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 typically syn<strong>the</strong>sised by almost all organisms from C2 units as a homologous series reach<strong>in</strong>g to C30 (with a marked dom<strong>in</strong>ance of even-over-odd carbon number). Correspond<strong>in</strong>gly, <strong>the</strong> derived series of alkanals and alkanols display a similar evenover-odd predom<strong>in</strong>ance. In contrast, <strong>the</strong> alkanes (<strong>for</strong>med by decarboxylation of <strong>the</strong> acids) show a predom<strong>in</strong>ance of odd carbon numbers. <strong>The</strong>se biological patterns of biomarker distribution carry through <strong>in</strong>to sediments. However, <strong>the</strong> odd-over-even dom<strong>in</strong>ance <strong>in</strong> <strong>the</strong> n-alkanes is gradually lost when <strong>the</strong>se compounds are subjected to geo<strong>the</strong>rmal alteration. In general, <strong>the</strong>se straight-cha<strong>in</strong>ed homologues do not carry such organism-specific <strong>in</strong><strong>for</strong>mation, s<strong>in</strong>ce <strong>the</strong>y are of such universal biosyn<strong>the</strong>sis. Only <strong>the</strong>ir 13 C/ 12 C compositions rema<strong>in</strong> significantly <strong>in</strong><strong>for</strong>mative, because <strong>the</strong>se are controlled by <strong>the</strong> respective ratios of <strong>the</strong> C2 precursor units which are, <strong>in</strong> turn, dependent upon <strong>the</strong> photosyn<strong>the</strong>tic patway of carbon dioxide fixation, as discussed <strong>in</strong> I.4.3. Impr<strong>in</strong>t of <strong>the</strong> Biosphere <strong>in</strong> <strong>the</strong> Geosphere Standard methods <strong>for</strong> exam<strong>in</strong><strong>in</strong>g liv<strong>in</strong>g biota, when applied to young immature sediments, reveal morphological evidence of decayed biota as microscopically recognisable organic debris and molecular evidence as detectable concentrations of simple and complex biochemicals of both orig<strong>in</strong>al and diagentically modified structures. For example, am<strong>in</strong>o acid analyses of teeth, bones and shells <strong>in</strong> young sediments display distributions characteristic of prote<strong>in</strong>s which reveal <strong>in</strong>creased racemisation with related temperature and time of burial, a measure that can even be used <strong>for</strong> dat<strong>in</strong>g and chronostratigraphy. Similar considerations apply to o<strong>the</strong>r types of molecules, e.g. steroids, triterpenoids, alkaloids and <strong>the</strong> whole suite of secondary metabolites syn<strong>the</strong>sised by <strong>the</strong> enzymes. <strong>The</strong> <strong>in</strong>itial record is very close to that of contemporary biochemistry, with small amounts of fairly well-preserved DNA, prote<strong>in</strong> and carbohydrates still detectable. Most of this <strong>in</strong>itial record is rapidly removed with<strong>in</strong> a few tens of years, pr<strong>in</strong>cipally by microbial action. However, removal and destruction is selective, patchy and <strong>in</strong>complete. Thus, <strong>the</strong> end result is that relatively easily biodegraded compounds and biopolymers survive <strong>in</strong> small to trace amounts <strong>for</strong> thousands to millions of years <strong>in</strong> aquatic sediments that have not been deeply buried and subjected to geo<strong>the</strong>rmal heat<strong>in</strong>g. Some depositional microenvironments e.g. frozen sediments, anoxic sediments, organic res<strong>in</strong>s, copal and amber, result <strong>in</strong> an exceptional preservation of biochemicals. Certa<strong>in</strong>ly, low temperature, <strong>the</strong> restriction of access af<strong>for</strong>ded by <strong>the</strong> close pack<strong>in</strong>g of clay particles, absence of reactive species such as free radicals, lack of nutrients (e.g. electron donors such as nitrate, sulphate), and lack of liquid water all have a preservative role. At <strong>the</strong> molecular level, <strong>the</strong> fundamental factors facilitat<strong>in</strong>g preservation seem to be restrictions on <strong>the</strong> mobility of molecules and <strong>in</strong>creased steric h<strong>in</strong>drance at reactive centres, both <strong>in</strong>tra- and <strong>in</strong>termolecularly. All molecules are ultimately degradable, whe<strong>the</strong>r by microbiota, reagents and heat, operat<strong>in</strong>g through time, ei<strong>the</strong>r s<strong>in</strong>gly or toge<strong>the</strong>r. But differences <strong>in</strong> <strong>in</strong>tramolecular bond strengths are fundamental controls on molecular persistence, especially <strong>in</strong> Fig. I.4.3.2.3/2. A homologous series of long, straight-cha<strong>in</strong>ed, alipathic lipid biomarkers, which are biosyn<strong>the</strong>tically related through reduction and decarboxylation steps. 59
SP-1231 60 References regard to non-biologically mediated reactions. Thus, novel aliphatic type and aromatic type biopolymers, whose skeletons are based essentially on carbon carbon bonds, have been detected recently <strong>in</strong> several species of aquatic unicellular algae, and have been shown to persist <strong>in</strong> significant amounts <strong>in</strong> both recently deposited aand ancient sediments. <strong>The</strong>se biomaterials, or ra<strong>the</strong>r <strong>the</strong>ir alteration products, may make up a significant portion of sedimentary organic matter. Bulk Terrestrial Organic Matter <strong>The</strong> bulk (circa 98%) of <strong>the</strong> organic matter held <strong>in</strong> <strong>the</strong> sedimentary rocks of <strong>the</strong> Earth’s crust is <strong>the</strong> <strong>in</strong>soluble, amorphous, heteropolymeric organic ‘kerogen’. In young, immature sediments it is rich <strong>in</strong> hydrogen and conta<strong>in</strong>s o<strong>the</strong>r elements such as oxygen, sulphur and nitrogen <strong>in</strong> lesser amounts, while <strong>in</strong> old, mature sediments it is highly carbonaceous, even graphitic, and is very low <strong>in</strong> hydrogen and elements o<strong>the</strong>r than carbon. Whatever its state, kerogen is almost entirely derived ultimately from biological debris which, when immature, conta<strong>in</strong>s component pieces of biomacromolecules and smaller metabolites crossl<strong>in</strong>ked <strong>in</strong>to <strong>the</strong> kerogen structure by carboncarbon, carbon-sulphur and o<strong>the</strong>r bonds. Some components may be trapped <strong>in</strong> <strong>the</strong> molecular <strong>in</strong>terstices, while o<strong>the</strong>rs may have suffered additional modification through oxidation and o<strong>the</strong>r processes. Experiments specifically designed to release molecular moieties from this heteropolymeric material have shown that <strong>the</strong> kerogen matrix has protected some components from diagenetic and o<strong>the</strong>r changes experienced by free unbonded biomarkers <strong>in</strong> <strong>the</strong> same sediment. However, <strong>the</strong> orig<strong>in</strong>al biosyn<strong>the</strong>tic order <strong>in</strong>corporated as molecular components of <strong>the</strong> kerogen is gradually obscured and destroyed by <strong>the</strong> diagenetic processes <strong>in</strong> <strong>the</strong> sediment. Indeed, deep burial with accompany<strong>in</strong>g rise <strong>in</strong> geo<strong>the</strong>rmal temperature, pressure and <strong>the</strong> passage of time eventually erases almost all of <strong>the</strong> orig<strong>in</strong>al biosyn<strong>the</strong>tic order as <strong>the</strong> carbonisation proceeds, e.g. <strong>the</strong> biosyn<strong>the</strong>tic dom<strong>in</strong>ance of odd-over-even carbon numbers <strong>in</strong> <strong>the</strong> C30 region of plant wax alkanes is gradually replaced by a smooth distribution at lower carbon numbers, as <strong>the</strong> molecules are broken down through <strong>the</strong>rmally-<strong>in</strong>duced carbon-carbon bond breakage. Improved analytical methodologies are now needed to maximise <strong>the</strong> retrieval of biomarker <strong>in</strong><strong>for</strong>mation from kerogens. We need to release <strong>the</strong> bound and trapped biochemical moieties with m<strong>in</strong>imal loss of order, both <strong>in</strong>tramolecular, <strong>in</strong> <strong>the</strong> <strong>for</strong>m of detailed molecular structures, and <strong>in</strong>termolecular <strong>in</strong> <strong>the</strong>ir relative abundances. Pyrolysis techniques are good candidates, especially where <strong>the</strong> duration of <strong>the</strong> heat<strong>in</strong>g step is extremely brief and recomb<strong>in</strong>ation of <strong>the</strong> pyrolysates is m<strong>in</strong>imised. <strong>The</strong> analysis of <strong>the</strong> pyrolysates by gas chromatography/mass spectrometry (GCMS) provides <strong>in</strong><strong>for</strong>mation on <strong>the</strong> nature of <strong>the</strong> orig<strong>in</strong>al organic material contributed to <strong>the</strong> kerogen. <strong>The</strong> pyrolysis provides a reflection of <strong>the</strong> orig<strong>in</strong>al biosyn<strong>the</strong>tic order <strong>in</strong> <strong>the</strong> structures of <strong>the</strong> compounds released, <strong>the</strong>ir abundance patterns and <strong>the</strong>ir isotopic distributions. Abyzov, S.S. (1993). Microorganisms <strong>in</strong> <strong>the</strong> Antarctic ice. In Antarctic Microbiology (Ed. E. Friedmann), Wiley & Sons, New York, USA, pp265-295. Amann, R., Ludwig, W. & Schleifer, K.-H. (1992). Identification and <strong>in</strong> situ detection of <strong>in</strong>dividual bacterial cells. FEMS Microbiol. Lett. 100, 45-50. Awramik, S.M., Schopf, J.W. & Walter, M.R. (1983) Filamentous fossil bacteria from <strong>the</strong> Archaean of Western Australia. In Developments and Interactions of <strong>the</strong> Precambrian Atmosphere, Lithosphere and Biosphere (Developments <strong>in</strong> Precambrian Geology 7) (Eds. B. Nagy, R. Weber, J.C. Guerrero & M. Schidlowski), Elsevier, Amsterdam, <strong>The</strong> Ne<strong>the</strong>rlands, pp249-266. Barns, S.M., Fundyga, R.E., Jeffries, M.W. & Pace, N.R. (1994). Remarkable archaeal diversity detected <strong>in</strong> a Yellowstone National Park hot spr<strong>in</strong>g environment. Proc. Natl. Acad. Sci. USA 91,1609-1613. Bartosch, S., Quader, H. & Bock. E. (1996). Confocal laser scann<strong>in</strong>g microscopy: A
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SP-1231 Exobiology
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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
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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
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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
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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 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
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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
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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
Page 103 and 104: Urey, H.C. (1968). Origin</
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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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