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 and radiation environment, as well as organic molecules <strong>in</strong> sediments. <strong>The</strong> search <strong>for</strong> possible biological oases will be connected with <strong>the</strong> detection of areas where liquid water still exists under <strong>the</strong> current conditions on that planet. By analogy with terrestrial ecosystems, potential protected niches have been postulated <strong>for</strong> Mars, such as sulphur-rich subsurface areas <strong>for</strong> chemoautotrophic communities, rocks <strong>for</strong> endolithic communities, permafrost regions, hydro<strong>the</strong>rmal vents, soil and evaporite crystals. Much <strong>in</strong><strong>for</strong>mation can be obta<strong>in</strong>ed from remote-sens<strong>in</strong>g global measurements, such as <strong>the</strong> seasonal atmospheric and surface water distribution, <strong>the</strong> m<strong>in</strong>eralogical <strong>in</strong>ventory and distribution, geomorphologic features obta<strong>in</strong>ed with high spatial resolution, <strong>the</strong>rmal mapp<strong>in</strong>g of potential volcanic regions to determ<strong>in</strong>e possible geo<strong>the</strong>rmally active sites, and trace gases such as H 2, H 2S, CH 4, SOx, and NOx. It is only after identify<strong>in</strong>g sites that are suitable environments <strong>for</strong> a potential martian biota that landers are required <strong>for</strong> more detailed characterisations. <strong>The</strong>n analyses of <strong>the</strong> elemental and isotopic compositions, of m<strong>in</strong>eral composition and structure, and of organic compounds can follow. Concern<strong>in</strong>g Mars, it is also very important to explore and understand <strong>the</strong> strong oxidative processes on <strong>the</strong> surface. If <strong>the</strong> <strong>in</strong> situ <strong>in</strong>vestigations confirm that <strong>the</strong> sites constitute potential biological oases, detailed searches <strong>for</strong> direct and <strong>in</strong>direct biomarkers should follow. <strong>The</strong> search protocol, i.e. what methods to apply, depend largely on <strong>the</strong> environmental conditions offered by <strong>the</strong> sites. Alternatively, <strong>the</strong> analysis of samples collected and returned to Earth should be considered. This would allow much more crucial tests <strong>for</strong> extant biology <strong>in</strong> ground-based laboratories. However, <strong>in</strong> <strong>the</strong> latter case, a str<strong>in</strong>gent planetary protection programme is required. I.4.3.1 Paleontological Evidence Given <strong>the</strong> validity of terrestrial analogues, <strong>the</strong> stratigraphic (sedimentary) record of any planet should serve as a potential store <strong>for</strong> both bodily remnants of <strong>for</strong>mer organisms and o<strong>the</strong>r possible traces of <strong>the</strong>ir life activities. This should also apply to Mars dur<strong>in</strong>g <strong>the</strong> early stages of its history, when <strong>the</strong> planet may have been ba<strong>the</strong>d <strong>in</strong> abundant water and environmental conditions on <strong>the</strong> surface did not differ much from those on <strong>the</strong> early Earth (Carr & Wänke, 1992; Carr, 1996). If all terrestrial planets – and notably Mars and Earth – had occupied comparable start<strong>in</strong>g positions <strong>in</strong> terms of solar distance, condensation history and primary endowment with matter from <strong>the</strong> parent solar nebula (Moroz & Mukh<strong>in</strong>, 1978), <strong>the</strong>n <strong>the</strong> surface conditions on both planets should have been very similar <strong>in</strong> <strong>the</strong>ir juvenile states. Specifically, with evidence at hand <strong>for</strong> a denser atmosphere and extensive aqueous activity on <strong>the</strong> martian surface dur<strong>in</strong>g <strong>the</strong> planet’s early history, a conv<strong>in</strong>c<strong>in</strong>g case can be made that <strong>the</strong> primitive martian environment was no less conducive to <strong>the</strong> <strong>in</strong>itiation of life processes and <strong>the</strong> subsequent emplacement of prolific microbial ecosystems, than <strong>the</strong> surface of <strong>the</strong> ancient Earth (McKay, 1991; McKay & Stoker, 1989). Even if <strong>the</strong> evolutionary pathways of both planets diverged dur<strong>in</strong>g <strong>the</strong>ir later histories, so that life became ext<strong>in</strong>ct on Mars as <strong>the</strong> gradual deterioration of surface conditions rendered <strong>the</strong> planet <strong>in</strong>hospitable to prote<strong>in</strong> chemistry <strong>in</strong> <strong>the</strong> widest sense, Mars could still have started off with a veneer of microbial (prokaryotic) life comparable to that exist<strong>in</strong>g on <strong>the</strong> Archaean Earth (Schopf, 1983; Schidlowski, 1993a). Given <strong>the</strong> apparent failure of <strong>the</strong> Vik<strong>in</strong>g life-detection experiment <strong>for</strong> <strong>the</strong> present martian regolith (Biemann et al., 1977), <strong>the</strong> prime objective of a search <strong>for</strong> life on Mars should be to seek evidence of ext<strong>in</strong>ct (fossil) life. <strong>The</strong> oldest Martian sediments would be appropriate targets <strong>for</strong> such ef<strong>for</strong>ts. In any search <strong>for</strong> ext<strong>in</strong>ct life on Mars, <strong>the</strong> basic problem would not rest with <strong>the</strong> cognitive aspects of <strong>the</strong> identification of <strong>the</strong> fossil evidence, but ra<strong>the</strong>r with <strong>the</strong> serendipity <strong>in</strong>evitably <strong>in</strong>volved <strong>in</strong> <strong>the</strong> selection and recovery of suitable sampl<strong>in</strong>g material from <strong>the</strong> vast stretches of potential host rocks exposed on <strong>the</strong> planetary surface. Among <strong>the</strong> two-thirds of <strong>the</strong> martian surface deemed to be covered by rocks older than 3.8 Gyr (McKay, 1986), <strong>the</strong>re are several occurrences of well-bedded I.4.3 Evidence of Ext<strong>in</strong>ct (Fossil) Extraterrestrial <strong>Life</strong> 49
SP-1231 Fig. I.4.3.1.1/1. Scheme of pr<strong>in</strong>cipal morphologies of lam<strong>in</strong>ated microbial ecosystems that thrive at <strong>the</strong> sediment-water <strong>in</strong>terface. <strong>The</strong>se structures subsequently lend <strong>the</strong>mselves to lithification <strong>in</strong> <strong>the</strong> <strong>for</strong>m of ‘stromatolites’ (see Fig. I.4.3.1.1/2). <strong>The</strong> mat<strong>for</strong>m<strong>in</strong>g is mostly made up of cyanobacteria. Fig. I.4.3.1.1/2. Typical microbialite (or ‘stromatolite) from <strong>the</strong> Precambrian Transvaal Dolomite (~2.3 Gyr) that is made up of a succession of superimposed fossil microbial mats. <strong>The</strong> bun-shaped and partially <strong>in</strong>terfer<strong>in</strong>g lam<strong>in</strong>ae represent consecutive growth stages of <strong>the</strong> primary microbial community. 50
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
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Page 23 and 24: SP-1231 24 Laboratory Investigation
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Page 27 and 28: temperatures of 95-106ºC. This sug
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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
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Page 43 and 44: SP-1231 Fig. I.4.2.3/1A (left). Net
Page 45: SP-1231 48 A detailed chemical anal
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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
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Page 79 and 80: II.2 The Planet Ma
Page 81 and 82: cover the range 4.
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Page 93 and 94: principal atmosphe
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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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