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

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I.2 Chemical Evolution <strong>in</strong> <strong>the</strong> <strong>Solar</strong> <strong>System</strong> Primitive terrestrial life probably emerged <strong>in</strong> water, with <strong>the</strong> first chemical systems able to transfer <strong>the</strong>ir molecular <strong>in</strong><strong>for</strong>mation and to evolve. Un<strong>for</strong>tunately, <strong>the</strong> direct clues that may help chemists to identify <strong>the</strong> molecules that participated <strong>in</strong> <strong>the</strong> emergence of life on Earth about 4000 million years ago have been erased by <strong>the</strong> effects of plate tectonics, <strong>the</strong> permanent presence of runn<strong>in</strong>g water, <strong>the</strong> unshielded UV radiation from <strong>the</strong> Sun and <strong>the</strong> oxygen produced by life itself. By analogy with contemporary life, it is generally believed that primitive life orig<strong>in</strong>ated from <strong>the</strong> process<strong>in</strong>g of reduced organic molecules by liquid water. Primitive life may also have started based on m<strong>in</strong>eral crystals. This idea has been developed by Cairns-Smith (1982) but still needs to be supported by experiments. I.2.1.1 Terrestrial Production of Reduced Organic Molecules Opar<strong>in</strong> (1924) suggested that <strong>the</strong> small reduced organic molecules needed <strong>for</strong> primitive life were <strong>for</strong>med <strong>in</strong> an early atmosphere dom<strong>in</strong>ated by methane. <strong>The</strong> idea was tested <strong>in</strong> <strong>the</strong> laboratory by Miller (1953) when he exposed a mixture of methane, ammonia, hydrogen and water to electrical discharges. In his <strong>in</strong>itial experiment, he obta<strong>in</strong>ed four of <strong>the</strong> 20 naturally occurr<strong>in</strong>g am<strong>in</strong>o acids, via <strong>the</strong> <strong>in</strong>termediary <strong>for</strong>mation of hydrogen cyanide and <strong>for</strong>maldehyde. Miller’s laboratory syn<strong>the</strong>sis of am<strong>in</strong>o acids occurs efficiently when a reduc<strong>in</strong>g gas mixture conta<strong>in</strong><strong>in</strong>g significant amounts of hydrogen is used. However, <strong>the</strong> true composition of <strong>the</strong> primitive Earth’s atmosphere rema<strong>in</strong>s unknown. <strong>The</strong> source of <strong>the</strong> primitive terrestrial atmosphere can be found <strong>in</strong> a comb<strong>in</strong>ation of <strong>the</strong> volatiles CO 2, N 2 and H 2O trapped <strong>in</strong> <strong>the</strong> rocks that comprised <strong>the</strong> bulk of <strong>the</strong> planet’s mass (<strong>in</strong>ternal reservoir) and a late-accret<strong>in</strong>g veneer of extraterrestrial material (external reservoir). <strong>The</strong> <strong>for</strong>mation of <strong>the</strong> <strong>in</strong>ternal reservoir is a natural consequence of <strong>the</strong> planet’s accretion. We<strong>the</strong>rhill (1990) suggested that collisions with planetary embryos <strong>in</strong> eccentric orbits should have thoroughly homogenised <strong>the</strong> rocky composition of Venus, Earth and Mars. <strong>The</strong> late-accret<strong>in</strong>g veneer contributed by some comb<strong>in</strong>ation of volatile-rich meteorites and comets should also have been uni<strong>for</strong>m from planet to planet. A volative-rich veneer may have replaced <strong>the</strong> atmosphere produced by <strong>the</strong> planet’s early accretion, which was subsequently blown off by <strong>the</strong> giant impact between <strong>the</strong> Earth and a Mars-sized planetary embryo that generated <strong>the</strong> Moon and caused <strong>the</strong> early atmosphere massive hydrodynamic loss. <strong>The</strong> abundance and isotopic ratios of noble gases neon, argon, krypton and xenon suggest that meteorites alone or <strong>in</strong> comb<strong>in</strong>ation with planetary rocks could not have produced <strong>the</strong> Earth’s entire volatile <strong>in</strong>ventory and that a significant contribution from icy planetesimals was required (Owen & Bar-Nun, 1995). <strong>The</strong> surface temperature of <strong>the</strong> early Earth was very much dependent upon <strong>the</strong> partial pressure of CO 2 <strong>in</strong> <strong>the</strong> atmosphere. <strong>The</strong> dom<strong>in</strong>ant view <strong>in</strong> recent years has been that <strong>the</strong> early atmosphere was a weakly reduc<strong>in</strong>g mixture of CO 2, N 2 and H 2O, comb<strong>in</strong>ed with lesser amounts of CO and H 2 (Kast<strong>in</strong>g, 1993). An efficient greenhouse effect <strong>in</strong>duced by a high CO 2 partial pressure was <strong>the</strong> most likely mechanism capable of compensat<strong>in</strong>g <strong>for</strong> <strong>the</strong> fa<strong>in</strong>t young Sun, <strong>the</strong> lum<strong>in</strong>osity of which was about 30% lower than today. <strong>The</strong> dom<strong>in</strong>ant s<strong>in</strong>k <strong>for</strong> atmospheric CO 2 is silicate wea<strong>the</strong>r<strong>in</strong>g on land and subsequent <strong>for</strong>mation and deposition of carbonate sediments <strong>in</strong> <strong>the</strong> ocean. However, a negative feedback system probably ma<strong>in</strong>ta<strong>in</strong>ed <strong>the</strong> Earth’s surface temperature above freez<strong>in</strong>g: if <strong>the</strong> temperature dropped below freez<strong>in</strong>g, silicate wea<strong>the</strong>r<strong>in</strong>g would slow down and volcanic CO 2 would accumulate <strong>in</strong> <strong>the</strong> atmosphere. Walker (1985) suggested that <strong>the</strong> CO 2 partial pressure could have been as high as I.2.1 Terrestrial Prebiotic Chemistry 19
SP-1231 20 10 bar be<strong>for</strong>e <strong>the</strong> emergence of <strong>the</strong> cont<strong>in</strong>ents because of limited silicate wea<strong>the</strong>r<strong>in</strong>g and cont<strong>in</strong>ental carbonate rocks storage. Under <strong>the</strong>se conditions, <strong>the</strong> early Earth could have been as hot as 85ºC. Recent analysis of samarium/neodium ratios <strong>in</strong> <strong>the</strong> ancient zircons <strong>in</strong>dicate that <strong>the</strong> cont<strong>in</strong>ent grew rapidly, so a dense CO 2 atmosphere may have been restricted to <strong>the</strong> first few hundreds million of years of <strong>the</strong> Earth’s history. Recently, Wächtershäuser (1994) has suggested that <strong>the</strong> carbon source <strong>for</strong> life was carbon dioxide. <strong>The</strong> energy source required to reduce <strong>the</strong> carbon dioxide is proposed to be provided by <strong>the</strong> oxidative <strong>for</strong>mation of pyrite (FeS 2) from iron sulphide (FeS) and hydrogen sulphide. Pyrite has positive surface charges and bonds <strong>the</strong> products of carbon dioxide reduction, giv<strong>in</strong>g rise to a 2D reaction system, a surface metabolism. Experiments are be<strong>in</strong>g per<strong>for</strong>med <strong>in</strong> order to test this new hypo<strong>the</strong>sis. Deep-sea hydro<strong>the</strong>rmal systems may also represent likely environments <strong>for</strong> <strong>the</strong> syn<strong>the</strong>sis of prebiotic organic molecules. Experiments have been carried out <strong>in</strong> order to test whe<strong>the</strong>r am<strong>in</strong>o acids can be <strong>for</strong>med under conditions simulat<strong>in</strong>g <strong>the</strong> hydro<strong>the</strong>rmal alteration of oceanic crust (We<strong>the</strong>rhill, 1990). I.2.1.2 Extraterrestrial Delivery of Organic Molecules to <strong>the</strong> Earth <strong>The</strong> import of extraterrestrial organic molecules is a subject of <strong>in</strong>creas<strong>in</strong>g <strong>in</strong>terest. <strong>The</strong> study of meteorites, particularly <strong>the</strong> carbonaceous chondrites, conta<strong>in</strong><strong>in</strong>g up to 5% by weight of organic matter, has allowed close exam<strong>in</strong>ation of extraterrestrial organic material. Eight prote<strong>in</strong>aceous am<strong>in</strong>o-acids have been identified <strong>in</strong> <strong>the</strong> Murchison meteorite amongst more than 70 am<strong>in</strong>o acids (Cron<strong>in</strong> & Pizzarello, 1983). <strong>The</strong>se am<strong>in</strong>o acids are asymmetric and <strong>the</strong> two handedness <strong>for</strong>ms L and D, are generally found <strong>in</strong> equal proportions. However, Engel (1990) reported that L-alan<strong>in</strong>e was 18% more abundant that D-alan<strong>in</strong>e <strong>in</strong> one sample of <strong>the</strong> Murchison meteorite. This ra<strong>the</strong>r surpris<strong>in</strong>g result has been recently confirmed by Cron<strong>in</strong> (1996). <strong>The</strong> latter found that one-handedness was <strong>in</strong> excess of about 10% <strong>for</strong> isoval<strong>in</strong>e, a-methyl norval<strong>in</strong>e and a-methyl isoleuc<strong>in</strong>e. <strong>The</strong>se excesses found <strong>in</strong> <strong>the</strong> Murchison meteorite may help us to understand <strong>the</strong> eventual emergence of a one-handed (homochiral) primitive life. Indeed, homochirality is now believed to be not just a consequence of life, but also a prerequisite <strong>for</strong> life, because stereoregular polymers such as a-sheet polypeptides do not <strong>for</strong>m with mixtures of am<strong>in</strong>o acids of both handedness. <strong>The</strong> excess of one-handed am<strong>in</strong>o acids found <strong>in</strong> <strong>the</strong> Murchison meteorite may, however, result from <strong>the</strong> process<strong>in</strong>g of <strong>the</strong> organic mantles of <strong>in</strong>terstellar gra<strong>in</strong>s by circularly polarised synchrotron radiation from a neutron star remnant of a supernova (Bonner & Rubenste<strong>in</strong>, 1987). A large collection of micrometeorites has been recently extracted from Antarctic old blue ice and analysed by Maurette (1995). A high percentage of unmelted chondritic micrometeorites of 50-100 mm size has been observed, <strong>in</strong>dicat<strong>in</strong>g that a large fraction crossed <strong>the</strong> terrestrial atmosphere without suffer<strong>in</strong>g drastic <strong>the</strong>rmal treatment. In this size range, <strong>the</strong> carbonaceous micrometeorites represent 80% of <strong>the</strong> samples and conta<strong>in</strong> 7% of carbon. <strong>The</strong>y may have brought about 10 20 g of carbon to <strong>the</strong> Earth over a period of 300 million years, correspond<strong>in</strong>g to <strong>the</strong> late terrestrial bombardment phase. This delivery represents more carbon than that engaged <strong>in</strong> <strong>the</strong> surficial biomass, i.e. about 10 18 g. Am<strong>in</strong>o acids such as a-am<strong>in</strong>o isobutyric acid have been recently identified <strong>in</strong> <strong>the</strong>se Antarctic micrometeorites, which may have functioned as t<strong>in</strong>y chondritic chemical reactors when reach<strong>in</strong>g oceanic water. For many decades, it was believed that primitive life emerged as a cell, thus requir<strong>in</strong>g boundary molecules such as phospholipids, catalytic molecules such as prote<strong>in</strong> enzymes and <strong>in</strong><strong>for</strong>mative molecules such as nucleic acids. Vesicle-<strong>for</strong>m<strong>in</strong>g fatty acids have been identified <strong>in</strong> <strong>the</strong> Murchison meteorite (Deamer, 1985). Primitive membranes could also have <strong>in</strong>itially been <strong>for</strong>med by simple isoprene derivatives (Ourisson & Nakatani, 1994). Selective condensation of am<strong>in</strong>o acids <strong>in</strong> <strong>the</strong> presence of liquid water has been experimentally documented. When hydrophobic and hydrophilic am<strong>in</strong>o acids coexist with<strong>in</strong> <strong>the</strong> same polypeptide cha<strong>in</strong>, <strong>the</strong> duality generates <strong>in</strong>terest<strong>in</strong>g topologies such as stereoselective and <strong>the</strong>rmostable a-sheet structures. Short peptides have also been shown to exhibit catalytic properties (Brack, 1993).
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: SP-1231 18 surface pressure of CO 2
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 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
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SP-1231 72 Galileo: images availabl
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SP-1231 74 TABLE I.6.2/1 EVIDENCE O
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SP-1231 I.6.3 What to Searc
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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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