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 Fig. I.4.3.2.2/1. Isotope age functions of organic carbon (C org) and carbonate carbon (C carb) compared with <strong>the</strong> isotopic compositions of <strong>the</strong>ir progenitor substances <strong>in</strong> <strong>the</strong> present environment (mar<strong>in</strong>e bicarbonate and biogenic matter of various parentage, cf. right box). Isotopic compositions are given as δ 13 C values <strong>in</strong>dicat<strong>in</strong>g ei<strong>the</strong>r a relative <strong>in</strong>crease (+) or decrease (-) <strong>in</strong> <strong>the</strong> 13 C/ 12 C ratio of <strong>the</strong> respective subtance (<strong>in</strong> ‰ difference) compared with that of <strong>the</strong> PDB (Peedee belemnite) standard, which def<strong>in</strong>es 0‰ on <strong>the</strong> δ-scale. Note that <strong>the</strong> δ 13 C org spread of <strong>the</strong> extant biomass is basically transcribed <strong>in</strong>to recent mar<strong>in</strong>e sediments and <strong>the</strong> subsequent record back to 3.8 Gyr, with <strong>the</strong> Isua values moderately reset by amphibolite-grade metamorphism [associated bars show values <strong>for</strong> carbonaceous <strong>in</strong>clusions <strong>in</strong> apatite gra<strong>in</strong>s from a 3.85 Gyr-old Isua banded iron-<strong>for</strong>mation (A1) and from o<strong>the</strong>r Isua iron-<strong>for</strong>mations carbon fixation. This is primarily <strong>the</strong> assimilation of CO 2 and <strong>the</strong> bicarbonate ion (HCO 3 - ) by plants and microorganisms that proceeds by a limited number of pathways which <strong>in</strong>variably discrim<strong>in</strong>ate aga<strong>in</strong>st <strong>the</strong> heavy carbon isotope ( 13 C), mostly as a result of a k<strong>in</strong>etic isotope effect <strong>in</strong>herent <strong>in</strong> <strong>the</strong> first irreversible enzymatic CO 2-fix<strong>in</strong>g carboxylation reaction (cf. O’Leary, 1981; Schidlowski et al., 1983). This latter reaction promotes <strong>the</strong> <strong>in</strong>corporation of CO 2 <strong>in</strong>to <strong>the</strong> carboxyl (COOH) group of an organic acid which, <strong>in</strong> turn, lends itself to fur<strong>the</strong>r process<strong>in</strong>g <strong>in</strong> subsequent metabolic pathways. As biochemical carbon-fix<strong>in</strong>g reactions are largely enzyme-controlled, and liv<strong>in</strong>g systems constitute dynamic states undergo<strong>in</strong>g rapid cycles of anabolism and catabolism, it is generally accepted that most biological isotope fractionations are due to k<strong>in</strong>etic ra<strong>the</strong>r than equilibrium effects. Because of <strong>the</strong> discrim<strong>in</strong>ation aga<strong>in</strong>st ‘heavy’ carbon <strong>in</strong> <strong>the</strong> common carbon-fix<strong>in</strong>g pathways, we observe a marked enrichment of 12 C <strong>in</strong> all <strong>for</strong>ms of biogenic (reduced) carbon as compared with <strong>the</strong> <strong>in</strong>organic feeder pool of oxidised carbon consist<strong>in</strong>g ma<strong>in</strong>ly of atmospheric CO 2 and dissolved mar<strong>in</strong>e bicarbonate ion (HCO 3 - ). In terms of <strong>the</strong> conventional notation, δ 13 C values of average biomass usually turn out to be 20- 30‰ more negative than those of mar<strong>in</strong>e bicarbonate, <strong>the</strong> most abundant <strong>in</strong>organic carbon species <strong>in</strong> <strong>the</strong> environment. (A2), accord<strong>in</strong>g to Mojzsis et al. (1996)]. <strong>The</strong> envelope <strong>for</strong> fossil organic carbon is an update of <strong>the</strong> database presented by Schidlowski et al. (1983), compris<strong>in</strong>g <strong>the</strong> means of some 150 Precambrian kerogen prov<strong>in</strong>ces as well as <strong>the</strong> currently available <strong>in</strong><strong>for</strong>mation on <strong>the</strong> Phanerozoic record. <strong>The</strong> negative spikes at 2.7 Gyr and 2.1 Gyr <strong>in</strong>dicate a large-scale <strong>in</strong>volvement of methane <strong>in</strong> <strong>the</strong> <strong>for</strong>mation of <strong>the</strong> respective kerogen precursors. Contributors to <strong>the</strong> contemporary biomass are (1) C3 plants, (2) C4 plants; (3) CAM plants; (4) eukayotic algae; (5 a,b) natural and cultures cyanobacteria; (6) groups of photosyn<strong>the</strong>tic bacteria o<strong>the</strong>r than cyanobacteria; (7) methanogenic bacteria; (8) methanotrophic bacteria. <strong>The</strong> δ 13 C org range <strong>in</strong> recent mar<strong>in</strong>e sediments is from De<strong>in</strong>es (1980) and based on some 1600 data po<strong>in</strong>ts (black <strong>in</strong>sert covers >90% of <strong>the</strong> database). 55
SP-1231 56 <strong>The</strong> isotopic difference thus established between organic (biogenic) carbon and <strong>the</strong> surficial bicarbonate-carbonate pool is largely reta<strong>in</strong>ed when organic and carbonate carbon enter newly-<strong>for</strong>med sediments. As is obvious from Fig. I.4.3.2.2/1, both <strong>the</strong> carbon isotope spreads of extant primary producers and those of mar<strong>in</strong>e carbonate and bicarbonate are basically transcribed <strong>in</strong>to <strong>the</strong> sedimentary record back to 3.5 Gyr or even 3.8 Gyr ago. This would imply that organic carbon and carbonate carbon had always been transferred from <strong>the</strong> surficial environment to <strong>the</strong> crust with relatively little change <strong>in</strong> <strong>the</strong>ir isotopic compositions. For example, <strong>the</strong> δ 13 C org spread <strong>in</strong> recent mar<strong>in</strong>e sediments (cf. Fig. I.4.3.2.2/1) faithfully <strong>in</strong>tegrates over <strong>the</strong> spread of <strong>the</strong> contemporary liv<strong>in</strong>g biomass with just <strong>the</strong> extremes elim<strong>in</strong>ated, <strong>in</strong>dicat<strong>in</strong>g that <strong>the</strong> effect of a later diagenetic overpr<strong>in</strong>t on <strong>the</strong> primary isotope values is ra<strong>the</strong>r limited (usually below 3‰) and, <strong>for</strong> <strong>the</strong> most part, gets lost with<strong>in</strong> <strong>the</strong> broad scatter of <strong>the</strong> orig<strong>in</strong>al values. Consequently, <strong>the</strong> k<strong>in</strong>etic isotope effect <strong>in</strong>herent <strong>in</strong> photosyn<strong>the</strong>tic carbon fixation is propagated from <strong>the</strong> biosphere <strong>in</strong>to <strong>the</strong> rock section of <strong>the</strong> carbon cycle almost unaltered, which opens up <strong>the</strong> possibility of trac<strong>in</strong>g <strong>the</strong> isotopic signature of this process back <strong>in</strong>to <strong>the</strong> geologic past. With <strong>the</strong>se relationships established, decod<strong>in</strong>g of <strong>the</strong> vast body of isotopic <strong>in</strong><strong>for</strong>mation stored <strong>in</strong> <strong>the</strong> sedimentary record is fairly straight<strong>for</strong>ward. <strong>The</strong>re is little doubt that <strong>the</strong> conspicuous 12 C-enrichment displayed by <strong>the</strong> data envelope <strong>for</strong> fossil organic carbon (Fig. I.4.3.2.2/1) constitutes a coherent signal of autotrophic carbon fixation over almost 4 Gyr of recorded Earth history as it ultimately rests with <strong>the</strong> process that gave rise to <strong>the</strong> biological precursor materials. Moreover, <strong>the</strong> long-term uni<strong>for</strong>mity of <strong>the</strong> signal attests to an extreme degree of conservatism of <strong>the</strong> basic biochemical mechanisms of carbon fixation. In fact, <strong>the</strong> ma<strong>in</strong>stream of <strong>the</strong> envelope <strong>for</strong> δ 13 C org depicted <strong>in</strong> Fig. I.4.3.2.2/1 can be most readily expla<strong>in</strong>ed as <strong>the</strong> geochemical manifestation of <strong>the</strong> isotope-discrim<strong>in</strong>at<strong>in</strong>g properties of one s<strong>in</strong>gle enzyme, namely, ribulose-1,5-bisphosphate (RuBP) carboxylase, <strong>the</strong> key enzyme of <strong>the</strong> Calv<strong>in</strong> Cycle. It is well known today that <strong>the</strong> carbon transfer from <strong>the</strong> <strong>in</strong>organic to <strong>the</strong> organic world largely proceeds via <strong>the</strong> RuBP carboxylase reaction that feeds CO 2 directly <strong>in</strong>to <strong>the</strong> Calv<strong>in</strong> Cycle as a 3-carbon compound (phosphoglycerate). Most autotrophic microorganisms and all green plants operate along this pathway of carbon assimilation; higher plants rely<strong>in</strong>g on it entirely are termed C3 plants. As a result, <strong>the</strong> bulk of <strong>the</strong> Earth’s biomass (both extant and fossil) bears <strong>the</strong> isotopic signature of C3 (or Calv<strong>in</strong> Cycle) photosyn<strong>the</strong>sis characterised by <strong>the</strong> sizeable fractionations of <strong>the</strong> RuBP carboxylase reaction that assigns a mean δ 13 C org range of -26±7‰ to most biogenic matter. Occasional negative offshoots from this long-term average (Fig. I.4.3.2.2/1) are commonly restricted to <strong>the</strong> Precambrian (Hayes et al., 1983; Schidlowski et al., 1983) and suggest <strong>the</strong> <strong>in</strong>volvement of methanotrophic pathways <strong>in</strong> <strong>the</strong> <strong>for</strong>mation of <strong>the</strong> respective kerogen precursors. Basically, <strong>the</strong>se excursions appear to be oddities conf<strong>in</strong>ed to side stages of <strong>the</strong> carbon cycle that have never affected <strong>the</strong> economy of <strong>the</strong> global cycle as a whole. <strong>The</strong> only apparent discont<strong>in</strong>uity <strong>in</strong> <strong>the</strong> δ 13 C org record depicted <strong>in</strong> Fig. I.4.3.2.2/1 is <strong>the</strong> break between <strong>the</strong> ~3.8 Gyr-old Isua metasediments from West Greenland and <strong>the</strong> whole of <strong>the</strong> post-Isua record. <strong>The</strong> observed isotope shift is, however, fully consistent with <strong>the</strong> predictable effects of an isotopic re-equilibration between coexist<strong>in</strong>g organic carbon and carbonate <strong>in</strong> response to <strong>the</strong> amphibolite-grade metamorphism experienced by <strong>the</strong> Isua suite. Both currently available <strong>the</strong>rmodynamic data on 13 C/ 12 C exchange between C org and carbonate carbon (C carb) as a function of <strong>in</strong>creas<strong>in</strong>g metamorphic temperatures and observational evidence from a host of geologically younger metamorphic terranes make it virtually certa<strong>in</strong> that <strong>the</strong> ‘normal’ sedimentary δ 13 C org and δ 13 C carb records had orig<strong>in</strong>ally extended back to 3.8 Gyr ago, and that <strong>the</strong> Isua anomaly is clearly due to a metamorphic overpr<strong>in</strong>t (Schidlowski et al., 1979; 1983). Apparently prist<strong>in</strong>e δ 13 C org values <strong>in</strong> <strong>the</strong> range -21‰ to -49‰ (average -7±3‰) have been recently reported <strong>for</strong> m<strong>in</strong>or carbon <strong>in</strong>clusions <strong>in</strong> apatite gra<strong>in</strong>s from 3.85 Gyr-old Isua banded iron-<strong>for</strong>mation (Mojzsis et al., 1996). Here, carbonaceous
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
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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 57 and 58: SP-1231 60 References regard to non
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Page 61 and 62: SP-1231 64 ities in</strong
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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 93 and 94: principal atmosphe
Page 95 and 96: Lines of evidence
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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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