Reanalysis of the Viking results suggests perchlorate and organics at mid-latitudes on Mars

Details Reanalysis of the Viking results suggests perchlorate and organics at mid-latitudes on Mars Reanalysis of the Viking results suggests perchlorate and organics at mid-latitudes on Mars

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p12a..04n&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P12A-04

Biology

[0406] Biogeosciences / Astrobiology And Extraterrestrial Materials, [0424] Biogeosciences / Biosignatures And Proxies, [6225] Planetary Sciences: Solar System Objects / Mars, [6297] Planetary Sciences: Solar System Objects / Instruments And Techniques

Scientific paper

The most comprehensive search for organics in the Martian soil was performed by the Viking Landers. Martian soil was subjected to a thermal volatilization process in order to vaporize and break organic molecules, and the resultant gases and volatiles were analyzed by gas chromatography-mass spectrometry. Only water at 0.1-1.0 wt% was detected with traces of chloromethane at 15 ppb in the Viking Landing site 1, and water at 0.05-1.0 wt% and carbon dioxide at 50-700 ppm with traces of dichloromethane at 0.04-40 ppb in the Viking Landing site 2. The abundance ratio of the 35Cl and 37Cl isotopes in these chlorohydrocarbons was 3:1, corresponding to the terrestrial isotopic abundance. Therefore, these chlorohydrocarbons were considered to be terrestrial contaminants although they had not been detected at those levels in the blank runs. Recently, perchlorate was discovered in the Martian Arctic soil by the Phoenix Lander. Here we show that when Mars-like soils from the Atacama Desert with 32±6 ppm of organic carbon are mixed with 1 wt% magnesium perchlorate and heated nearly all the organics present are decomposed to water and carbon dioxide, but a small amount are chlorinated forming 1.6 ppm of chloromethane and 0.02 ppm of dichloromethane at 500○C. A chemical kinetics model was developed to predict the degree of oxidation and chlorination of organics in the Viking oven. The isotopic distribution of 35Cl and 37Cl for Mars is not known. Studies on Earth indicate that there is no isotopic fractionation of chlorine in the mantle or crust, despite the fact that it is significantly depleted on the planet as compare to solar abundances. The 37Cl/35Cl isotopic ratio in carbonaceous chondrites is similar to the Earth’s value, which suggests that the terrestrial planets, including Mars, were all formed from a similar reservoir of chlorine species in the presolar nebulae and that there was no further isotopic fractionation during the Earth’s differentiation or late accretion of volatiles. Consequently, 37Cl/35Cl ratio should be the same on Mars as well as on the Earth. Re-interpretation of the Viking results therefore suggests ≤0.1% perchlorate and 1.5-6.5 ppm organic carbon at the landing site 1, and ≤0.1% perchlorate and 0.7-2.6 ppm organic carbon at the landing site 2. The detection of organics on Mars is important to assess locations for future experiments to detect life itself. We suggest that future missions to Mars should include life detection experiments.

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