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

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SP-1231 Fig. I.3.2.1/1 Parental relationships between liv<strong>in</strong>g be<strong>in</strong>gs, based on sequence comparison of ribosomal RNA. Extreme halophiles are <strong>in</strong> red and hyperhermophiles are <strong>in</strong> black. <strong>The</strong> short branches of hyper<strong>the</strong>rmophiles <strong>in</strong>dicate that sequences of <strong>the</strong>ir rRNA evolve more slowly than similar sequences from organisms liv<strong>in</strong>g at lower temperatures (mesophiles). This universal tree is usually <strong>in</strong>terpreted as argu<strong>in</strong>g <strong>in</strong> favour of a <strong>the</strong>rmophilic last universal common ancestor. However, this tree should be regarded with caution. For example, it has been shown recently that microsporidia (represented by Vairimorpha and Encephalitozoon <strong>in</strong> <strong>the</strong> figure) are <strong>in</strong> fact fungi (yeast). <strong>The</strong> difference <strong>in</strong> rate of evolution between <strong>the</strong> various l<strong>in</strong>eages can <strong>in</strong>deed produce artifacts. 28 BACTERIA Euryotes ARCHAEA Plants Animals fungi Microsporidia Crenotes EUCARYA Protistes Halophiles extremes Hyper<strong>the</strong>rmophiles microbe Pyrodictium occultum <strong>in</strong> shallow water near a beach of Vulcano Island <strong>in</strong> Italy (Stetter, 1982). This record held <strong>for</strong> 15 years and was only recently surpassed, although by only 3ºC. <strong>The</strong> new record holder, Pyrolobus fumarii, is a deep-sea microbe, aga<strong>in</strong> described by Stetter’s group (Blöchl et al., 1997). <strong>The</strong> claim <strong>for</strong> organisms liv<strong>in</strong>g at 250ºC under a pressure of 350 atm was an artifact, as demonstrated by Trent (1984). It should be stressed that <strong>the</strong> actual upper limit is, of course, unknown, but <strong>the</strong> barrier at 110-113ºC has so far survived <strong>the</strong> <strong>in</strong>tensive search <strong>in</strong> deep-sea vents <strong>for</strong> organisms grow<strong>in</strong>g at higher temperatures. Many microbes liv<strong>in</strong>g at temperatures close to or above <strong>the</strong> boil<strong>in</strong>g po<strong>in</strong>t of water, <strong>the</strong> hyper<strong>the</strong>rmophiles, have been found <strong>in</strong> all places with volcanic activity, ei<strong>the</strong>r terrestrial or mar<strong>in</strong>e (Stetter, 1996). In particular, <strong>the</strong>re are also large microbial populations at depth with<strong>in</strong> hydro<strong>the</strong>rmal vents, at ocean floor spread<strong>in</strong>g centres, where <strong>the</strong>y exploit <strong>the</strong> reduced chemicals <strong>in</strong> <strong>the</strong> hot vent fluid <strong>for</strong> energy and growth. <strong>The</strong>y are all procaryotes, i.e. small unicellular microorganisms without a nuclear membrane (chromosomal DNA be<strong>in</strong>g directly <strong>in</strong> contact with <strong>the</strong> cell cytoplasm). Procaryotes are divided <strong>in</strong>to two phylogenetically dist<strong>in</strong>ct doma<strong>in</strong>s (or empires): Bacteria and Archaea (Fig. I.3.2.1/1.). Interest<strong>in</strong>gly, <strong>the</strong> most hyper<strong>the</strong>rmophilic organisms at present all belong to <strong>the</strong> Archaea, <strong>the</strong> upper temperature limit <strong>for</strong> Bacteria be<strong>in</strong>g only (!) 95ºC (Stetter, 1996). It should be noted that Archaea liv<strong>in</strong>g at 110-113ºC actually have optimal growth
temperatures of 95-106ºC. This suggests that one or several critical factors prevent terrestrial life from proliferat<strong>in</strong>g efficiently even at 110ºC. This limit<strong>in</strong>g factor cannot be <strong>the</strong> requirement <strong>for</strong> liquid water, because <strong>the</strong>re are pressurised environments with liquid water at higher temperatures (<strong>the</strong> chimneys of hydro<strong>the</strong>rmal vents), but <strong>the</strong>y are sterile (Trent, 1984). An important factor prevent<strong>in</strong>g life at temperatures well above 110ºC is <strong>the</strong> <strong>the</strong>rmal <strong>in</strong>stability of some covalent bonds <strong>in</strong>volved <strong>in</strong> biological molecules. Prote<strong>in</strong>s can be very stable at high temperatures – some prote<strong>in</strong>s from hyper<strong>the</strong>rmophiles are still active <strong>in</strong> vitro after <strong>in</strong>cubation at 140ºC. To achieve such <strong>the</strong>rmostability, evolution has <strong>in</strong>creased <strong>the</strong> number of non-covalent <strong>in</strong>teractions between am<strong>in</strong>o-acids that ma<strong>in</strong>ta<strong>in</strong> <strong>the</strong> folded structure of <strong>the</strong> polypeptide cha<strong>in</strong>. <strong>The</strong>y have also managed to get rid of <strong>the</strong> am<strong>in</strong>o-acid asparag<strong>in</strong>e from <strong>the</strong>ir surface – asparag<strong>in</strong>e be<strong>in</strong>g rapidly deam<strong>in</strong>ated at such temperature (Hensel et al., 1992). <strong>The</strong> DNA double-helix is also very stable (at least up to 107ºC) as long as <strong>the</strong> two strands cannot freely rotate around each o<strong>the</strong>r, which corresponds to <strong>the</strong> <strong>in</strong>tracellular situation (Marguet & Forterre, 1994). However, DNA is chemically degraded at high temperature by different mechanisms, <strong>the</strong> most important be<strong>in</strong>g <strong>the</strong> removal of pur<strong>in</strong>e bases (depur<strong>in</strong>ation) and <strong>the</strong> subsequent cleavage of <strong>the</strong> strands at apur<strong>in</strong>ic sites (L<strong>in</strong>dahl, 1993). Accord<strong>in</strong>gly, powerful repair mechanisms should exist <strong>in</strong> hyper<strong>the</strong>rmophiles. In contrast to prote<strong>in</strong>s and DNA, RNA is highly unstable at high temperatures, because of <strong>the</strong> presence of a reactive OH group <strong>in</strong> <strong>the</strong> 2' position of <strong>the</strong> ribose moiety that can <strong>in</strong>duce <strong>the</strong> cleavage of <strong>the</strong> strands. It has been estimated, <strong>for</strong> example, that an RNA molecule of 2000 nucleotides (a typical length <strong>for</strong> a messenger RNA) should be cleaved <strong>in</strong> two pieces <strong>in</strong> about 2 s at 110ºC (Forterre, 1995). Regions sensitive to heatdestruction appear to be protected <strong>in</strong> <strong>the</strong> transfer and ribosomal RNA of hyper<strong>the</strong>rmophiles by methylation of <strong>the</strong> reactive oxygen. <strong>The</strong> actual stability of RNA <strong>in</strong> vivo is, however, unclear s<strong>in</strong>ce <strong>the</strong>rmodegradation is strongly <strong>in</strong>creased by physiological concentrations of magnesium but protected by monovalent salts. This po<strong>in</strong>t is under study and is of utmost importance <strong>in</strong> <strong>the</strong> problem of life at high temperatures. Many metabolites are also highly unstable at temperatures near <strong>the</strong> boil<strong>in</strong>g po<strong>in</strong>t of water, <strong>in</strong> particular <strong>the</strong> ribonucleotides which conta<strong>in</strong> energy-rich bonds, such as ATP (<strong>the</strong> energy supplier of <strong>the</strong> cell) or <strong>the</strong> substrates <strong>for</strong> DNA and RNA replication. Some energy-rich covalent bonds are already unstable <strong>in</strong> <strong>the</strong> temperature range typical of <strong>the</strong>rmophilic life (80-110ºC), but hyper<strong>the</strong>rmophiles have successfully developed orig<strong>in</strong>al strategies to bypass <strong>the</strong>se limitations. One of <strong>the</strong>m is channell<strong>in</strong>g successive reactions of a given biochemical pathway <strong>in</strong>to a set of physically associated enzymes. Ano<strong>the</strong>r is to <strong>in</strong>crease considerably <strong>the</strong> aff<strong>in</strong>ity and activity of enzymes deal<strong>in</strong>g with <strong>the</strong>rmolabile substrates. An important limit<strong>in</strong>g factor prevent<strong>in</strong>g life above 110ºC could be related to membrane permeability. Biological membranes become leaky at high temperature, allow<strong>in</strong>g <strong>the</strong> free passage of ions (Driessen et al., 1996). This is not compatible with <strong>the</strong> <strong>for</strong>mation of ionic gradients across membranes, which is a prerequisite to <strong>the</strong> build<strong>in</strong>g of energy transduction systems <strong>in</strong> liv<strong>in</strong>g organisms. In conclusion, unless future work on subterranean organisms breaks <strong>the</strong> upper temperature limit (see I.3.6), we are faced with two possibilities: ei<strong>the</strong>r terrestrial life has been unable <strong>in</strong> <strong>the</strong> course of its history to design strategies <strong>for</strong> liv<strong>in</strong>g at temperatures above 110ºC, or such design is impossible because of <strong>the</strong> <strong>in</strong>tr<strong>in</strong>sic limitations of carbon chemistry-based life on Earth. I.3.2.2. Low Temperatures <strong>Life</strong> is extremely diverse <strong>in</strong> <strong>the</strong> ocean at temperatures of 2ºC. Liv<strong>in</strong>g organisms, especially microorganisms, are also present <strong>in</strong> <strong>the</strong> frozen soils of arctic and alp<strong>in</strong>e environments (Russel, 1992). However, <strong>the</strong>ir optimal growth temperatures are usually well above <strong>the</strong> temperature of <strong>the</strong> site of isolation. Those organisms with optimal growth temperatures below 15ºC and m<strong>in</strong>imal growth temperatures below 0ºC are limits of life under extreme conditions/I.3 29
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: I.3 Limits of Life
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
Page 53 and 54: SP-1231 56 The iso
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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
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Page 75 and 76: SP-1231 78 I.7.3 Organic Chemistry
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II.1 Introduction The</stro
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II.2 The Planet Ma
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cover the range 4.
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The depletion of c
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valley networks (Fig. II.2.5/2) are
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plate tectonic evidence has been ob
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Table II.2.7.4/1. Potential Mars <s
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II.3 The Martian M
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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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