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 2. <strong>the</strong> labelled release experiment tested catabolic activity, e.g. digestion of food, as a metabolic capability to release radioactively labelled carbon from organic nutrient compounds (Lev<strong>in</strong> & Straat, 1977); 3. <strong>the</strong> gas-exchange experiment tested respiration as metabolic production of gaseous byproducts <strong>in</strong> <strong>the</strong> presence of water and nutrients (Oyama et al., 1977). In <strong>the</strong> first <strong>in</strong>stance, all three Vik<strong>in</strong>g biology experiments gave results <strong>in</strong>dicative of active chemical or even biological processes when samples of martian soil were subjected to <strong>in</strong>cubation under <strong>the</strong> conditions that were imposed on <strong>the</strong>m. However, when tak<strong>in</strong>g <strong>the</strong> data toge<strong>the</strong>r with all o<strong>the</strong>r Vik<strong>in</strong>g results, non-biological processes have generally been <strong>in</strong>terpreted to be responsible <strong>for</strong> <strong>the</strong> observed reactions (Kle<strong>in</strong>, 1979). <strong>The</strong> strongest argument aga<strong>in</strong>st extant life was <strong>the</strong> complete lack of any organic residues <strong>in</strong> <strong>the</strong> martian soil samples <strong>in</strong>vestigated by gas chromatographymass spectrometry (GCMS). <strong>The</strong> detection limit was <strong>in</strong> <strong>the</strong> range of ppb <strong>for</strong> compounds conta<strong>in</strong><strong>in</strong>g three carbon atoms or more, and <strong>in</strong> <strong>the</strong> range of ppm <strong>for</strong> molecules with one or two carbon atoms. <strong>The</strong> conclusion is that extant life is absent at <strong>the</strong> two Vik<strong>in</strong>g sites. So far, <strong>the</strong> mechanisms underly<strong>in</strong>g <strong>the</strong> reactions of <strong>the</strong> biology experiments are not known. <strong>The</strong> most likely explanation suggests <strong>the</strong> presence of highly reactive peroxides produced <strong>in</strong> <strong>the</strong> soil by <strong>the</strong> <strong>in</strong>tense unfiltered solar UV radiation, which is also responsible <strong>for</strong> <strong>the</strong> lack of organics <strong>in</strong> <strong>the</strong> samples (reviewed <strong>in</strong> Horneck, 1995). I.4.2.4. Chemical Signatures and Biomarkers Microorganisms that are isolated and enriched from <strong>the</strong>ir primary habitats can be subjected to a thorough biochemical analysis, such as determ<strong>in</strong>ation of <strong>the</strong> total biomass and <strong>the</strong> content of prote<strong>in</strong>s, nucleic acids, lipids, sugars etc. <strong>The</strong> detailed characterisation of <strong>the</strong> pattern of biomarker mixtures from a sample permits <strong>the</strong> assessment of <strong>the</strong> major contribut<strong>in</strong>g species and <strong>the</strong>ir metabolic state, e.g. actively metabolis<strong>in</strong>g, dormant or ext<strong>in</strong>ct. Phospholipids, which are part of every bacterial membrane, have been considered as valuable <strong>in</strong>dicators of <strong>the</strong> liv<strong>in</strong>g biomass that is present because <strong>the</strong>y ma<strong>in</strong>ta<strong>in</strong> a relatively constant portion of <strong>the</strong> cell mass and dis<strong>in</strong>tegrate fairly rapidly after cellular death (Chapelle, 1993). <strong>The</strong> pattern of lipids is also diagnostic <strong>for</strong> different species of a microbial community, such as phototrophic cyanobacteria, photosyn<strong>the</strong>tic bacteria, bacterial heterotrophs and achaea (Summons et al., 1996). Isotopic ratios are ano<strong>the</strong>r valuable set of chemical biomarkers. For <strong>in</strong>stance, <strong>the</strong> enzymatic uptake of <strong>in</strong>organic carbon dur<strong>in</strong>g photosyn<strong>the</strong>sis is typically associated with a discrim<strong>in</strong>ation aga<strong>in</strong>st 13 C relative to 12 C (e.g. Schidlowski, 1993a). It is <strong>in</strong>terest<strong>in</strong>g to note that <strong>the</strong> δ 13 C values of average biomass usually range about 20- 30‰ more negative than those of <strong>in</strong>organic carbon. <strong>The</strong>se low δ 13 C values are ma<strong>in</strong>ta<strong>in</strong>ed ra<strong>the</strong>r conservatively over <strong>the</strong> whole history of life on Earth (see Fig. I.4.3.2.2/1 and relevant discussion <strong>in</strong> section I.4.3.2.2.). By <strong>the</strong> comb<strong>in</strong>ation of compound-specific isotopic <strong>in</strong><strong>for</strong>mation with diagnostic molecular structure a powerful tool is available <strong>for</strong> characteris<strong>in</strong>g <strong>the</strong> constituents of a microbial community (Summons et al., 1996). Optical handedness of monomers, e.g. am<strong>in</strong>o acids <strong>in</strong> prote<strong>in</strong>s and sugars <strong>in</strong> nucleic acids, is a s<strong>in</strong>e qua non <strong>for</strong> ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g <strong>the</strong> secondary, tertiary and quaternary structure of <strong>the</strong> polymers and is <strong>the</strong>re<strong>for</strong>e considered as a basic requirement <strong>for</strong> terrestrial life. <strong>The</strong>re<strong>for</strong>e, evidence <strong>for</strong> optical activity <strong>in</strong> a class of compounds that may make up polymers represents an <strong>in</strong>terest<strong>in</strong>g biomarker. Measurements of optical activity can be done <strong>in</strong> extracts by gas chromatography (Pollock et al., 1970) or directly with <strong>the</strong> sample collected us<strong>in</strong>g polarised light and a light detector (MacDermott et al., 1996). A non-<strong>in</strong>trusive <strong>in</strong> situ high-precision analysis of <strong>the</strong> distribution of organic and <strong>in</strong>organic components of samples conta<strong>in</strong><strong>in</strong>g liv<strong>in</strong>g microorganisms is obta<strong>in</strong>ed by Fourier trans<strong>for</strong>m Raman spectroscopy (Edwards et al., 1995). From <strong>the</strong> vibrational bands, <strong>the</strong> presence of organic compounds, such as chlorophyll and calcium oxalate di- and monohydrate, was detected <strong>in</strong> different zones of endolithic communities. 47
SP-1231 48 A detailed chemical analysis of martian surface soil was per<strong>for</strong>med dur<strong>in</strong>g <strong>the</strong> Vik<strong>in</strong>g missions. <strong>The</strong> elemental composition was determ<strong>in</strong>ed by X-ray fluorescence, which analysed <strong>the</strong> composition <strong>for</strong> elements heavier than Mg. It was shown that <strong>the</strong> most essential major elements that make up <strong>the</strong> biological matter, such as C, H, O, N, P, K, Ca, Mg and S, are present on <strong>the</strong> surface of Mars. However, organic compounds were not detected by GCMS, which was capable of detect<strong>in</strong>g organic residues <strong>in</strong> martian soil down to ppb <strong>for</strong> compounds conta<strong>in</strong><strong>in</strong>g three or more C and to ppm <strong>for</strong> compounds with one or two C (Biemann et al., 1977). I.4.2.5 Indirect F<strong>in</strong>gerpr<strong>in</strong>ts of <strong>Life</strong> Dur<strong>in</strong>g its >3.5 Gyr history, life on Earth has substantially modified <strong>the</strong> terrestrial lithosphere, hydrosphere and atmosphere. Examples are <strong>the</strong> fossil deposits of petroleum and coal, <strong>the</strong> sediments of shell limestone, <strong>the</strong> coral reefs and <strong>the</strong> deposits of banded iron <strong>for</strong>mation, biom<strong>in</strong>eralisation and biowea<strong>the</strong>r<strong>in</strong>g, which all bear witness to biological activity <strong>in</strong> <strong>the</strong> geological past. Stromatolites and related biosedimentary build-ups are examples of microbial biom<strong>in</strong>eralisation, which are widespread <strong>in</strong> <strong>the</strong> geological record (Walter, 1976). Composition and dynamic cycles of <strong>the</strong> terrestrial hydrosphere and atmosphere are decisively <strong>in</strong>fluenced by <strong>the</strong> terrestrial biosphere. Examples are <strong>the</strong> water, CO 2 and nitrogen cycles. <strong>The</strong> albedo of our planet is also modified by <strong>the</strong> surface vegetation. Concern<strong>in</strong>g <strong>the</strong> water cycle, evapotranspiration, especially of <strong>the</strong> tropic ra<strong>in</strong> <strong>for</strong>est, is an important biogenic effect contribut<strong>in</strong>g to <strong>the</strong> release of water. <strong>The</strong> atmospheric O 2 is largely a product of photosyn<strong>the</strong>tic activity that began <strong>in</strong> <strong>the</strong> early history of life with cyanobacteria as <strong>the</strong> ma<strong>in</strong> primary producers. Photodissociation of O 2 <strong>in</strong> <strong>the</strong> upper layers of our atmosphere has led to <strong>the</strong> build-up of <strong>the</strong> UV-protective ozone layer <strong>in</strong> <strong>the</strong> stratosphere. Concern<strong>in</strong>g <strong>the</strong> CO 2 cycle, <strong>the</strong> mar<strong>in</strong>e phytoplankton constitutes a large CO 2 s<strong>in</strong>k, which is essential <strong>for</strong> ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g <strong>the</strong> steady-state. Accord<strong>in</strong>g to <strong>the</strong> Gaia hypo<strong>the</strong>sis, proposed by Lovelock, <strong>the</strong> composition, oxidative-reductive state and temperature of <strong>the</strong> atmosphere are actively regulated by life activities (Lovelock, 1979). However, it is also possible that life<strong>for</strong>ms have conquered ecological microniches, where a symbiosis of photosyn<strong>the</strong>tic primary producers and heterotrophic consumers may produce a steady-state where <strong>the</strong> <strong>in</strong>flux and efflux of carbon are equal (Morita, 1975). <strong>The</strong>se ‘hidden’ econiches would not necessarily contribute to global cycl<strong>in</strong>g. <strong>The</strong>re exist several biogenic m<strong>in</strong>erals, which have dist<strong>in</strong>ctive crystallographies, morphologies and isotopic ratios that make <strong>the</strong>m dist<strong>in</strong>guishable from <strong>the</strong>ir abiotically-produced counterparts of <strong>the</strong> same chemical composition. Those m<strong>in</strong>erals that result from genetically-controlled m<strong>in</strong>eralisation processes and that are <strong>for</strong>med with<strong>in</strong> a pre<strong>for</strong>med organic framework have non-<strong>in</strong>terchangeable characteristics, such as orientation of <strong>the</strong> crystallographic axes and <strong>the</strong> microarchitecture. Examples are <strong>the</strong> skeletons of <strong>the</strong> unicellular mar<strong>in</strong>e Acantharia (composed of strontium sulphate), <strong>the</strong> shells of amorphous silicate of diatoms, and <strong>the</strong> biogenic magnetite <strong>for</strong>med by bacteria (Schwartz et al., 1992). It is important to note that m<strong>in</strong>erals produced under biologically-controlled processes are not necessarily <strong>in</strong> equilibrium with <strong>the</strong> extracellular environment. In contrast, those m<strong>in</strong>erals that are <strong>for</strong>med extracellularly by biologically-<strong>in</strong>duced m<strong>in</strong>eralisation processes can be less easily dist<strong>in</strong>guished from <strong>the</strong>ir abiotically-<strong>for</strong>med counterparts. <strong>The</strong>y are <strong>for</strong>med <strong>in</strong> an open environment and are <strong>in</strong> equilibrium with this environment. I.4.2.6 Conclusions <strong>The</strong> search <strong>for</strong> signatures <strong>in</strong>dicative of extant life on Mars or on any o<strong>the</strong>r celestial body of exobiological <strong>in</strong>terest <strong>in</strong> our <strong>Solar</strong> <strong>System</strong> can only be one of <strong>the</strong> f<strong>in</strong>al steps <strong>in</strong> <strong>the</strong> quest <strong>for</strong> extraterrestrial life. For future exobiology exploration of Mars, a stepwise approach might be <strong>the</strong> most promis<strong>in</strong>g <strong>in</strong>vestigative strategy (Chicarro et al., 1989). In particular, be<strong>for</strong>e any search-<strong>for</strong>-extant-life experiment, more data are required on martian geology (paleolakes, volcanism, hydro<strong>the</strong>rmal vents, carbonates), climate (hydrosphere, duration of phases that allow liquid water), <strong>the</strong> past and present
Page 1 and 2: 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: SP-1231 Fig. I.4.2.3/1A (left). Net
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
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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
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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
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Page 93 and 94: principal atmosphe
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Lines of evidence
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indicated by carbo
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quest for
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Anders, E. (1989). Prebiotic organi
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Urey, H.C. (1968). Origin</
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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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