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

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morphological and biochemical signatures of extraterrestrial life: utility of terrestrial analogues/I.4 material sealed <strong>in</strong> a m<strong>in</strong>eral matrix and hosted by a carbonate-free lithology had obviously escaped high-temperature isotope exchange, <strong>the</strong>reby preserv<strong>in</strong>g its orig<strong>in</strong>al biogenic δ 13 C org spread with probably little alteration. I.4.3.2.3 Molecular Biomarkers (‘Chemical Fossils’) <strong>in</strong> Sediments As mentioned earlier, biological markers, or ‘biomarkers’, are terms used <strong>for</strong> <strong>the</strong> wide variety of organic compounds that are derived from liv<strong>in</strong>g organims and can be found <strong>in</strong> sediments. Such compounds have also been termed ‘chemical fossils’ but <strong>the</strong> most commonly-used term is that of biomarker (Peters & Moldowan, 1993; Engel & Macko, 1983; Killops & Killops, 1993). Those biomarkers show<strong>in</strong>g little or no change <strong>in</strong> structure from those of <strong>the</strong> orig<strong>in</strong>al biochemical compounds are easily recognised <strong>in</strong> extracts of sediments, but compounds with strongly diagenetically or catagenetically altered structures, where<strong>in</strong> <strong>the</strong> carbon skeletons differ from those of <strong>the</strong> orig<strong>in</strong>al biological compounds yet carry sufficient residual portions of <strong>the</strong> orig<strong>in</strong>al structures, may still be useful biomarkers. Biomarkers are extractable from sediments and fossils by organic solvents, <strong>the</strong> most studied be<strong>in</strong>g of <strong>the</strong> lipid type, although certa<strong>in</strong> pigments, e.g. metal porphyr<strong>in</strong>s, are also referred to as biomarkers. <strong>The</strong> key to <strong>the</strong> use of biomarkers is frequently <strong>the</strong>ir complexity of structure, s<strong>in</strong>ce this represents a major <strong>in</strong>heritance of orig<strong>in</strong>al biologically-derived <strong>in</strong><strong>for</strong>mation. In addition, <strong>the</strong> molecular distributions <strong>in</strong> terms of <strong>the</strong>ir relative amounts can be highly <strong>in</strong><strong>for</strong>mative, as <strong>the</strong> orig<strong>in</strong>al biosyn<strong>the</strong>sis imparts characteristic distributions of <strong>the</strong> different compounds. However, subsequent diagenetic alterations and m<strong>in</strong>eralisation modify those patterns. Hence, <strong>the</strong> biochemist generally prefers to work with unaltered, orig<strong>in</strong>al molecular biomarkers, because <strong>the</strong>se compounds carry <strong>the</strong> full orig<strong>in</strong>al biochemical <strong>in</strong><strong>for</strong>mation and exhibit an extremely high degree of order both <strong>in</strong> structure and <strong>in</strong> relative abundances. Molecular Characteristics of <strong>the</strong> Biosphere What is <strong>the</strong> recognisable record of <strong>the</strong> past biosphere <strong>in</strong> <strong>the</strong> surface of <strong>the</strong> Earth? In general terms, <strong>the</strong> biosyn<strong>the</strong>tic impr<strong>in</strong>t of a liv<strong>in</strong>g organism is seen <strong>in</strong> <strong>the</strong> carbon skeletons, <strong>the</strong> functionalities and <strong>the</strong> stereochemistries of <strong>the</strong> compounds it manufactures under enzymic, molecular mach<strong>in</strong>ery. <strong>The</strong> fundamental scheme of life’s universal, but extremely precise, molecular mach<strong>in</strong>ery is simply: DNA ====> RNA ====> Prote<strong>in</strong>s ====> Metabolites DNA and RNA consist of genetically determ<strong>in</strong>ed sequences of five different nucleotide bases, with <strong>the</strong> DNA sequence determ<strong>in</strong><strong>in</strong>g <strong>the</strong> RNA sequence. It, <strong>in</strong> turn, controls <strong>the</strong> syn<strong>the</strong>sis of prote<strong>in</strong>s from some 20 homochiral alpha am<strong>in</strong>o acids. <strong>The</strong> prote<strong>in</strong>s consist of short (10-10 3 am<strong>in</strong>o acids) polypeptide cha<strong>in</strong>s and normally generate <strong>the</strong>ir own specific <strong>in</strong>tramolecular crossl<strong>in</strong>ks, <strong>the</strong>reby def<strong>in</strong><strong>in</strong>g <strong>the</strong> precise shape and location of reactive functionalities required <strong>for</strong> enzymatic activity. Prote<strong>in</strong>s <strong>in</strong> turn catalyse <strong>the</strong> syn<strong>the</strong>sis of specific metabolites and homologous metabolic series. Enzymatic syn<strong>the</strong>sis (metabolism) and breakdown (catabolism) of molecules provides <strong>for</strong> all <strong>the</strong> needs of <strong>the</strong> organisms. <strong>The</strong> metabolites comprise a large range of molecules that <strong>in</strong>clude, <strong>in</strong>ter alia, <strong>the</strong> build<strong>in</strong>g blocks to syn<strong>the</strong>size a variety of cellular biochemicals, such as lipids, polysaccharides, and co-enzymes. Chiral centres such as those at a s<strong>in</strong>glem tetrahedrally substituted carbon atom are almost always syn<strong>the</strong>sised with a s<strong>in</strong>gle chirality. Very unusually, an organism may make both, while, <strong>in</strong> o<strong>the</strong>r rare <strong>in</strong>stances, one enantiometer may be made by one species and <strong>the</strong> o<strong>the</strong>r by a different species. Cholesterol, a sterol widely distributed <strong>in</strong> eukaryotes, is an example of a highly ordered carbon skeleton <strong>in</strong> which <strong>the</strong> overall shape is dependent upon <strong>the</strong> stereochemistry of <strong>the</strong> r<strong>in</strong>g fusions and of <strong>the</strong> methyl and hydroxyl substituents of <strong>the</strong> r<strong>in</strong>gs. It has eight chiral centres, as <strong>in</strong>dicated <strong>in</strong> Fig. I.4.3.2.3/1. <strong>The</strong> structure is highly specific, both <strong>in</strong> <strong>the</strong> gross skeleton and <strong>in</strong> <strong>the</strong> stereochemistry, so that it makes an excellent biomarker, one which has been traced back hundreds of millions of years <strong>in</strong> immature sediments. It is also a carrier of 57
SP-1231 Fig. I.4.3.2.3/1. Cholesterol as an example of a biomarker molecule. (A) is a conventional l<strong>in</strong>e draw<strong>in</strong>g of <strong>the</strong> molecular structure <strong>in</strong> which <strong>the</strong> stereochemistry of <strong>the</strong> r<strong>in</strong>g junctions and C-atom substitutions is <strong>in</strong>dicated, <strong>in</strong> part, by <strong>the</strong> thick and <strong>the</strong> dotted l<strong>in</strong>es at <strong>the</strong> chiral carbon centres denoted by an asterisk. (B) is a simple 3D representation of <strong>the</strong> structure, show<strong>in</strong>g <strong>the</strong> flat tabular shape created from <strong>the</strong> four r<strong>in</strong>gs. 58 abundant isotopic <strong>in</strong><strong>for</strong>mation, due to <strong>the</strong> biosyn<strong>the</strong>sis pathway. Thus carbon isotope measurements can help to def<strong>in</strong>e <strong>the</strong> orig<strong>in</strong> of <strong>the</strong> cholesterol found <strong>in</strong> such sediments, s<strong>in</strong>ce its δ 13 C value will reflect that of <strong>the</strong> carbon pool from which <strong>the</strong> <strong>in</strong>dividual C5 units were assembled. Normally, of course, <strong>the</strong> gross averaged isotopic composition <strong>for</strong> <strong>the</strong> whole compound is measured, but <strong>the</strong>re is considerable scope <strong>for</strong> <strong>the</strong> study of <strong>the</strong> <strong>in</strong>tramolecular variations <strong>in</strong> isotopic composition. <strong>The</strong> biosyn<strong>the</strong>tic reaction sequences will generate <strong>in</strong>tramolecular order by processes that <strong>in</strong>volve : 1. <strong>The</strong> use of pre-syn<strong>the</strong>sised units; 2. <strong>The</strong> sequential coupl<strong>in</strong>g of such units; 3. <strong>The</strong> control of functionality and of any cyclisation steps <strong>in</strong>troduced; 4. Stereochemical controls of all steps, <strong>in</strong>clud<strong>in</strong>g <strong>the</strong> chirality of any asymetrically substituted carbon or o<strong>the</strong>r centres; 5. <strong>The</strong> isotopic fractionations <strong>in</strong>troduced dur<strong>in</strong>g <strong>the</strong> fixation of <strong>in</strong>organic carbon, <strong>the</strong> coupl<strong>in</strong>g of units, and <strong>in</strong> functionality changes. Intermolecular order is also <strong>in</strong>troduced by <strong>the</strong> control of <strong>the</strong> amounts of <strong>the</strong> different compounds biosyn<strong>the</strong>sised. For a homologous series of compounds, this frequently results <strong>in</strong> characteristic molecular distribution patterns whereby homologues maximise around a particular carbon number, with <strong>the</strong> amounts of <strong>the</strong> o<strong>the</strong>r homologues decreas<strong>in</strong>g rapidly to lower and higher carbon number. Molecular distributions of this type are characteristic <strong>in</strong>dications of <strong>the</strong> action of liv<strong>in</strong>g organisms. This is true of <strong>in</strong>dividual species, whole groups of families, orders and, <strong>in</strong> fact, throughout <strong>the</strong> k<strong>in</strong>gdoms of liv<strong>in</strong>g organisms. Thus, straight-cha<strong>in</strong> carbon compounds are syn<strong>the</strong>sised almost universally by <strong>the</strong> acetate pathway, operated by an evolutionary highly conserved polyketide enzyme system. <strong>The</strong> result is an assemblage of long-cha<strong>in</strong> compounds distributed over a range of carbon numbers that often maximises at a s<strong>in</strong>gle carbon number and displays a marked odd-over-even or even-over-odd dom<strong>in</strong>ancy. <strong>The</strong>se distributions and accompany<strong>in</strong>g carbon isotope characteristics are seen <strong>in</strong> extracts of sediments from <strong>the</strong> present time right back to <strong>the</strong> Archean period. Thus, <strong>the</strong> contemporary biota use enzyme systems <strong>in</strong>herited more or less unchanged from <strong>the</strong> Archean. Similar carbon number distribution systematics apply to o<strong>the</strong>r biosyn<strong>the</strong>tic molecular assemblages such as isoprenoids and oxygenated aromatics. Hence, a number of characteristic biosyn<strong>the</strong>tic patterns of carbon compound abundances are obvious <strong>in</strong> <strong>the</strong> extracts of both recent and ancient sediments. Abiotic reaction systems are not capable of produc<strong>in</strong>g such highly ordered distribution patterns. Fur<strong>the</strong>rmore, <strong>the</strong>re are with<strong>in</strong> each organic compound fur<strong>the</strong>r impr<strong>in</strong>ts of <strong>the</strong>ir biosyn<strong>the</strong>sis <strong>in</strong> <strong>the</strong> isotopic values of each carbon atom. Such impr<strong>in</strong>ts must also reside <strong>in</strong> any atoms o<strong>the</strong>r than carbon <strong>in</strong> <strong>the</strong> structures, notably hydrogen, oxygen and sulphur, but this area of knowledge rema<strong>in</strong>s almost entirely unexplored. Fig. I.4.3.2.3/2 is an example of such a homologous series. <strong>The</strong> four different groups of compounds (acids, aldehydes, alcohols and hydrocarbons) are all biosyn<strong>the</strong>tically related through reduction and decarboxylation steps. <strong>The</strong> alkanoic acids are
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SP-1231 Exobiology
Page 3 and 4: Cover Fossil coccoid bacteria, 1 µ
Page 5 and 6: 4 I.5 Potential Non-Martian Sites <
Page 7 and 8: 6 6.2 Imaging of F
Page 9 and 10: The Exobio
Page 11 and 12: pyrolysis, or similar techniques, f
Page 13 and 14: Ignasi Casanova, Institute
Page 15 and 16: I AN EXOBIOLOGICAL VIEW OF THE SOLA
Page 17 and 18: SP-1231 18 surface pressure of CO 2
Page 19 and 20: SP-1231 20 10 bar befor</st
Page 21 and 22: SP-1231 22 kilometres in</s
Page 23 and 24: SP-1231 24 Laboratory Investigation
Page 25 and 26: I.3 Limits of Life
Page 27 and 28: temperatures of 95-106ºC. This sug
Page 29 and 30: (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
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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 and 82: cover the range 4.
Page 83 and 84: The depletion of c
Page 85 and 86: valley networks (Fig. II.2.5/2) are
Page 87 and 88: plate tectonic evidence has been ob
Page 89 and 90: Table II.2.7.4/1. Potential Mars <s
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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SP-1231 Fig. II.4.1.1/1. A Gilbert-
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SP-1231 Fig. II.4.1.3/1. Th
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SP-1231 114 Sinuou
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SP-1231 Fig. II.4.3/1. Ages of seve
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SP-1231 118 orbital characteristics
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SP-1231 120 II.4.5 The</str
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SP-1231 122 II.4.6 Rovers and Drill
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SP-1231 124 Site 2: Apolin<
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SP-1231 126 Site 4: Chasma area, ea
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SP-1231 128 Sharp, M., Parkes, J.,
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SP-1231 Fig. II.5.1/1. The<
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SP-1231 132 II.5.3 Isotopic Analysi
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SP-1231 134 chemical composition de
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SP-1231 Fig. II.5.7.1/1. Th
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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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Exobiology in the Solar System & The Search for Life on Mars - ESA

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