Engineering

Stromatolites are one of the most important forms of fossil evidence for microbial life on early Earth (Schopf et al., 1971). They are formed when layers of microbial organisms at the shallow bottom of a lake or tide pool are periodically covered with sediment or precipitating salts (e.g. carbonate). The photosynthetic organisms that form the basis of the community must migrate through sediment toward the light in order to survive. If life emerged on Mars, it is possible that stromatolites were formed in lakes and marine lagoons. Recently the Mars Reconnaissance Orbiter mapping found a regional rock layer with near-infrared spectral characteristics that are consistent with the presence of magnesium carbonate in the Nili Fossae region (Ehlmann et al., 2008). The Nili Fossae is a fracture in the surface of Mars that has been eroded and partly filled in by sediments and clay-rich ejecta from a nearby crater. It is located at 22° N, 75° E and has an elevation of 0.6 km. The carbonate-bearing rocks outcrops in the Nili Fossae region could have formed in (1) the subsurface by groundwater percolating through fractures in the ultramafic rock and altering olivine or (2) in shallow lakes from waters enriched in Mg2+ relative to other cations by percolation through ultramafic olivine-bearing rocks. In the latter scenario, it is possible that these carbonate outcrops could have been deposited in association with microbial activity. The purpose of this work is to chemically characterize a modern stromatolite by thermal volatization (TV), a method that has been widely used in past missions (Viking and Phoenix) and will also be used in future missions (Mars Science Laboratory and ExoMars) in the search for life on Mars. Alchichica is a volcanic crater lake situated in an enclosed basin within the El Seco Valley at 19° 24' 13" N, 97° 24' 0" W, and 2.345 km above sea level in Mexico. It has an area of 1.81 km2 , a mean depth of 38.5 m and a maximum depth of 64 m. The lake is hyposaline (8.5-10 g l-1 ) and alkaline (pH 8.9-9.1 and alkalinity of 37 meq l-1 ) with sodium and chloride being the dominant ions but also with bicarbonate and carbonate ions. These conditions are favorable for active carbonate deposition that results in the formation of distinctive stromatolite structures in the littoral region of the lake. These structures generate extended submerged carbonaceous platforms down to a depth of 15 m that slope steeply to the bottom of the lake where soft sediments prevail (Escobar-Briones et al., 1998). The lake is populated by 18 species of cyanobacteria (Tavera and Komárek, 1996) which are the primary producers in the food web a of the lake fixing not only carbon (Escobar-Briones et al., 1998) but also nitrogen (Falcón eto al, 2002). The stromatolite samples were freeze-dried and then were finely grounded with an agate mor-tar mill for analyses. The mineral composition of the sample is essentially hydromagnesite (Mg5 (CO3 )4 (OH)2 , 4 H2 O)) with traces of quartz (SiO2 ) as determined by X-rays diffraction. This is also consistent with differential thermal analysis and thermal gravimetry of the sample. Analysis of the sample by TV-mass spectrometry (MS) using the Phoenix Lander Protocol (Navarro-González et al., 2009) indicates that the major volatile released is carbon dioxide a from 350 C to 1000° C with broad peaks centered at 400° C and 600° C. Organics are also re-° leased from 380° C to 750° C as monitored by a characteristic organic fragment with m/z=41 and that is attributed to the allyl cation: [CH2 CHCH2 ]+ . Analysis of the sample by TV-gas chromatography (GC)-MS using the Viking Lander Protocol but at 750° C (Navarro-González a et al., 2009) demonstrates that a complex suite of organics are released such as 1,3-butadiene, furan, propanal, 2-propenenitrile, 1,3 cyclopentadiene, 2-methyl-1,3-butadiene, propanenitrile, methylfuran, butanal, benzene, 1-methyl-cyclohexene, 1-H-pyrrole, pyridine, methylbenzene, 2,5-dimethylfuran, 4-methylpyridine, ethylbenzene, dimethylbenzene, styrene, methylbenze-neamine, 2,4-hexadienal, phenol, 3-methyl-2-cyclopenten-1-one, benzonitrile, cyclopropyliden-emethylbenzene, 2,3-dihydro-1-H-indene, 1-methylethenylbenzene, 3-methylphenol, and 1-propenyl-benzene. The carbon isotopic composition of the organic fraction of the stromatolites is deter-mined by a δ 13 C=-24.28 whereas the inorganic fraction, hydromagnesite, is determined by a δ 13 C=5.86. Our data shows two interesting chemical signatures that can used to detect extant life in Martian carbonates: (1) organics are not oxidized by mineral matrix during TV-MS and TV-GC-MS analysis; and (2) distinctive carbon isotopic fractionation occurs in the or-ganic and inorganic fraction of stromatolites. Future work should center on studying fossilized stromatolites to determine if these chemical signatures are lost with time. References: Ehlmann, B.L., Mustard, J.F., Murchie, S.L. Poulet, F., Bishop, J.L., Brown, A.J., Calvin, W.M., Clark, R.N., Des Marais, D.J., Milliken, R.E., Roach, L.H., Roush, T.L., Swayze, G.A., and Wray, J.J.: 2008, Orbital identification of carbonate-bearing rocks on Mars. Science 322, 1828-1832. Escobar-Briones, E., Alcocer, J., Cienfuegos, E., and Morales P.: 1998, Carbon stable isotopes ratios of pelagic and litoral communities in Alchichica crater-lake, Mexico, Internat J Salt Lake Res 7, 345-355. Falcón, L.I., Escobar-Briones, E., and Romero, D.: 2002, Nitrogen fixation patterns displayed by cyanobacterial consortia in Alchichica crater-lake, Mexico, Hydrobiol 467, 71-78. Navarro-González, R., Navarro, K.F., de la Rosa, J., Molina, P., Iñiguez, E., Miranda, L.D., a n Morales, P., Cienfuegos, E., Coll, P., Raulin, F., Amils, R. and McKay, C.P.: 2006. The limitations on organic detection in Mars-like soils by thermal volatilization-gas chromatography-MS and their implications for the Viking results. Proc Natl Acad Sci USA 103, 16089-16094. Navarro-González, R., Iñiguez, E., de la Rosa, J. and McKay, C.P.: 2009, Characterization of a n organics, microorganisms, desert soils and Mars-like soils by thermal volatilization coupled to mass spectrometry and their implications for the search of organics on Mars by Phoenix and future space missions. Astrobiology 9, 703-715, doi: 10.1089/ast.2008.0284. Schopf, J.W., Oehler, D.Z. Horodyski, R.J. and Kvenvolden K.A.:1971, Biogenicity and signif-icance of the oldest known stromatolites. J Paleontol 45, 477-485. Tavera, R., and Komárek J.: 1996, Cyanoprokaryotes in the volcanic lake of alchichica, Puebla a State, Mexico, Algological Studies 83, 511-538.