1: Extraction + derivatization

Step 2: « one-pot/one-step » derivatization

Figure 1 :

 1 Figure 1: Chemical derivatization reaction between MTBSTFA and glycine. The amino function is silylated by the reagent in addition of DMF solvent when heated for a few minutes at 75°C. The labile hydrogen is replaced by the silyl functional group.

  References: [1] Grotzinger, J.P., et al. (2013) Sci Express 343, 1242771-1242714. [2] Mahaffy P.R. et al. (2012) Space Sci Rev, 401-478. [3] C. Malespin, 49 th LPSC (2018). [4] Glavin, D. et al. (2013), JGR 118, 1955-1973. [5] Freissinet, C. et al. (2015) JGR. [6] A. J. Williams, 49 th LPSC (2018), #1112. Acknowledgments: Support from the post-doc grant "Using Organic Molecule Detections in Mars Analog Environments to Interpret the Results of the SAM investigation on the MSL mission" to S. S. Johnson from NASA GSFC.

Figure 2 :

 2 Figure 2: Comparison of the chromatograms obtained after the analysis of the OM2 sample in three steps: SAM-like pyrolysis: Step 0, solid-liquid extraction + MTBSTFA derivatization: step 1, "one-pot/onestep" extraction + derivatization (step 2).

  Planetary Science Conference 2018 (LPI Contrib. No. 2083)


A Buch

I Belmahdi

C Szopa

C Freissinet

D Glavin

P Coll

M Cabane

M Millan

J Eigenbrode

R Navarro-Gonzalez

J C Stern

V Pinnick

D Coscia

S Teinturier

M Morisson

M Stambouli

T Dequaire

P Mahaffy

POTENTIAL SOURCES OF ARTIFACTS AND BACKGROUNDS GENERATED BY THE SAMPLE PREPARATION OF SAM

Introduction: Sample Analysis at Mars (SAM) is one of the instruments of the MSL mission. Three analytical devices are onboard SAM: the Tunable Laser Spectrometer (TLS), the Gas Chromatography (GC) and the Mass Spectrometer (MS). To adapt the nature of a sample to the analytical devices used, a sample preparation and gas processing system implemented with (a) a pyrolysis system, (b) wet chemistry: MTBSTFA and TMAH (c) the hydrocarbon trap (silica beads, Tenax® TA and Carbosieve G) and the injection trap (Tenax® GR composed of Tenax® TA and 30% of graphite) are employed to concentrate volatiles released from the sample prior to GC-MS analysis [1]. This study investigates several propositions for chlorinated hydrocarbon formation detected in the SAM background by looking for: (a) all products coming from the interaction of Tenax® and perchlorates, (b) also between some soil sample and perchlorates and (c) sources of chlorinated hydrocarbon precursors.

Here we report on the detection of chlorohydrocarbon compounds and their potential origin.

SAM Results: Volatile compounds and abundant chlorinated hydrocarbons have been detected with SAM when analyzing samples collected in several sites explored by Curiosity rover [2][3][4]. Majority of the volatile compound come from the degradation of the MTBSTFA under hight temperature. Some of the chlorohydrocarbon compounds are produced during pyrolysis by the reaction of Martian oxychlorine compounds (perchlorate (ClO 4 -) or chlorate (ClO 3 -)) in the samples with terrestrial carbon from a derivatization agent (MTBSTFA) used in SAM [2].

Chlorobenzene cannot be formed by the reaction of MTBSTFA and perchlorates [2] and two other reaction pathways for chlorobenzene were therefore proposed :

(1) reactions between the volatile thermal degradation products of perchlorates (e.g. O 2 , Cl 2 and HCl) and Tenax® (see figure 1) and (2) the interaction of permingchlorates (T>200°C ) with OM from Mars's soil such as benzenecarboxylates [5]. The oxychlorine compound in Sheepbed starts releasing O 2 at temperatures around 200°C, the same temperature where we also see evidence of masses (e.g. 78, 112) in EGA con-sistent with aromatic hydrocarbons being released from the samples. By Product From Thermal Degradation of Tenax®: Tenax® GR has been heated at different temperature from 200°C to 600°C. Released organic compounds has been monitored by GC-MS by using a Restek column RTX5SilMS (30mX0.25mmX0.25µm). At 300°C and under, no aromatic compounds have been formed by the thermal degradation of Tenax. At 350°C a large range of aromatic compounds produced by Tenax® GR (benzene, phenyl, biphenyl, terphenyl derivatives and others) have been identified. After 400°C, among the other compounds already detected, phenylethyne is identified. However in SAM experiment, trap is heated at 300°C [1] and released some by-products at this temperature. Indeed, benzene, toluene, phenylethyne, styrene, biphenyl and chlorobenzene have been detected by SAM GC-MS [2]. Presence of perchlorate in the martian soil has to be taken in account to explain that production.

Influence of the Martian Perchlorate:

To simulate the SAM experiment, perchlorate (Ca(ClO₄)₂ → HCl , Cl₂ , O₂) [2] have been heated, in our laboratory built reactor, separately from the Tenax® GR, which is located in the injector. Results show that perchlorates have an impact on the degradation of Tenax® GR. We have detected a large range of aromatic compounds: thermal degradation products, chlorinated and oxidized compounds (Figure 1). Benzene and chlorobenzene have been formed by the thermal degradation of Tenax® at 300°C in presence of perchlorate. At 400°C, in addition of benzene and chlorobenzene, toluene, phenylethyne, styrene and biphenyl are formed by the interaction of Tenax® GR and perchlorate. Whatever the temperature, the addition of Ca(ClO 4 ) 2 raise the quantity of hydrocarbon products until 18mg (CaClO 4 ). Then, the amount of hydrocarbon products decrease as they are degraded, chlorinated or oxidized. We have also proved that the increase of the amount of chlorobenzene CBZ formed is related to the increase of the temperature. Quantity of CBZ released is directly correlated with the quantity of Ca(ClO 4 ) 2 . More we have perchlorate more the quantity of CBZ is importante. Nevertheless it is important to note that on SAM the quantity of CBZ produced by Tenax is still inferior at the quantity detected on Mars.

1952.pdf 47th Lunar and Planetary Science Conference (2016)

Conclusions: From Tenax® GR thermal degradation, we observe a production of benzene, toluene, styrene and biphenyl below 350°C and phenylethyne below 450°C. Within the range of temperature studied we have observed no impact of the derivatization agents (MTBSTFA/DMF) on the Tenax® GR by thermal degradation. But in the presence of calcium perchlorates production of benzene and chlorobenzene have been detected at 300°C after interaction with Tenax® GR. Chlorobenzene produced by this way take part of the SAM backround but does not explain the presence of CBZ detected on the Martian soil. Other process which imply oxychlorine and indigenous compounds can explain the CBZ formation [START_REF] Freissinet | pdf 47th Lunar and Planetary Science Conference[END_REF]. The absence of detection of other aromatic compounds detected in SAM background at 300°C could be explained by the presence of other minerals (such as iron oxides) in Martian soil which are able to facilitate the release O₂ (e.g. hematite).

Figure 1 -

 1 Figure 1 -summary of the species detected in lab in SAM condition in presence or in absence of perchlorate.Nevertheless it is important to note that on SAM the quantity of CBZ produced by Tenax is still inferior at the quantity detected on Mars.Conclusions: From Tenax® GR thermal degradation, we observe a production of benzene, toluene, styrene and biphenyl below 350°C and phenylethyne below 450°C. Within the range of temperature studied we have observed no impact of the derivatization agents (MTBSTFA/DMF) on the Tenax® GR by thermal degradation. But in the presence of calcium perchlorates production of benzene and chlorobenzene have been detected at 300°C after interaction with Tenax® GR. Chlorobenzene produced by this way take part of the SAM backround but does not explain the presence of CBZ detected on the Martian soil. Other process which imply oxychlorine and indigenous compounds can explain the CBZ formation[START_REF] Freissinet | pdf 47th Lunar and Planetary Science Conference[END_REF]. The absence of detection of other aromatic compounds detected in SAM background at 300°C could be explained by the presence of other minerals (such as iron oxides) in Martian soil which are able to facilitate the release O₂ (e.g. hematite).


Determination of the Possible Sources of Chlorinated Hydrocarbons Detected During Viking and MSL Missions

A. Buch 1 , I.Belmahdi 1 , C. Szopa 2 , C. Freissinet 3,4 , D. Glavin 3 , K. Miller 5 , R. Summons 5 , P. Francois 6 , P. Coll 6 , S. Teinturier 3 , J. Eigenbrode 3 , A. R. Navarro-Gonzalez 7 , A. McAdam 3 , J. Stern 3 , D. Coscia 2 , T. Dequaire 6 , M. Millan 2 , J.Y. Bonnet 2 , P. Mahaffy 3 and M. Cabane 2 . arnaud.buch@ecp.fr. 1 LGPM, Ecole Centrale de Paris, Châtenay-Malabry 2 LATMOS, Univ. Pierre et Marie Curie, Univ. Versailles Saint-Quentin & CNRS, Paris, 3 NASA GSFC, Greenbelt, MD, USA 4 NASA PPA, Oak Ridge, Tennessee, USA, 5 Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, Massachussetts, USA, 6 LISA, Univ. Paris-Est Créteil, Univ. Denis Diderot & CNRS Créteil, France, 7 Universidad Nacional Autónoma de México, México,

Interest of exploration on Mars

Mars is interesting given that its early history is similar to one on the Earth. In fact, volcanoes were still active, the environment was wetter and warmer and the magnetosphere still existed.

Source of organic Mars: Various sources of endogenous organic matter (OM) could have existed including (a) abiotic production via hydrothermal vents, volcanism and atmospheric synthesis and (b) biotic synthesis. Currently, the sources of extraterrestrial organic compounds that should be delivered to Mars are known: carbon-rich meteorites, micrometeorites, comets and interstellar dust particles.

Stabilization of the OM takes place through three mechanisms described in the article of Lützow ( 1): (1) First, the selective preservation of OM is described as a phenomenon of accumulation of some compounds because of their resistance against the environment. (2) The second path which allows the persistence of OM is the space isolation of OM from environmental stress. (3) The last way to stabilize the OM is intermolecular interactions between minerals or metal ions with OM. One of the primary objectives of the Mars Science Laboratory (MSL) mission is to search for environments on the Martian surface that have preserved OM.

Structure and aim of SAM

Sample Analysis at Mars (SAM) is one of the instruments of the MSL mission. Three analytical devices are onboard SAM: the Tunable Laser Spectrometer (TLS), the Gas Chromatograph (GC) and the Mass Spectrometer (MS) (2).

Solid sample preparation: To adapt the nature of a sample to the analytical devices used, a sample preparation and gas processing system implemented with (a) a pyrolysis system, (b) wet chemistry: MTBSTFA and TMAH (c) the hydrocarbon trap (silica beads, Tenax® TA and Carbosieve G) which is employed to concentrate volatiles released from the sample prior to the GC-MS analyses.

Detection of chlorinated hydrocarbons

All chlorinated hydrocarbons detected during the Viking I and II missions and MSL (Rocknest (RN), John Klein (JK), Cumberland (CB) and Confidence Hill (CH)) are listed in Table 1.

Viking landers (1976):

The origin of chloromethane and dichloromethane was explained at the time by terrestrial contamination from the instruments (3). In a recent paper from Navarro-González (4), these results have been reinterpreted and chlorinated compounds could have been the product of the reaction of perchlorates identified by Phoenix (5) with martian organic carbon present in the sample or terrestrial organic carbon in the instrument or sample handling chain.

MSL (2011):

A diverse range of chlorinated hydrocarbons have been detected with SAM after GCMS analysis of samples collected from several sites explored by Curiosity rover (Table 1). Some of these chlorohydrocarbons are produced during pyrolysis by the reaction of martian oxychlorine compounds in the samples with terrestrial carbon from a derivatization agent (MTBSTFA) present in SAM (6,7). Chlorobenzene (CBZ) cannot be formed by the reaction of MTBSTFA and perchlorates (6) and two other reaction pathways for CBZ were therefore proposed : (1) reactions between the volatile thermal degradation products of perchlorates (e.g. O2, Cl2 and HCl) and Tenax® and (2) the interaction of perchlorates (T > 200 °C) with OM from Mars's soil such as benzenecarboxylates [START_REF] Freissinet | LPSC XXXXV Abstract 2796[END_REF][START_REF] Steininger | [END_REF]. Among all the sample analyzed by SAM, JK and CB sites are interesting; smectites (phyllosilicates -18 to 22 wt %) and quartz (0.1 to 1 wt %) were detected at the two sites (10) and could have an important role in the preservation of OM (1).

Objectives

This study investigates several hypotheses for chlorinated hydrocarbon formation by looking for: (a) all products coming from the interaction of Tenax® and perchlorates, (b) products between various soil sample and perchlorates and (c) sources of chlorinated hydrocarbon precursors.

Experiments and methods:

To answer some of our remaining questions, laboratory experiments were done in several solid matrices which

Viking

2066.pdf 46th Lunar and Planetary Science Conference (2015)

were brought directly in contact with perchlorates and heated.

Solid matrices

Four solid matrices were analyzed. Fused silica: It is used as an organics-free sample. Tenax®: This polymer absorbent can release organic compounds when heated to temperatures > 300 °C) (11). JSC-Mars1: JSC-1 is a martian regolith simulant collected in volcanic active area in Hawaii [START_REF] Allen | LPSC XXVII Abstract 1797[END_REF]. Fontainebleau Sand: Fontainebleau sand is almost pure silica (quartz). This sand has been previously heated to 700 °C during 24 h in our laboratory.

Experiments GC-MS Analysis

In this work, we have performed a direct (i.e. solid matrix and perchlorates are mixed together in the injector) and indirect (i.e. perchlorate is in the reactor which is upstream from the injector where the solid samples are placed). The abundances of Ca-perchlorate (3 or 9 wt %) used in these experiments are much higher than SAM perchlorate abundances estimated to be 0.3-0.5 wt % at Rocknest (6). The GC-MS used in experiment is a Thermo Trace GC Ultra with Restek Rtx-5 Sil-MS column (30m×0.25mm×0.25µm), its injector is Optic 3 from ATASGL, and the MS is a Thermo DSQII. The helium flow was maintained constant at 1 mL/min (split 10 mL/min). The temperature of the column was started and held 8 min at 35 °C, then increased at 7 °C/min to a final temperature of 300 °C and held 2 min at this temperature.

Results and discussions:

In the light of the results presented in Table 3, we can conclude that the chlorinated hydrocarbons can originate from two origins:

Martian origin Once the OM and perchlorates are raised to high temperature, chlorinated compounds are produced, even in the sample which contains 3 wt % (B2). Precursors of chlorinated hydrocarbons might be preserved on the surface of Mars as the persistent molecules found in the sample of Fontainebleau's sand (A3).

SAM origin

Chlorobenzene is also product by the reaction of perchlorates and Tenax® as other compounds (e.g. No. 9-24) Conclusion. Chlorinated compounds highlighted by SAM on Mars could have several origins: from perchlorate oxidation of MTBSTFA, Tenax® and/or Martian organics. However, due to the higher concentration of chlorobenzene in the Mars sample compared to the blank, this compound is interpreted to be from martian origin. The other line of supporting evidence for a martian chlorobenzene source is the detection of m/z 112 and 114 at ~3:1 ratio in the EGA data that is attributed to CBZ. Based on the SAM gas flow design the CBZ released from the sample cannot be attributed to reaction products from Tenax and the hydrocarbon trap. Then, to determine the origin of chlorobenzene, it is necessary to go further in laboratory investigations by using other way of analyses (13).

Composition of sample

Table 1 :

 1 major chlorinated hydrocarbon detected during the Viking and MSL mission. MXT-CLP column (nd: not detected).

			MSL
		RN	JK	CB	CH
	CH3Cl CH2Cl2 nd nd nd nd nd	CH3Cl CH2Cl2 CHCl3 CCl4 nd C4H7Cl C6H5Cl	CH3Cl CH2Cl2 CHCl3 CCl4 nd ndC6H5Cl	CH3Cl CH2Cl2 CHCl3 CCl4 C3H6Cl2 C4H7Cl C6H5Cl	CH3Cl CH2Cl2nd nd nd C4H7Cl C6H5Cl

Table 3 :

 3 Chlorinated compounds detected in laboratory.

Table 2 :

 2 Samples use in GC-MS studies

2066.pdf 46th Lunar and Planetary Science Conference (2015)


Maeva Millan
email: maeva.millan@latmos.ipsl.fr

Cyril Szopa

A Buch

Imène Belmahdi

Daniel P Glavin

Caroline Freissinet

Jennifer L Eigenbrode

P Douglas Archer

B Sutter

Onboard The

Rover M Millan

R E Summons

R Navarro-González

P Coll

M Cabane

P Mahaffy

à la diffusion de documents scientifiques de niveau

recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Influence of Oxychlorine Phases During the Pyrolysis of Organic Molecules: Implications for the Quest of Organics on Mars with the SAM Experiment Onboard the Curiosity Rover

Introduction: One among the main objectives of the Sample Analysis at Mars (SAM) experiment is the in situ molecular analysis of gases evolving from solid samples heated up to ~850°C, and collected by Curiosity on Mars surface/sub-surface in Gale crater. With this aim, SAM [1] uses a gas-chromatograph coupled to a quadrupole mass spectrometer (GC-QMS) devoted to separate, detect and identify both volatile inorganic and organic compounds.

SAM detected chlorinated organic molecules [2,3] produced in evolved gas analysis (EGA) experiments. Several of these were also detected by the Viking experiments in 1976 [4]. SAM also detected oxychlorine compounds that were present at the Phoenix landing site. The oxychlorines may be prevelant over much of the martian surface [5].

The C1 to C3 aliphatic chlorohydrocarbons (chloromethane and di-and trichloromethane) detected by SAM were attributed to reaction products occurring between the oxychlorines phases and the organic compounds coming from SAM instrument background [2]. But SAM also showed the presence of a large excess of chlorobenzene and C2 to C4 dichloroalkanes among the volatile species released by the Cumberland sample of the Sheepbed mudstone. For the first time in the history of the Mars exploration, this proved the presence of Mars indigenous organic material at the Mars' surface [3]. However, the identification of the precursor organic compounds of these chlorohydrocarbons is difficult due to the complexity of the reactions occurring during the sample pyrolysis.

Laboratory pyrolysis experiments have demonstrated that oxychlorines phases such as perchlorates and chlorates, decomposed into dioxygen and volatile chlorine bearing molecules (HCl and/or Cl 2 ) during the pyrolysis. These chemical species can then react with the organic molecules present in the martian solid samples through oxidation, chlorination and oxychlorination processes [2,6,7].

Objectives: Previous laboratory analog work has examined a limited set of organic-chlorine reactions that may explain the SAM detected chlorinated hydrocarbons such the chlorobenzene [7]. But the small number of chemical species investigated, including some carboxylic acids and hydrocarbons, does not exclude others parent molecules. Therefore, we perfomed a series of systematic laboratory experiments to help understanding the influence of oxychlorine phases on organic molecules during SAM pyrolysis, and try to define the possible organic molecules parent to the species detected by SAM.

Sample selection and preparation:

Three types of organic molecules potentially present on Mars were evaluated: polycylic aromatic hydrocarbons, amino acids, and carboxylic acids. The organics were mixed with various perchlorates and chlorates salts (Fe, Ca, Mg, K and Na), that may be present in martian solid samples. To focus on the reactivity between oxychlorines and organic compounds only, solid samples were simulated using fused silica as an inert sample matrix and 1wt.% of both oxychlorines and organics were added to the silica. The perchlorate and chlorate salts concentration correspond to Gale Crater mean value detected by SAM [3]. The solid volume analyzed is also similar to the one analyzed by the SAM experiment.

2124.pdf Lunar and Planetary Science XLVIII (2017)

Pyrolysis-GC-QMS experiments: Laboratory Experiments were performed on a GC-QMS using a Restek Rxi-5 column (30 m x 0.25 mm x 0.25 µm) and a Frontier pyrolyser. The pyrolyser is mounted upstream of the column, in the GC injector. Simulated samples were pyrolyzed in flash-pyrolysis up to 900°C and under SAM-pyrolysis conditions using a 35°C.min -1 ramp temperature. This covers the SAM pyrolysis temperature range and enables a comparison of both pyrolysis modes. The ratio of organic molecules to oxychlorine were varied to evaluate how ratio changes could influence the amount, nature and type of reaction products.

Results:

The reaction products detected in both pyrolysis modes (flash and ramp pyrolysis) were analyzed and identified by GC-MS. It is evident that the destruction and/or the evolution of the organic compounds by the oxychlorine phases depends not only on the nature of both organic and inorganic compounds (perchlorate and chlorate) but also on the pyrolysis mode used. The figure 1 shows the chromatogram obtained after the pyrolysis of naphthalene mixed with 1wt.% of calcium perchlorates (Ca-PCL). We observed that the naphthalene is still detected, even after its interaction with the Ca-PCL (Figure 1).

These results show that it is sometimes possible to correlate the reaction products and the parent chemical family and/or the organics. For example, we detected chloronaphthalene after the pyrolysis of the oxychlorines with the naphthalene by using both pyrolysis modes (Figure 1).

This data also demonstrates that some of the molecules, or molecular families studied, could be the precusors of the chlorohydrocarbons detected by both Viking and SAM experiments. Some chlorobenzene is detected after the flash-pyrolysis of the organic compounds mixed with the oxychlorines such naphthalene with Ca-PCL for example (Figure 1). Moreover, C1 to C4 chloroalkanes are only formed after the SAM-like pyrolysis of PAH and amino acids mixed with the oxychlorine phases (Figure 1). Figure 1: chromatograms obtained after the pyrolysis of the naphthalene mixed with the calcium perchlorates (Ca-PCL) at 1wt.%. Pyrolysis parameters (flashpyrolysis and ramp temperature at 35°C.min -1 ) are represented in light and dark orange respectively. The nature of some chlorohydrocarbons formed depends on the pyrolysis modes used.

Conclusion:

This work facilitates the interpretation of the SAM detections, especially regarding the chlorinated compounds found on Mars. The reaction products obtained after the pyrolysis of the organics mixed with the perchlorates and/or chlorates improved the understanding of the behavior of organic matter during SAM pyrolysis experiments on Mars where chlorination, oxidation or oxychlorination are taking place. These experiments are also helpful to prepare future analyses and the best pyrolysis procedures to be done with the MOMA GC-MS experiment that will be onboard the Exomars 2020 mission.

Acknowledgments: French Space Agency (CNES) support for SAM-GC development and exploitation.


Michel Cabane

Patrice Coll
email: pcoll@lisa.u-pec.fr

Cyril Szopa

David Coscia

A Buch

Samuel Teinturier

Rafael Navarro-González

Jean-Pierre Goutail

Christophe Montaron

Jean-Baptiste Rigal

R Navarro-Gonzalez

J-P Goutail

J-B Rigal

J-J Correia

V Guerrini

P Poinsignon

M-S Clerc

M Meftah

L Soldani

F Mettetal

M Jerôme

C Philippon

A Galic

J Sablairolles

S Triqueneaux

D Chazot

B Toffolo

F Y Rakoto

A Gaboriaud

D Glavin

F Raulin

P François

O Poch

R Sternberg

P Mahaffy

Initial Performances/Observations/Results of the SAM Gas Chromatograph (SAM-GC) at Rocknest Site

Introduction: among the SAM suite of instruments [1], SAM-GC (Gas Chromatograph) is designed to support analytical sequences devoted to identify and quantify volatiles evolved from the thermal/chemical treatment of any soil sample delivered to SAM. On November 6 th , 2012, SAM-GC was commissioned in Curiosity at the Rocknest site. SAM-GC was the last instrument onboard Curiosity to be commissioned. Four SAM-GC runs have now been performed, identified as 25033 (no sample run), 25038 (scoop 5, sample 1), 25041 (scoop 5, sample 2), 25044 (scoop 5, sample 3), It is an appropriate time to report on the GC performance, observations and preliminary impact on Mars evolution/habitability.

Performance of SAM-GC:

The first instrumental goal addressed by SAM-GC is the separation of the constituents of a complex gaseous mixture. To analyze the Rocknest samples, GC column n°5 was used. In column 5, compounds were separated on the MXT-CLP column (30 m x 0.25 mm I.D.). The column was set at an initial temperature of 50°C followed by heating to 220°C at 10°C/min. The separation performance conforms well to ground qualification data and will be addressed more extensively in this paper. Figure 1 reveals these separation capabilities for two different analytical runs, where 25033 refers to an analytical sequence where no solid sample was delivered to SAM, and 25038 refers to an analytical run where scoop five, solid sample #1 was delivered. The overlap of the main signatures on the two runs emphasizes the reproducibility of the separation performance, in which the difference between the peak areas reveals the difference in the amount of compounds analyzed with GC.

From a quantitative point of view the GC is equipped with autonomous detectors known as nanoTCDs or TCDs (Thermal Conductivity Detectors). All GC columns, except column 1, incorporate a TCD that provides a signal independent of the mass spectrometer. The TCDs can detect chemical species down to the part per million level, with a 10 5 dynamic linear range. This detector, which generates a simple physical signal, provides an additional level of detection redundancy to the SAM QMS (Quadripole Mass Spectrometer). From a qualitative point of view, the chromatographic peak shapes derived from the TCDs can be utilized to better fit the GCMS chromatographic signatures to particular species. This is particularly useful as the QMS temporally under samples the GC signal, illustrates from laboratory calibrations when to expect a given compound to elute. For example, the figure shows that dodecane elutes at around 9.5 minutes. If a sample contains dodecane, a peak should be found in this time range.

In such a framework we were searching for several organics from different chemical families in the gases evolved from the Rocknest soil samples, and especially aromatics. This is addressed on figure 4 where signals indicating the release of organics should be seen if any compound of this chemical family is present. Preliminary results: one important result supported by SAM during the Rocknest campaign is clearly the presence of perchlorates in the soil. Since perchlorates have been detected with Phoenix lander [5] and then recently with Curiosity [6,2,3], the re-interpretation of the Viking data has to be looked at differently [7]. It is now more and more assumed that perchlorates, whatever their chemical nature is (this is still under investigation), are probably distributed throughout the entire Mars environment. This is a key parameter for future exploration of Mars, for Curiosity in the short term, and at much longer term for the development of future analytical instruments (payload of ExoMars mission for example).

The absence of PAH's release is also interesting, when it is known that micrometeorites containing PAHs still bring this organic material to the Mars surface today. This lack of detection defines an upper limit on the content of PAHs in the martian soil at the Curiosity site, but it could also be indicative of the presence of chemical mechanisms that process this type of material at the surface. In this last case, the investigation of the Gale crater environment with Curiosity instrumentation should help to determine which type of mechanisms are occurring on Mars.

References:

Figure 1 :

 1 Figure 1: TCD (GC detector) signal overview for two different analyses of Rocknest campaign. Major signatures are common to the two chromatograms with same retention times, showing the accurate and repeatable separation of constituents of the gaseous mixture as a function of time.

Figure 2 :

 2 Figure 2: TCD (GC detector) signal overview of the same detected volatile for three different analyses of the sand of Rocknest. The time axis was rescaled for direct comparison.

  Planetary Science Conference (2013)especially when peaks are eluted[1]. An illustration of these acquired TCD signals are shown on Figure2, where signals from three different runs of Rocknest samples are plotted together. Note the excellent Gaussian shape of the peak related to the eluted compound in all of the runs. This type of chromatographic peaks is known as a symmetrical normal error peak. This type of peak facilitates the treatment of equations governing chromatographic performance. In many cases, tailing or fronting peaks are observed. However, Figure2, which is representative of the signal shape measured by the TCD, does not have these tailing characteristics.Observations partially supported by SAM-GC, and preliminary results: Most of the analytical sequences used to address the content of Rocknest Soil samples were Solid Sample -Evolved Gases Analyses (SS-EGA, see [1]), with part of the released gases analyzed by Solid Sample -Gas Chromatograph Mass Spectrometer analysis (SS-GCMS, see[1]). The separation capacity of the GC plays an important role for tracking and identifying volatiles contained at the trace level in the gases evolved from Rocknest samples. As an illustration this separation strongly supports the identification of several methyl-chlorohydrocarbons [2], including specific ones as chloromethylpropene [3], and it confirms the identification of HCN in the gases evolved from the solid sample[4]. The separation performance of the GC allows the search for species in the chromatogram in a time range where the relevant signatures are expected. Figure3

Figure 3 :

 3 Figure 3: laboratory analysis of a pentane, hexane, benzene, octane, nonane, decane, undecane and dodecane (from the left to the right) mixture on a MXT-CLP wall coated open tubular chromatographic column, operating under He gas flow (deltaP = 1 bar), starting temperature of 40°C for 5 min then a heating rate of 10°C/min until 250°C.

Figure 4 :

 4 Figure 4: SS-GCMS analysis of the Rocknest soil focusing on single m/z ions related to simple 2 and 3 rings PAHs: naphthalene, anthracene and phenanthrene. No signal appears above the background level, assessing the non detection of these compounds in the gases evolved from the sample.

  [1] Mahaffy, P. et al. (2012) Space Sci Rev, 170, 401-478. [2] Glavin, D. et al. (2013), this meeting. [3] Eigenbrode, J. et al. (2013), this meeting. [4] Stern, J. et al. (2013), this meeting. [5] Hecht, M. H. et al. (2009), Science, 32, 64-67. [6] Sutter, B. et al., (2013) this meeting. [7] Navarro-Gonzalez, R. (2010), J. Geophys. Res. 115, E12010.

44th Lunar and Planetary Science Conference (2013)

Acknowledgements: SAM-GC team acknowledges support from the French Space Agency (CNES), French National Programme of Planetology (PNP), National French Council (CNRS), Pierre Simon Laplace Institute, Institut Universitaire de France (IUF) and ESEP Labex.


C A Malespin

C Freissinet

P R Mahaffy

S Teinturier

D P Glavin

A Mcadam

R Williams

A Buch

C Szopa

D Cosica

E Raaen

C Johnson

W Brinckerhoff

B Prats

R Gonzales

The Sample Analysis at Mars (SAM) Wet Chemistry Experiment

Introduction:

The Sample Analysis at Mars (SAM) instrument suite is currently operating on the Curiosity rover in Gale Crater, Mars. SAM carries nine sealed wet chemistry cups, seven which contain a mixture of N-methyl-N-(tertbutyldimethylsilyl) trifluoroacetamide (MTBSTFA) and dimethylformamide (DMF) for derivatization and two that are filled with tetramethylammonium hydroxide (TMAH) and methanol for thermochemolysis [1]. Here we describe the testing and experimental procedure for the upcoming use of the MTBSTFA derivatization experiment on Mars.

Since MTBSTFA derivatization was not tested on the SAM flight instrument, the experiment needed to be optimized on Earth using the SAM testbed (TB). In preparation for the puncture of a wet chemistry cup on Mars, a high fidelity testing campaign using Mars analog samples from Lake Hoare, Antarctica was conducted using the TB, located at NASA GSFC. The TB is an identical copy of the flight instrument, and is described in detail elsewhere, [1,2].

Overview of wet chemistry. One of the primary goals of the SAM investigation is the search for organic compounds on Mars. The wet chemistry cups onboard SAM are intended to transform complex organic molecules into more volatile derivatized products that can be readily analyzed by GCMS. The seven derivatization cups contain 0.5 mL of a mixture of MTBSTFA and DMF in a 4:1 ratio. These cups also contain trace standards (3-fluorovaline and pyrene) for calibration purposes. The MTBSTFA derivatization reaction with an amino acid is shown in Figure 1.

Three sealed MTBSTFA cups, identical to the flight cups, were loaded into the TB, which were used for the testing campaign. A more detailed description of SAM and the wet chemistry hardware can be found in Mahaffy et al [1].Here we focus on the experimental procedure and parameters used.

Experimental procedure: The SAM wet chemistry experiment is based on the "standard" SAM EGA/GCMS sequence which nominally accepts ~120 mg of sample for analysis. For the wet chemistry experiment, the sequence is divided into three sequential parts, which all occur on the same Sol.

Part 1-Prep and background. In a nominal SAM EGA sequence (i.e. without derivatization), the cup being used to accept sample is preconditioned before use by heating to 900 C under He flow in the pyrolysis oven.

Since the MTBSTFA would be pyrolyzed at 900°C and The timing and order of events was critical in the development. It is important to begin heating the sample as quickly as possible after the rover drops off portions into the punctured wet chemistry cup since MTBSTFA will slowly evaporate from the cups after puncture at Mars pressure at temperatures above 9 C. Typically the SAM SMS is at -10 C during puncture. In order to minimize the time the punctured cup was exposed to the open environment, the sample drop off from the rover was moved to the middle of the SAM experiment. Prior to receiving a sample, the wet chemistry cup is punctured onboard SAM using two puncture needles, one for each foil capping the solvent.

The cup is then moved to the inlet to accept sample from the rover arm. After sample delivery, the cup is put into the SAM oven under helium flow in preparation for analysis.

Part 3-Pyrolysis EGA and GCMS of sample. Part 3 begins minutes after part 2 ends with the cup in the oven. The sample is heated using the standard SAM 35 C.min -1 pyrolysis ramp, up to ~900 C. In order to avoid saturating the SAM hydrocarbon trap with MTBSTFA/DMF fluid, only a portion of the evolved gas is captured for analysis. On Mars, the remainder of the gas is vented out an exhaust located on the side of the rover, but on the TB, the vented gas

2369.pdf Lunar and Planetary Science XLVIII (2017)

is collected on an external cold trap for subsequent analysis using a lab GCMS. The fraction of gas captured below 250 C, where the majority of the MTBSTFA/DMF vapor exists, is lower than at higher temperatures. The fraction of evolved gas collected is a script parameter, allowing for a a higher percentage of analytes to be targeted at mid and high temperatures, and allowing a quick change of those temperature cuts depending on the sample.

To help further mitigate clogging the SAM hydrocarbon trap and GC columns, during the low temperature (< 250 C) portion of the pyrolysis ramp the SAM hydrocarbon trap is kept at 80 C. Laboratory studies have shown that at this temperature MTBSTFA is not efficiently trapped, while still allowing any complex derivatized organics to be collected (such as derivatized amino acids). Once the sample temperature reaches 250 C, the trap is quickly cooled down to 0 C to help freeze out and collect any analytes, derivatized and non-derivatized.

The collected gas on the hydrocarbon trap is used for a split column GCMS analysis. SAM has 6 GC columns, the wet chemistry experiment utilizes columns 1 and 4 for GCMS. Column 1 is an MXT20 column without an adsorption injection trap (IT) and targetting light to medium molecular weight mid-polar organics. Column 4 is a ChirasilDex column with an IT used to focus and the analytes as quickly as possible into the column.

The SAM HC trap is heatetd to 320 C and the desorbed gases are split between columns 1 and 4. GCMS analysis column 1 begins immediately at the end of the EGA since it has no injection trap to retain released gas. However, organics are still trapped at the front-end of this column due to a combination between He flow and temperature. The column is ramped up from ambient to 250 C at a rate of 10 C min -1 . Upon completion of GC1, the IT of GC 4 is flashed to inject gas into the column for GCMS. GC4 reaches a maximum temperature of 190 C using a similar 10 C.min -1 ramp rate.

The entire three part procedure was validated, both for science and engineering purposes, using filled MTBSTFA/DMF cups on the SAM TB. We used Lake Hoare samples as an analog for the wet chemistry campaign, due to it being a well characterized organic rich sample[3] that has also been previously analyzed using the SAM TB.

Summary of SAM TB Lake Hoare results: EGA and GCMS of Lake Hoare sediment cores have been performed on the SAM TB, without the introduction of any wet chemistry reagents. We use this sample as a control 'blank' under SAM-like conditions for comparison with the MTBSTFA experiment.

A detailed analysis of the Lake Hoare results, with and without MTBSTFA/DMF, will be discussed in a later paper, but we briefly note several compounds of interest in Figure 2. Propanenitrile, which is released from the sample, is seen in both data sets, albeit reduced by a factor of two in the derivatization run. Benzene, found in almost all SAM runs because it is released from IT trap, is shown as a standard compound for comparison. We also detected various MTBSTFA by-products such as N-methyl-2,2,2trifluoroacetamide (TFMA) and 1,3-bis(1,1-dimethylethyl)-1,1,3,3 tetramethyldisiloxanetetramethyldisiloxane, along with the whole MTBSTFA molecule since it has not yet been measured by SAM on Mars. This clearly shows there are sufficient reagents for reactions to occur and produce derivatized compounds, as seen by the tertbutyldimethylsilyl (TBDMS)-phenol peak in the upper plot.

Path Forward: The SAM TB wet chemistry campaign successfully demonstrated that using the proposed experimental procedure will produce sufficient reagent fluid/vapor to react with the sample, without saturating the HC trap or GC columns.

Development work to incorporate this procedure with the larger rover handoff plan is currently underway. Wet chemistry allows for a new capability to be employed on the surface, potentially opening the possibility for a larger set of organics to be detected.

Experience with derivatization on SAM is further being applied to optimize the protocols planned for the Mars Organic Molecule Analyzer(MOMA) investigation on the ExoMars rover. References: [1] Mahaffy, P.R. et al (2012) Space Sci Rev. 170, 401-478. [2] Malespin, C.A., et. al. (2015), LPSC, abst # 2558[3] Bishop, J, et al. (1996), Geochim Cosmochim, 765-785 Acknowledgements: The authors thank the SAM and MSL operations and science teams.

Figure 1 .

 1 Figure 1. The MTBSTFA cup (top) and reaction with an amino acid (bottom) that displaces the hydroxyl hydrogen with the methylated silica group to produce a volatile derivative.

Figure 2 .

 2 Figure 2. Lake Hoare GCMS plot run using wet chemistry procedure(top) compared to Lake Hoare sample run without MTBSTFA (bottom). Select compounds of interest showing differences in signal intensity as a result of reactions with MTBSFTA reagent.


William B Brinckerhoff

Veronica T Pinnick

Ryan Danell

Friso H W Van Amerom

Ricardo D Arevalo

Andrej Grubisic

Xiang Li

Caroline Freissinet

Zhiping Chu

Marco Castillo

Mars Organic Molecule Analyzer (MOMA) Flight Model Qualification

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

MARS ORGANIC MOLECULE ANALYZER (MOMA) FLIGHT MODEL QUALIFICATION. William B.

Brinckerhoff [START_REF]3 rd Conf. Early Mars[END_REF] , Veronica T. Pinnick 1 , Ryan M. Danell 2 , Friso H. W. van Amerom 3 , Ricardo D. Arevalo, Jr. 1 , Andrej Grubisic 1 , Xiang Li 1 , Caroline Freissinet 1 , Zhiping Chu 1 , Marco Castillo 1 , Chris Johnson 1 , Cyril Szopa 4 , Fabien Stalport 5 , Arnaud Buch 6 , Tristan Allain 5 , Noel Grand 5 , Francois Raulin 5 , Walter Goetz 7 , Harald Steininger 7 , Fred Goesmann 7 , and the MOMA Team 1-8 , 1 NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771; 2 Danell Consulting, Inc., Winterville, NC; 3 Mini Mass Consulting, Hyattsville, MD; [START_REF]3 rd Conf. Early Mars[END_REF] Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Guyancourt, France; [START_REF] Goesmann | Astrobiology[END_REF] Laboratoire Interuniversitaire des Systèms Atmosphériques (LISA), Univ. Paris-Est, Créteil, France; 6 Laboratoire de Génie des Procédés et Matériaux (LGPM), École Centrale Paris, Chatenay-Malabry, France; 7 Max Planck Institut für Sonnensystemforschung (MPS), Göttingen, Germany; 8 Laser Zentrum Hannover e.V. (LZH), Hannover, Germany.

Introduction:

The Mars Organic Molecule Analyzer (MOMA) investigation on the 2020 ExoMars rover will examine the molecular composition of samples acquired from depths of up to two meters below the martian surface, where organics may have been protected from radiative and oxidative degradation [1,2]. The clay-rich rock exposures accessible in Oxia Planum, the preferred landing zone, will provide an excellent opportunity to sample ancient material for organics analysis. MOMA incorporates both pyrolysisgas chromatography/mass spectrometry (pyr-GCMS) and laser desorption mass spectrometry (LDMS) modes of operation, and is designed to work in concert with the other investigations in the rover's Pasteur Payload, particularly the Raman [3] and MicrOmega [4] instruments that analyze common drill samples presented on a carousel. With two modes, MOMA detects compounds over a wide range of molecular weight, volatility, and mineralological association [5].

Implementation of GCMS and LDMS Modes: GCMS mode is operated analogously to the Sample Analysis at Mars (SAM) investigation on Curiosity. Samples are sealed into ovens and heated to 850° C. Analyte gases, entrained in He, flow through the GC system to the MS. The GC includes two cooled hydrocarbon traps (Tenax and Carbosieve) that are opened after major water evolution (above 100° C) but prior to organic thermodesorption (from ~300° C up to > 500° C). Trapped organics are injected onto one of four columns and eluted over a temperature ramp to the MS electron ionization (EI) source. GCMS analyzes compounds of high-to-moderate volatility (enthalpies of vaporization !H V ≤ 50 kJ mol -1 ) such as alkanes, amines, and lighter carboxylic and amino acids and aromatic species. Derivatization agent present in some ovens enables detection of the higher !H V and polar species over the full GCMS m/z range of 50-500 Da.

In LDMS mode, molecules are desorbed and ionized directly from powder samples with a pulsed UV laser (266 nm, 1 ns duration) at Mars ambient pressures. Parent molecular cations, and their fragments, enter the MS through a fast aperture valve that closes after ions are trapped, permitting the ion trap pressure to reduce to <10 -3 Torr where the detectors can be operated. LDMS mode is designed to analyze compounds of moderate-to-low volatility (enthalpies of vaporization !H V ≥ 40 kJ mol -1 ) such as heavier carboxylic acids, aromatic species, chain-like compounds, and macromolecular organics. The nature of LDI permits some fraction of large "parent" molecules to desorb intact. As previously demonstrated, the pulsed LDMS mode is not strongly affected by the potentially oxidizing effects of heat-evolved perchlorates, simplifying analysis of nonvolatile organics [6].

Flight System Intergration and Test: The various flight instrument subsystems have been functionally tested and qualified at their respective institutions in preparation for flight model (FM) integration, test, and delivery to the rover in 2017. The flight MS completed earlier [7] has been integrated with an engineering model of the laser and with a commercial (Thermo) GC for both bench and Mars environmental chamber testing (Fig. 1) under stringent contamination and planetary protection controls.

2385.pdf Lunar and Planetary Science XLVIII (2017)

Preliminary tests of the GCMS mode using calibration gases have demonstrated as-expected functional performance in for resolution, acquisition rate, sensitivity, and fragmentation patterns. A four-component hydrocarbon mix (butane, pentane, hexane, and benzene) was injected at the appropriate concentrations and rate onto the 20 m " 0.25 mm " 0.25 µm Restek CLP GC column, matching Mars-like MOMA pyrolysis and GC operation, to test GC-specific trapping and retention characteristics (Figure 2). The spectrum in the lower plot was found to closely match the fragmentation pattern known for hexane during its column elution. The limit of detection (LOD) requirement of 1 pmol s -1 is readily met and will be revisited for different compounds and with higher fidelity GC configurations. The flight Tapping Station, which seals pyrolysis ovens on the carousel, and the flight GC have been assembled (Figure 3) and are undergoing functional testing. A flight GCMS coupling is planned, but limited to the activities critical to verify MOMA/GCMS without significant contamination exposure. Additional end-to-end characterization will be conducted using the engineering units and, as in the case of the Sample Analysis at Mars (SAM) investigation, a testbed model assembled from spare flight components. The flight model has been tested in LDMS mode, initially with the engineering unit laser, verifying ion transmission, sensitivity, resolution, and accuracy requirements over the mass range up to 1000 Da. A calibration target of solid CsI will be delivered to Mars with MOMA and used to track and correct any drift of m/z position or mass bias of sensitivity. Only the most trace-level concentrations of selected organics are used, rarely, to perform critical verification of the MOMA limit of detection for nonvolatile compounds in the FM. Figure 4 demonstrates the flight MS readily produces an accurate spectrum of the aromatic Rhodamine 6G calibrant, present at 50 femtomoles per mm 2 , with a signal-to-noise ratio over 100 at the base peak of m/z 443. As such the observed performance in this test is more than 1000 times better than the overall LDMS requirement of 1 pmol mm -2 Additional tests of the FM with other selected gases and samples are underway to complete the verification of all operational parameters, including use of MS/MS and SWIFT protocols [8]. In parallel, the engineering unit LDMS is used to characterize and develop libraries of the response to a range of Mars-relevant mineral matrices [9]. Following a thorough cleaning and continued background and procedural blank tracking scans, the flight laser will be integrated to the flight ion trap MS, and the full system will undergo final Mars thermal vacuum testing prior to delivery to the rover.

Fig. 1

 1 Fig. 1 Flight linear ion trap MS assembly with laser in Mars environment simulation chamber to match mechanical, thermal, pressure, and contamination level that MOMA will experience on the ExoMars rover. Configuration supports either external commercial GC, via gas feedthroughs, or flight model GC within the existing mechanical structure.

Fig. 2

 2 Fig. 2 Flight MOMA GCMS total ion chromatogram (top) shows nominal detection of hydrocarbons in He. The hexane GC peak is sampled with multiple EI spectra such as shown (bottom) with its characteristic fragmentation pattern.

Fig. 3

 3 Fig.3 The flight models of the Tapping Station (left; to seal pyrolysis ovens) and the GC subsystem (right) have been assembled and are undergoing "ultra-clean" functional tests.

Fig. 4

 4 Fig.4 Flight mass spectrometer in laser desorption/ionization mode achieves ~femtomole per mm 2 limit of detection per spectrum for a nonvolatile aromatic organic compound.


Rover W Goetz
email: goetz@mps.mpg.de

F Goesmann

W B Brinckerhoff

F Raulin

C Szopa

C Freissinet

A Buch

S Siljeström

J R Brucato

R M Danell

S A Getty

H Mißbach

H Steininger

A Grubisic

V T Pinnick

F Stalport

M D Schulte

D P Glavin

X Li

F H W Van Amerom

J L Vago

THE CHALLENGE TO SEARCH FOR ORGANICS AND BIOSIGNATURES ON MARS BY THE EXOMARS-2020

Introduction:

The ESA/Roskosmos Exomars-2020 rover shall be launched on July 24, 2020 and land in Western Arabia Terra [1], Mars, in April 2021. The goals of the mission, in order of priority, are: (i) to search for signs of past and present life on Mars, and (ii) to characterize the water/geochemical environment as a function of depth in the shallow subsurface (ESA, 2016: http://exploration.esa.int/jump.cfm?oid=45082). These goals shall be accomplished over 7-8 months of mission operations and within less than 5 km from the rover's landing site by the following set of tools & instruments: (1) remote sensing instruments to characterize surface (NIR reflectance spectrometer & cameras) and subsurface (radar & neutron spectrometer) nearby the rover, (2) a drill to acquire samples down to 2 m below the surface, and (3) analytical instruments (MicrOmega, a microscopic NIR reflectance spectrometer, RLS [Raman Laser Spectrometer] & MOMA [Mars Organic Molecule Analyzer]) to characterize crushed drill cores [1]. This abstract demonstrates how these instruments are going to be used, which science questions will be addressed, and which questions are beyond the scope of this mission.

Analytical instruments onboard ExoMars-2020: Figure 1 shows the spatial arrangement of the analytical instruments (MicrOmega, RLS, MOMA) in the body of the rover. A subsurface drill core is deposited in the sample port, then crushed and eventually delivered to the refillable sample container that is mounted on the sample carousel. Then it can be investigated by the analytical instruments in this order: (i) MicrOmega [2] to detect mafic and alteration phases (as well as some specific organic molecules) at high resolution 20 μm/px within a Field of View 5 x 5 mm 2 , (ii) RLS [3] to identify mineral phases and organic compounds within an area 50 μm across, and (iii) MOMA [4,5] to detect and identify volatile and refractory organics within an area ~300 μm across. Organic blanks (Figure 1) will define the instruments' background (terrestrial organic contaminants). All parts of the rover that are in contact with Martian samples (sample path, refillable container, ovens etc.) are sterile (as per Category-4b Planetary Protection rules [1]) as a result of the above stated mission goals. organic analysis technique on landed missions so far, leads to (partial) combustion of organic compounds due to oxygen release by heated perchlorate (up to 1 wt% in surface rocks & soils). (iii) Pyrolysis does not allow for efficient extraction of organic compounds from their host minerals (e.g. clays). ExoMars-2020 addresses these problems. Concerning (i): Samples can be acquired down to a depth of 2 meters which is the typical spatial scale for transitioning from radiationbathed to radiation-free subsurface. Concerning (ii-iii): MOMA offers next to pyrolysis several other experimental techniques (Figure 2) that are insensitive (or much less sensitive) to the presence of perchlorates: (a) LDI (Laser Desorption & Ionization) and (b) several forms of low-T chemical processing of Martian samples (addition of a derivatization agent) that increases the volatility of certain key organic compounds such that they become detectable by Gas Chromatography-Mass Spectrometry (GC-MS). Indeed, LDI has been shown to be insensitive to perchlorates [10]. The same applies to the derivatization techniques, either because they occur at temperatures below the decomposition of perchlorates (such as DMF-DMA, Figure 2) or the derivatization agent protects organics by scavenging short-lived oxidizing radicals that are released during thermal decomposition of perchlorates.

MOMA experiments and their interpretation: MOMA is a highly versatile instrument that can be run in many different ways, some of them presented in Figure 3. Interpretation of mass spectra may be straightforward, but interpretation of MOMA data on a high level is challenging. MOMA will search primarily for signs of ancient life (as preserved in sediments) [4,5]. In some cases MOMA may be able to distinguish between current and extinct life, although this capability is beyond the instrument requirements.

Assuming positive detection of organic material MOMA's most important goals will be [4,5]: (1) to assess if the abundances of macromolecular carbon, poly-aromatic hydrocarbons (PAH) & kerogen (as potentially detected by LDI) vary differently on a given spatial scale than those in meteorites, and (2) to assess if potential organic compounds (detected by any MOMA operational mode, Figure 2) are biogenic or abiogenic. Strong chemical biosignatures include (a) an even-to-odd bias in fatty-acids carbon-chain lengths within a fairly limited range of molecular weights, (b) an enantiomeric excess of chiral molecules (such as amino acids or sugars), and (c) the detection of specific high-molecular-weight organic compounds, e.g. terpenoids (including hopanes) or peptides [4]. Several (or even all) of these biosignatures and a favorable geologic context will be required to make the claim of signs of ancient life in Martian sediments.

Figure 1

 1 Figure 1 ExoMars-2020 rover with sample port and analytical instruments in the rover body. Modified from [1].

Figure 2

 2 Figure 2 Overview of MOMA operational modes. Left part: LD-MS mode (grain surface analysis), right part: GC-MS modes (grain bulk analysis), three of them involving a derivatization agent (DMF-DMA or MTBSTFA or TMAH; N,N-dimethylformamide-dimethyl acetal, N,N-methyltertbutyl-dimethylsilyltrifluoroacetamide, tetramethylammonium hydroxide). "Mars samples" (red): aliquots of crushed drill core. Blue boxes: potential organic target compounds.

Figure 3

 3 Figure 3 MOMA decision tree. Modified from [4]. Black-filled rectangles refer to MOMA techniques (Figure 2). Red framed boxes mark the end of an analytical thread.