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

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II THE SEARCH FOR LIFE ON MARS
II.1 Introduction <strong>The</strong> Vik<strong>in</strong>g Lander experiments failed to detect any organic matter <strong>in</strong> <strong>the</strong> martian soil, ei<strong>the</strong>r at <strong>the</strong> surface or from samples collected a few centimetres below <strong>the</strong> surface. <strong>The</strong> <strong>in</strong>dications were that strong oxidation processes were at work at <strong>the</strong> surface. Subsequent <strong>the</strong>oretical studies have shown that photochemical processes, as well as <strong>the</strong> effects of oxidants such as hydrogen peroxide, are likely to be responsible <strong>for</strong> <strong>the</strong> destruction of all such material <strong>in</strong> <strong>the</strong> surface region. That will be true whe<strong>the</strong>r <strong>the</strong> organics are of an <strong>in</strong>tr<strong>in</strong>sic martian orig<strong>in</strong> or have been imported via carbonaceous meteorite and micrometeorite bombardment over geological time. In order, <strong>the</strong>re<strong>for</strong>e, to have any chance of detect<strong>in</strong>g and analys<strong>in</strong>g <strong>in</strong>tr<strong>in</strong>sic organic material on Mars, it will be necessary to obta<strong>in</strong> samples from several centimetres below <strong>the</strong> chemically-affected ‘r<strong>in</strong>d’ that covers <strong>the</strong> surface rocks. Similarly, if we are to explore <strong>the</strong> true regolith, it will be necessary to drill deep below <strong>the</strong> surface <strong>in</strong> order to escape <strong>the</strong> likely effects of <strong>the</strong> oxidant diffusion reactions. How deep is unknown and probably cannot be accurately modelled, because <strong>the</strong> effects of regolith ‘churn<strong>in</strong>g’ are highly uncerta<strong>in</strong>. <strong>The</strong> approach adopted <strong>in</strong> this study was to aim <strong>for</strong> subsurface drill<strong>in</strong>g to <strong>the</strong> maximum depth that is technically feasible on a given mission. Typically, this will be at least 1 m. <strong>The</strong> chances of a successful detection of organics are greatly improved if <strong>the</strong> land<strong>in</strong>g site is selected to be a major sedimentary bas<strong>in</strong>, composed of highly compacted dense material. Not only will <strong>the</strong> oxidant penetration be more limited but also, by analogy with <strong>the</strong> situation on Earth, such sites are more likely to have hosted life at some earlier epoch. <strong>The</strong>se comb<strong>in</strong>ed aspects (oxidant-free samples collected from sedimentary doma<strong>in</strong>s) must be satisfied if <strong>the</strong>re is to be any reasonable chance of f<strong>in</strong>d<strong>in</strong>g organics on Mars and of <strong>in</strong>creas<strong>in</strong>g <strong>the</strong> likelihood of observ<strong>in</strong>g fossils of ext<strong>in</strong>ct life. To improve those chances fur<strong>the</strong>r, mobility is necessary to extend <strong>the</strong> locations <strong>for</strong> such sampl<strong>in</strong>g. Toge<strong>the</strong>r, <strong>the</strong>se three requirements are fundamental to any future search <strong>for</strong> evidence of life on Mars. While excit<strong>in</strong>g <strong>in</strong> itself, simply detect<strong>in</strong>g some basic organic compounds or observ<strong>in</strong>g some morphology set that looks like bacteria are unlikely on <strong>the</strong>ir own to confirm unambiguously that life once existed on Mars. Much of <strong>the</strong> discussion <strong>in</strong> Part I, which is extended here, is concerned with establish<strong>in</strong>g a m<strong>in</strong>imum set of <strong>in</strong>terlock<strong>in</strong>g measurements and observations that, taken toge<strong>the</strong>r, are likely to provide conv<strong>in</strong>c<strong>in</strong>g evidence <strong>for</strong> or aga<strong>in</strong>st <strong>the</strong> existence of ext<strong>in</strong>ct, or extant, life <strong>in</strong> <strong>the</strong>se samples. <strong>The</strong> core of <strong>the</strong>se experiments concerns <strong>the</strong> search <strong>for</strong> <strong>in</strong>dicators or requirements <strong>for</strong> extant or ext<strong>in</strong>ct life. <strong>The</strong>se analyses will <strong>in</strong>clude not only <strong>the</strong> search <strong>for</strong> carbonaceous residues and organics, but also particular <strong>in</strong>organic m<strong>in</strong>erals such as carbonates and phosphates, which can serve as biomarkers. <strong>The</strong> detection of water and its depth profile is obviously important. So, too, is <strong>the</strong> accurate determ<strong>in</strong>ation of <strong>the</strong> isotopic ratios <strong>for</strong> hydrogen, carbon, oxygen, sulphur and nitrogen, which can give clear evidence of biologically mediated fractionations. Besides <strong>the</strong> chemical and spectroscopic equipment needed <strong>for</strong> those analyses, microscopes are required to study m<strong>in</strong>eral structures and to search <strong>for</strong> evidence of fossils, on scales rang<strong>in</strong>g from about 1 µm up to 1 mm. Observations and visual selection is also a prerequisite <strong>for</strong> subsequent sophisticated chemical and spectroscopic analysis, especially of localised areas. It can be argued that <strong>in</strong> situ analysis could never match <strong>the</strong> results us<strong>in</strong>g samples returned from Mars, so why not wait until that opportunity arises? Apart from <strong>the</strong> S. Lee (Univ. Colorado), J. Bell (Cornell Univ.), M. Wolff (Space Science Inst.) and NASA. 81
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SP-1231 Exobiology
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Cover Fossil coccoid bacteria, 1 µ
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4 I.5 Potential Non-Martian Sites <
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6 6.2 Imaging of F
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The Exobio
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pyrolysis, or similar techniques, f
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Ignasi Casanova, Institute
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I AN EXOBIOLOGICAL VIEW OF THE SOLA
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SP-1231 18 surface pressure of CO 2
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SP-1231 20 10 bar befor</st
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SP-1231 22 kilometres in</s
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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
Page 53 and 54: SP-1231 56 The iso
Page 55 and 56: SP-1231 Fig. I.4.3.2.3/1. Cholester
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: SP-1231 78 I.7.3 Organic Chemistry
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
Page 97 and 98: indicated by carbo
Page 99 and 100: quest for
Page 101 and 102: Anders, E. (1989). Prebiotic organi
Page 103 and 104: Urey, H.C. (1968). Origin</
Page 105 and 106: SP-1231 Fig. II.4.1.1/1. A Gilbert-
Page 107 and 108: SP-1231 Fig. II.4.1.3/1. Th
Page 109 and 110: SP-1231 114 Sinuou
Page 111 and 112: SP-1231 Fig. II.4.3/1. Ages of seve
Page 113 and 114: SP-1231 118 orbital characteristics
Page 115 and 116: SP-1231 120 II.4.5 The</str
Page 117 and 118: SP-1231 122 II.4.6 Rovers and Drill
Page 119 and 120: SP-1231 124 Site 2: Apolin<
Page 121 and 122: SP-1231 126 Site 4: Chasma area, ea
Page 123 and 124: SP-1231 128 Sharp, M., Parkes, J.,
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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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