Exobiology in the Solar System & The Search for Life on Mars - ESA

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SP-1231 86 Table II.2.3/1. Bulk Composition of Mars Derived from SNC Meteorites. (Dreibus & Wänke, 1987). Mantle/Crust % ppm MgO 30.2 K 305 Al 2 O 3 3.02 Rb 1.06 SiO 2 44.4 Cs 0.07 CaO 2.45 F 32 TiO 2 0.14 Cl 38 FeO 17.9 Co 68 Na 2 O 0.5 Ni 400 P 2 O 5 0.16 Cu 5.5 Cr 2 O 3 0.76 Zn 62 MnO 0.46 Ga 6.6 ppb Br 145 I 32 Mo 118 In 14 La 480 Tl 3.6 W 105 Th 56 U 16 Core (21.7% of Mars mass) % Fe 77.8 Ni 7.6 Co 0.36 S 14.24 H 2O added dur<strong>in</strong>g accretion = 3.4%; H 2O reta<strong>in</strong>ed <strong>in</strong> <strong>the</strong> mantle = 36 ppm, equivalent to a surface layer 130 m deep (100% degass<strong>in</strong>g). Fig. II.2.3/2. Estimated elemental abundances <strong>in</strong> <strong>the</strong> Martian mantle as derived from SNC meteorites. <strong>The</strong> higher abundances of moderately volatile (Na, Ga, P, K, F, Rb, Zn) and volatile elements (Cl, Br) compared to those <strong>in</strong> <strong>the</strong> terrestrial mantle is obvious. Note <strong>the</strong> depletion of all chalcophile elements (In, Co, Cu, Ni) <strong>in</strong> <strong>the</strong> martian mantle. Note especially <strong>the</strong> high mantle abundance of P on Mars and its depletion <strong>in</strong> <strong>the</strong> Earth’s mantle. <strong>The</strong> similar abundances of several geochemically (W to Rb <strong>in</strong> Fig. II.2.3/2) very different elements <strong>in</strong> <strong>the</strong> mantle of Mars is fur<strong>the</strong>r strong evidence <strong>for</strong> <strong>the</strong> <strong>for</strong>mation of <strong>the</strong> <strong>in</strong>ner planets from two compositionally different components (R<strong>in</strong>gwood, 1977; 1979; Wänke, 1981; Dreibus & Wänke, 1984). <strong>The</strong>se components are: Component A: Highly reduced and free of all elements with equal or higher volatility than Na, but conta<strong>in</strong><strong>in</strong>g all o<strong>the</strong>r elements <strong>in</strong> C1 abundance ratios. Fe and all siderophile elements are metallic, and even Si might be partly present <strong>in</strong> metallic <strong>for</strong>m. Component B: Oxidised and conta<strong>in</strong><strong>in</strong>g all elements (<strong>in</strong>clud<strong>in</strong>g <strong>the</strong> volatiles) <strong>in</strong> C1 abundances. Fe and all siderophile (Co, Ni, Cu, Ga, W, etc) and lithophile elements are present ma<strong>in</strong>ly as oxides. As seen from Table II.2.3/1 and Fig. II.2.3/2, <strong>the</strong> SNC meteorites <strong>in</strong>dicate abundances of moderately volatile (Na, K, Rb, Zn) and volatile elements (Cl, Br, I) <strong>in</strong> <strong>the</strong> martian mantle <strong>in</strong> excess of those <strong>in</strong> <strong>the</strong> terrestrial mantle. A mix<strong>in</strong>g ratio of components A:B of 60:40 is obta<strong>in</strong>ed <strong>for</strong> Mars, compared to 85:15 <strong>for</strong> Earth. <strong>The</strong>re are, however, a number of elements supposedly derived from component B, which <strong>in</strong> <strong>the</strong> Earth’s mantle have abundances similar to those of Fe, Na, Ga, K, F and Rb, but <strong>in</strong> <strong>the</strong> martian mantle have considerably lower abundances (Co, Ni, Cu, In). <strong>The</strong>se elements all have strong chalcophile character. <strong>The</strong>ir low abundances are taken to <strong>in</strong>dicate a homogeneous accretion of Mars. Hence, contrary to an <strong>in</strong>homogeneous accretion of <strong>the</strong> Earth, as frequently favoured, on Mars <strong>the</strong> two components had <strong>the</strong> chance to equilibrate and were probably supplied almost simultaneously to <strong>the</strong> grow<strong>in</strong>g planet. <strong>The</strong> high abundance of component B, which supplied large amounts of sulphur, was obviously responsible <strong>for</strong> FeS becom<strong>in</strong>g a major phase and at its segregation extracted all chalcophile elements accord<strong>in</strong>g to <strong>the</strong>ir sulphide-silicate partition coefficients. <strong>The</strong> sulphide-silicate equilibrium <strong>in</strong> <strong>the</strong> martian mantle <strong>in</strong>dicates its saturation with FeS. <strong>The</strong> mantle’s FeO content is about a factor of 2 higher than that of <strong>the</strong> terrestrial mantle. As a consequence, <strong>the</strong> sulphur abundance <strong>in</strong> <strong>the</strong> martian mantle is expected to be substantially above <strong>the</strong> S abundance <strong>in</strong> <strong>the</strong> Earth’s mantle, because <strong>the</strong> solubility of FeS <strong>in</strong> silicates <strong>in</strong>creases with <strong>the</strong> FeO content. Hence, <strong>the</strong> observed high concentrations of sulphur <strong>in</strong> mantle-derived magmas as represented by <strong>the</strong> shergottites (sulphur content 600-2800 ppm) is not surpris<strong>in</strong>g. <strong>The</strong> water abundance given <strong>in</strong> Table II.2.3/1 is derived from element correlations observed <strong>in</strong> shergottites and cosmochemical constra<strong>in</strong>ts. Two quite different approaches yielded an almost identical value. It might, however, be an upper limit only <strong>for</strong> <strong>the</strong> water <strong>in</strong> <strong>the</strong> martian mantle.
<strong>The</strong> depletion of chalcophile elements <strong>in</strong> <strong>the</strong> martian mantle has been used to <strong>in</strong>fer a homogeneous accretion scenario <strong>for</strong> Mars. In <strong>the</strong> same way as discussed above <strong>for</strong> sulphur, homogeneous accretion had <strong>the</strong> consequence that water added to <strong>the</strong> grow<strong>in</strong>g planet by component B reacted with <strong>the</strong> metallic Fe of component A, oxidis<strong>in</strong>g it to FeO, while <strong>the</strong> generated hydrogen escaped. Hence, we should expect a very dry Martian mantle, because water, although added <strong>in</strong> large quantities to <strong>the</strong> planet dur<strong>in</strong>g accretion, was reduced to H 2 except <strong>for</strong> trace amounts. Two of <strong>the</strong> seven known shergottites, Shergotty and Zagami, have compositions very similar to that of <strong>the</strong> Vik<strong>in</strong>g soil (Table II.2.4/1). <strong>The</strong> o<strong>the</strong>r SNC meteorites differ considerably <strong>in</strong> <strong>the</strong>ir chemical compositions. <strong>The</strong> large concentrations of sulphur (3.5%) and chlor<strong>in</strong>e (0.8%) <strong>in</strong> <strong>the</strong> Vik<strong>in</strong>g soil do not seem to be noticeably accompanied by respective cations. <strong>The</strong> most likely cations <strong>for</strong> <strong>the</strong> sulphates, Mg and Ca, have even higher concentrations <strong>in</strong> Shergotty and Zagami than <strong>in</strong> <strong>the</strong> Vik<strong>in</strong>g soil. This suggests direct <strong>in</strong>troduction of SO 2 or H 2S and probably also HCl to <strong>the</strong> martian regolith via gas-solid reactions. With respect to <strong>the</strong> contradictory evidence <strong>for</strong> a dry martian mantle, supported by <strong>the</strong> low water content of SNC meteorites but opposed by <strong>the</strong> martian erosional surface features (which seem to require large amounts of water), <strong>the</strong> measurements of <strong>the</strong> oxygen isotopes (Karlsson et al., 1991) and <strong>the</strong> H/D ratio (Watson et al., 1991) of water extracted from SNC meteorites is of great <strong>in</strong>terest. It was observed that, <strong>for</strong> all <strong>the</strong> SNC meteorites analysed and apart from <strong>the</strong> presence of terrestrial contam<strong>in</strong>ation, a large fraction of <strong>the</strong> water is not derived from <strong>the</strong> martian mantle, but obviously represents martian surface water: <strong>the</strong> oxygen isotope composition is up to three times fur<strong>the</strong>r away from <strong>the</strong> terrestrial isotope fractionation l<strong>in</strong>e than <strong>the</strong> oxygen <strong>in</strong> <strong>the</strong> silicates of SNC meteorites. At <strong>the</strong> high temperatures of magma generation <strong>in</strong> <strong>the</strong> martian mantle, isotopic equilibration between oxygen of <strong>the</strong> silicates and of water would certa<strong>in</strong>ly have been established. Hence, only a fraction of <strong>the</strong> water found <strong>in</strong> SNC meteorites can be mantle-derived and <strong>the</strong> o<strong>the</strong>r non-terrestrial part must come from <strong>the</strong> martian surface. <strong>The</strong> oxygen isotopes of <strong>the</strong> surface component might have been created by non-l<strong>in</strong>ear isotope fractionation by non-<strong>the</strong>rmal escape of oxygen to space (see Section II.3). <strong>The</strong> contradictory evidence of a dry martian mantle (Table II.2.3/1) and <strong>the</strong> geo- <strong>the</strong> planet mars/II.2 II.2.4 <strong>The</strong> Geochemistry of <strong>the</strong> Martian Surface Layers Fig. II.2.4/1. A 70 km-wide view of layered deposites <strong>in</strong> <strong>the</strong> north polar cap region. Below <strong>the</strong> ice cap <strong>the</strong>re are successive layers of dust and ice, each about 50 m thick. <strong>The</strong> layer<strong>in</strong>g may be due to period variations <strong>in</strong> <strong>the</strong> orbit of Mars caus<strong>in</strong>g climatic changes. Erosion of <strong>the</strong> 500 m-high cliffs, possibly by w<strong>in</strong>ds, produced <strong>the</strong> dark ripple-textured areas at <strong>the</strong> base. Table II.2.4/1. Comparison of <strong>the</strong> % Compositions of <strong>the</strong> Shergotty Meteorite (Dreibus et al., 1982) and Martian Soil (Clark et al., 1976). Shergotty Mars Soil SiO 2 51.4 43.0 FeO 19.4 16.2 CaO 10.0 5.8 MgO 9.28 6.0 Al 2O 37.06 7.2 TiO 2 0.87 0.6 Na 2O 1.29 n.d. P 2O 5 0.80 n.d. S 0.13 3.5 Cl 0.01
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SP-1231 Exobiology
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Cover Fossil coccoid bacteria, 1 µ
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4 I.5 Potential Non-Martian Sites <
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6 6.2 Imaging of F
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The Exobio
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pyrolysis, or similar techniques, f
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Ignasi Casanova, Institute
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I AN EXOBIOLOGICAL VIEW OF THE SOLA
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SP-1231 18 surface pressure of CO 2
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SP-1231 20 10 bar befor</st
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SP-1231 22 kilometres in</s
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SP-1231 24 Laboratory Investigation
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I.3 Limits of Life
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temperatures of 95-106ºC. This sug
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(mainly found <str
Page 31 and 32: cannot exclude fin
Page 33 and 34: Responses to Cosmic Radiation <stro
Page 35 and 36: acid to identify the</stron
Page 37 and 38: Horneck, G. (1993). Responses of Ba
Page 39 and 40: SP-1231 Fig. I.4.2.2/1. Size distri
Page 41 and 42: SP-1231 Fig. I.4.2.2/4. Evaporite w
Page 43 and 44: SP-1231 Fig. I.4.2.3/1A (left). Net
Page 45 and 46: SP-1231 48 A detailed chemical anal
Page 47 and 48: SP-1231 Fig. I.4.3.1.1/1. Scheme of
Page 49 and 50: SP-1231 Fig. I.4.3.1.2/1. A: Compar
Page 51 and 52: SP-1231 54 I.4.3.2.1 Sedimentary Or
Page 53 and 54: SP-1231 56 The iso
Page 55 and 56: SP-1231 Fig. I.4.3.2.3/1. Cholester
Page 57 and 58: SP-1231 60 References regard to non
Page 59 and 60: SP-1231 62 Klein,
Page 61 and 62: SP-1231 64 ities in</strong
Page 63 and 64: SP-1231 Fig. I.5.1.1/1. A 34×42 km
Page 65 and 66: SP-1231 Fig. I.5.2.1/1. Titan’s a
Page 67 and 68: SP-1231 Fig. I.5.2.2/2. Modelled Ti
Page 69 and 70: SP-1231 72 Galileo: images availabl
Page 71 and 72: SP-1231 74 TABLE I.6.2/1 EVIDENCE O
Page 73 and 74: SP-1231 I.6.3 What to Searc
Page 75 and 76: SP-1231 78 I.7.3 Organic Chemistry
Page 77 and 78: II.1 Introduction The</stro
Page 79 and 80: II.2 The Planet Ma
Page 81: cover the range 4.
Page 85 and 86: valley networks (Fig. II.2.5/2) are
Page 87 and 88: plate tectonic evidence has been ob
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Page 91 and 92: II.3 The Martian M
Page 93 and 94: principal atmosphe
Page 95 and 96: Lines of evidence
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Page 101 and 102: Anders, E. (1989). Prebiotic organi
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Page 113 and 114: SP-1231 118 orbital characteristics
Page 115 and 116: SP-1231 120 II.4.5 The</str
Page 117 and 118: SP-1231 122 II.4.6 Rovers and Drill
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Page 123 and 124: SP-1231 128 Sharp, M., Parkes, J.,
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SP-1231 138 optical microscope. Bot
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SP-1231 140 protons of discrete ene
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SP-1231 142 atures should be per<st
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SP-1231 Fig. II.5.7.7/1. Example of
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SP-1231 146 soils. In this approach
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SP-1231 148 discrimin</stro
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SP-1231 150 determin</stron
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SP-1231 Fig. II.5.10.2/2. Enantiome
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SP-1231 154 References achiral colu
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II.6 Team III: The
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in width and heigh
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Searchin</
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minerals or oxidat
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A second group of rocks, however, m
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SCIENTIFIC METHOD AND REQUIREMENTS
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The sample materia
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surprising. To ach
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The main</
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Consideration of a diamond wire-saw
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Schoonen, M.A. & Barnes, H.L. (1991
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SP-1231 180 II.7.2 The</str
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SP-1231 Fig. II.7.4/1. Look
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SP-1231 184 II.7.5 A Possible <stro
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Team IV was asked to examin
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