Perchlorates on Mars: Implications for the Detection of Organics on Mars

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1 Proceedings of The National Conference On Undergraduate Research (NCUR) 2012 Weber State University, Ogden Utah March 29 31, 2012 Perchlorates on Mars: Implications for the Detection of Organics on Mars Daniel J. Pacheco Department of Chemistry San Jose State University One Washington Square. San Jose, California USA Faculty Advisors: Richard C. Quinn (NASA Ames Research Center/SETI), Cynthia Phillips (SETI Institute), Monika Kress (SJSU/SETI) Abstract In 1976, the Viking Mars Landers analyzed surface samples for the presence of organic compounds using gas chromatography-mass spectroscopy (GC-MS). On Mars, trace levels of organics, including chloromethane and dichloromethane, where detected. However, these species were attributed to preflight system contamination (Biemann et al. 1976). In 2008, perchlorate was detected in Martian soil using the Thermal and Evolved-Gas Analyzer and Wet Chemical Laboratory on the NASA Mars Phoenix Lander (Hecht et al. 2009). This has prompted the reinvestigation of the source of the halogenated hydrocarbons detected by the Viking GC-MS. Recent work has concluded that the origin of the chloromethanes in the Viking GC-MS results was the reaction of perchlorates and Martian soil organics during sample heating in the GC-MS analyses (Navarro-Gonzales et al. 2010). This has prompted the investigation of alternative mechanisms for the formation of chloromethanes in the Viking GC-MS experiments that do not involve the presence of soil organics including, the reaction of perchlorates with terrestrial organic contamination and Fischer-Tropsch type synthesis. By conducting simulations identical to the GC-MS experiments performed on the Viking Landers, we seek to resolve the debate on whether or not Viking soils contained detectable levels of organic compounds. Keywords: Mars, Astrochemistry, GC-MS 1. Introduction In 1976, the Viking landers arrived on Mars. One of the objectives of the Viking mission was to perform molecular analysis of the Mars surface composition. Through the process of thermal volatilization, organic compounds could be separated by gas chromatography (GC) and could be detected via mass spectroscopy (MS). In the Viking GC, molecules having lower boiling points can be eluted out of the GC column at a faster rate, while molecules with higher boiling points are retained and eluted at later times. A mass spectrometer was then used to measure the mass to charge ratio of ionized fragments after they eluted from the GC column. Based on the formation of specific fragmentation patterns, it is possible to identify molecules 1. During the voyage to Mars, a blank experiment was carried out by heating an empty oven to 500 C. Low levels of contaminants were detected in the GC-MS system 1. During sample analysis on Mars, the Viking Lander 1 GC-MS detected the presence of chloromethane 1 while the Viking Lander 2 GC-MS detected dichloromethane. The origin of the chlorinated hydrocarbon is debated. In 2010, Navarro-Gonzalez et al. reported that the origin of chloromethane and dichloromethane resulted from the combustion of perchlorate and soil organics 2. Biemann et al. concluded that the chlorinated compounds were not from perchlorate or soil organics, but were due to system contamination 1.

2 In May 2008, the Phoenix spacecraft landed on the Martian surface. The objective of the Phoenix mission was to assess the habitability, and to characterize the Martian soil. Phoenix carried multiple soil analysis instruments including the Wet Chemistry Laboratory (WCL). The WCL, which was a part of the Microscopy Electrochemistry and Conductivity Analyzer (MECA), consisted of four cells used to analyze the Martian soil. MECA detected the presence of perchlorate in the Martian soil. Another Phoenix instrument used to analyze the Martian soil was the Thermal and Evolved Gas Analyzer (TEGA). TEGA provided supporting evidence for perchlorate on Mars, through the detection of oxygen (ionized mass fragment of mass 32) during the thermal volatilization of soil samples 4,5. In addition to perchlorate, the presence of carbonates in the soil in amounts of 3-5% by weight was detected by the TEGA and WCL. Samples heated in the TEGA instrument exhibited an endothermic peak between 725 C and 820 C, which is consistent with the presence of calcium carbonate 5. This investigation focuses on potential mechanisms for the production of chloromethane and dichloromethane in the Viking GC-MS experiment. Two possible methane and chloromethane formation mechanisms were examined: 1) the combustion of acetone contamination in the presence of soil perchlorate and 2) the combustion of soil carbonates in the presence of soil catalyst. 2. Methodology 2.1 Materials instrumentation A custom built GC-MS system was used to replicate the performance of the Viking GC-MS 6. Thermally evolved gases in hydrogen carrier gas were separated with a poly-metaphenoxylene coated 2,6-diphenyl-para-phenylene oxide (Tenax, 2m, 60/80 mesh) GC column 7. Prior to sample product elution into a quadrupole MS (Stanford Research Systems), the hydrogen carrier gas was removed using a palladium/silver H 2 separation tube 8. A Pyroprobe 2000 with a 1500 GC interface (CDS Analytical Inc.) was used to heat samples held in a 50µL quartz tube and to introduce the evolved volatiles into to the GC chemicals Magnesium perchlorate, acetone, calcium nitrate and nickel nitrate from Sigma Aldrich were used as received. Nickel Calcium Carbonate, was synthesized from an aqueous solution 0.5 M in nickel (II) nitrate hexahydrate and calcium nitrate tetrahydrate. The solution was then added dropwise to a 1M solution of sodium bicarbonate at 80ºC while stirring at a constant rate and bubbling in CO 2 continuously. The precipitate was then collected via vacuum filtration and dried Methods Nickel-calcium carbonate mixed with magnesium perchlorates (5:1 by wt.) and acetone samples were analyzed using the Viking GC-MS protocols. Samples were sequentially heated for 30 seconds at 200, 350, or 500 C. A H 2 carrier gas flow rate of 2mL/min was used with the GC column held at a temperature of 50 C for 10 minutes followed by a linear ramp to 200 C at a rate of 8.33 C/min. The thermal volatilization of carbonates and perchlorate (5:1 ratio) was carried out using the protocol. 3. Results The following data was collected using a GC-MS as described above. The spectrum displayed in Figure 1 and Figure 2 display background subtracted results from the combustion of perchlorate with acetone. Upon the combustion of these two reactants, methane (M=16) is produced, as indicated by the fragmentation pattern at 15,14,13,12. The peak at 16 was not observed after the background was subtracted, due to higher levels of water (=18, 17, 16) that were initially present. 871

3 Pyrolysis of Actone and Perchlorate Pressure (torr) 2.00E E E E E E E E E E Figure 1. Mass spectrum of the combustion of acetone and perchlorate. Based on fragmentation patterns, the peaks at =43 and =58 are associated with acetone. Peaks at =15 and =14 due to methane and are associated with the formation of methane during acetone decomposition 3.00E Pressure( torr) 2.50E E E E E Figure 2. Mass spectrum ( 47 to 60) of product from the combustion of acetone and perchlorate at 500 C. Based on fragmentation patterns, the peaks at =50 and =52 due to chloromethane. As seen in the spectrum above, the ratio of peak heights of =50 to =52 is ~3:1 872

4 1.40E E-08 Pressure(torr) 1.00E E E E E Figure 3. Mass spectrum of the combustion of carbonate and perchlorate. Based on fragmentation patterns, the peaks at =50 and =52 are attributed to chloromethane. The ratio of peak heights of =50 to =52 is ~3:1 and corresponds to the relative isotopic abundance of 35 Cl and 37 Cl 4. Conclusion During the combustion of acetone and perchlorate, Fragmentation patterns formed that allowed species to be identified. Peaks at =43 and =58 are consistent with the fragmentation pattern of acetone. Peaks at =15 and =14 are associated with the formation of methane. In addition chloromethane was observed when acetone and perchlorate were pyrolyzed and the combustion of carbonate with perchlorate has also resulted in the formation of chloromethane. This indicates that the reaction of acetone with perchlorate and the reaction between carbonates with perchlorate are consistent with the formation of chloromethane in the Viking GC-MS experiment. 5. Acknowledgments The author wishes to express his appreciation to Dr. Richard C. Quinn, Dr. Monika Kress, Dr. Cynthia B. Phillips, Dr. Olenka C. Hubickyj, and the Undergraduate Research at SETI Institute in Astrobiology (URSA) Program Funded by the NASA Astrobiology: Exobiology and Evolutionary Biology Program (award number NNX09AM93G) and the NASA EPOESS program. 6. References 1. K. Biemann, J. Oro, P. Toulmin III, L. E. Orgel, A. O. Nier, D. M. Anderson, P. G. Simmonds, D. Flory, A. V. Diaz, D. R. Rushneck, J. A. Biller, Search for Organic and Volatile Inorganic Compounds in Two Surface Samples from the Chryse Planitia Region of Mars, Journal of Geophysical Research 194 (1976): R. Navarro González, E. Vargas, J. Rosa, A. C. Raga, and C. P. McKay, Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars. Journal of Geophysical Research 115 (2010):

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