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

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provided that a depth below <strong>the</strong> strong oxidis<strong>in</strong>g conditions can be reached. This approach offers <strong>the</strong> possibility of determ<strong>in</strong><strong>in</strong>g a depth gradient of redox-sensitive geochemical parameters. This is particularly true <strong>for</strong> sulphur. In addition, <strong>the</strong> presence of sulphur phases <strong>in</strong> different oxidation states and <strong>the</strong>ir subsequent isotopic analysis will yield <strong>in</strong><strong>for</strong>mation on <strong>the</strong> mode of redox reactions, specifically a possible participation of biologically-controlled processes. <strong>The</strong> analytical approach could <strong>in</strong>clude previously applied techniques such as stepped combustion and isotopic analyses of result<strong>in</strong>g SO 2 fractions and sulphur isotope measurements us<strong>in</strong>g an ion microprobe. <strong>The</strong> full analytical spectrum, however, is still possible only <strong>in</strong> Earthbased laboratories, requir<strong>in</strong>g a sample-return mission. Spacecraft analytical facilities might, however, obta<strong>in</strong> sufficient data through combustion followed by isotope ratio mass spectrometry of sulphur-bear<strong>in</strong>g compounds (i.e. m<strong>in</strong>eral phases). It should be possible to per<strong>for</strong>m this type of analysis on surface and subsurface material. Sample material would have to be crushed (better: pulverised) be<strong>for</strong>e deliver<strong>in</strong>g it to <strong>the</strong> furnace. S<strong>in</strong>ce <strong>the</strong> sample material will likely conta<strong>in</strong> o<strong>the</strong>r oxidisable components, a chromatographic separation of <strong>the</strong> gas mixture (such as <strong>in</strong> elemental analysers with a conflow-setup) is a prerequisite <strong>in</strong> order to obta<strong>in</strong> reasonably pure SO 2 <strong>for</strong> mass spectrometric analyses. A temperature-based separation, analogous to <strong>the</strong> stepped combustion approach, might be used to study different m<strong>in</strong>eral phases of different sulphur oxidation states. In conclusion, a MODULUS-like <strong>in</strong>strumentation, based on chemical trans<strong>for</strong>mations, <strong>in</strong>clud<strong>in</strong>g stepped combustion and pyrolysis, coupled to gas chromatography -mass spectrometry techniques can fulfil most of <strong>the</strong> scientific objectives described <strong>in</strong> II.5.3. <strong>The</strong> analysis of <strong>in</strong>organic and organic compounds can be efficiently carried out by gas chromatography coupled to pyrolysis and mass spectrometry (PYR-GC-MS). II.5.9.1 Gas Chromatography (GC) Gas chromatography is a powerful technique <strong>for</strong> analys<strong>in</strong>g complex mixtures of gases or volatilisable constituents, and it will be a crucial component of <strong>the</strong> proposed exobiology <strong>in</strong>strumentation package. Schematically, a gas chromatograph <strong>in</strong>cludes: (i) a carrier gas reservoir and pressure controller; (ii) an <strong>in</strong>jection subsystem, where a sample of <strong>the</strong> mixture to be analysed is <strong>in</strong>troduced and carried by <strong>the</strong> carrier gas <strong>in</strong>to (iii) one or several (<strong>in</strong> series or <strong>in</strong> parallel) GC columns – <strong>the</strong> heart of <strong>the</strong> GC system, where <strong>the</strong> separation occurs – and (iv) a detector, connected at <strong>the</strong> exit of <strong>the</strong> column, able to <strong>in</strong>dicate systematically elution of compounds (o<strong>the</strong>r than <strong>the</strong> carrier gas) from <strong>the</strong> column(s). <strong>The</strong> carrier gas is usually He, N 2, Ar or H 2. It flows cont<strong>in</strong>uously through <strong>the</strong> column, carry<strong>in</strong>g <strong>the</strong> different constituents at different rates, depend<strong>in</strong>g on <strong>the</strong>ir chemical structures and molecular weights. <strong>The</strong> column can be a tube of a few metres length and one to a few mm <strong>in</strong>ternal diameter, packed with an absorbent (GSC: Gas Solid Chromatography) or with an absorbent impregnated with a stationary phase, a liquid of very low vapour pressure (GLC: Gas Liquid Chromatography). It can also be a longer (10-100 m) and open tube (wall-coated open tubular), with smaller <strong>in</strong>ternal diameter (0.5-0.1 mm), coated with a film of a solid absorbent (GSC) or of liquid phase (GLC). <strong>The</strong> chemical nature of <strong>the</strong> absorbent or <strong>the</strong> liquid phase is directly related to <strong>the</strong> chemical nature of <strong>the</strong> compounds to be analysed. <strong>The</strong> best universal detector is probably a mass spectrometer, able to provide <strong>the</strong> mass signature of each eluted compound and hence allow<strong>in</strong>g a secure identification of each GC peak. <strong>The</strong> GC-MS coupl<strong>in</strong>g requires that most of <strong>the</strong> carrier gas be removed from <strong>the</strong> sample be<strong>for</strong>e it enters <strong>the</strong> mass spectrometer. In <strong>the</strong> case where multi-GC columns are used <strong>in</strong> parallel, specific GC detectors must be used (<strong>in</strong> addition to MS coupl<strong>in</strong>g, as planned on Rosetta’s COSAC <strong>in</strong>strument). Nano-<strong>the</strong>rmal conductivity detectors (nano-TCDs) are commercially available <strong>in</strong> Europe, but <strong>the</strong>y need to be space qualified. Detectors able to team II: <strong>the</strong> search <strong>for</strong> chemical <strong>in</strong>dicators of life/II.5 II.5.9 Molecular Analysis 147
SP-1231 148 discrim<strong>in</strong>ate chiral compounds have recently been described (Bodenhoefer et al., 1997) (see Section II.5.10, below). <strong>The</strong>y also need to be studied. II.5.9.2 Mass Spectroscopy (MS) A variety of mass spectrometers (magnetic, quadrupole, time-of-flight, ion trap) are able to differentiate and analyse <strong>the</strong> result<strong>in</strong>g ions, depend<strong>in</strong>g on <strong>the</strong>ir mass-to-charge ratio. <strong>The</strong> ion trap spectrometer is best suited <strong>for</strong> use <strong>in</strong> conjunction with a gas chromatograph, because its optimum pressure conditions are much higher (about 10 -3 mbar) than those required with <strong>the</strong> o<strong>the</strong>r MS types (typically 10 -6 mbar or lower). O<strong>the</strong>r mass spectrometers could also be used if a powerful pump<strong>in</strong>g device is available (see below). II.5.9.3 Pyrolysis (PYR) Pyrolysis is a powerful technique <strong>for</strong> <strong>the</strong> analysis of non-volatile compounds of low <strong>the</strong>rmal stability, <strong>in</strong> particular organic compounds of high molecular weight, such as macromolecules or oligomers, when <strong>the</strong> <strong>the</strong>rmal pulse fractures <strong>the</strong> bonds between separate entities to produce recognisable sub-units (Irw<strong>in</strong>, 1982; Meuzelaar et al., 1982). <strong>The</strong>re are many pyrolysis variants, most of which readily lend <strong>the</strong>mselves to space applications. <strong>The</strong> simplest <strong>for</strong> space application is furnace pyrolysis, us<strong>in</strong>g a small oven. Combustion can also convert non-volatile material <strong>in</strong>to gaseous species. This method, when used <strong>in</strong> stepped fashion, can dist<strong>in</strong>guish between different <strong>for</strong>ms, e.g. organics vs carbonate, sulphides vs sulphates. Because it is quantitative, it gives absolute abundances of elements of biological significance (e.g. C, H, N, S) and allows bulk isotopic compositions to established. It should be po<strong>in</strong>ted out that <strong>the</strong> use of pyrolysis techniques with small ovens could also allow differential <strong>the</strong>rmal analysis measurements to be carried out. This is a powerful tool <strong>for</strong> determ<strong>in</strong><strong>in</strong>g <strong>the</strong> nature of <strong>the</strong> source of volatiles (such as H 2O, SO 2, CO 2), which are released dur<strong>in</strong>g stepped heat<strong>in</strong>g of <strong>the</strong> samples, and thus to <strong>in</strong>fer, <strong>in</strong> particular, <strong>the</strong> molecular structure of <strong>in</strong>organic compounds. II.5.9.4 Available PYR-GC-MS Techniques Chemical sensors based on GC and MS <strong>in</strong>strumentation have already been used <strong>in</strong> atmospheric probes and surface landers <strong>for</strong> analys<strong>in</strong>g extraterrestrial environments, <strong>in</strong>clud<strong>in</strong>g Venus and Mars surface materials. Until recently, <strong>the</strong>y <strong>in</strong>volved only chromatographic packed columns with ma<strong>in</strong>ly magnetic, <strong>the</strong>n quadrupole, MS. <strong>The</strong> Titan atmosphere Huygens Probe of <strong>the</strong> Cass<strong>in</strong>i/Huygens mission launched on 15 October 1997 to reach <strong>the</strong> Saturnian system <strong>in</strong> 2004 <strong>in</strong>cludes a GC-MS <strong>in</strong>strument (Niemann et al., 1997) coupled to an aerosol collector and pyrolyser (ACP) (Israel et al., 1997). It will determ<strong>in</strong>e <strong>the</strong> nature and abundance of <strong>the</strong> organic and <strong>in</strong>organic compounds present <strong>in</strong> Titan’s atmospheric gas phase and aerosols. This will be <strong>the</strong> second GC-MS <strong>in</strong>strument, and PYR-GC-MS <strong>in</strong>strument, ever flown <strong>in</strong> a planetary mission (after Vik<strong>in</strong>g). <strong>The</strong> GC subsystem uses three GC columns <strong>in</strong> parallel: two wall-coated open tubular (WCOT) columns and one micro-packed capillary column based on up-to-date chromatographic technology. One WCOT capillary is 10 m-long (MXT type, Restek) of very narrow bore (0.15 mm), with special sta<strong>in</strong>less steel tub<strong>in</strong>g (Silcosteel) (Afflalaye et al., 1996). It allows separation of C3 and higher hydrocarbons and nitriles. <strong>The</strong> o<strong>the</strong>r (14 m × 0.18 mm <strong>in</strong>ternal diameter) uses an adsorbent material newly developed <strong>for</strong> capillary GC applications: glassy carbon. It allows <strong>the</strong> quantitative analysis of C1-C3 hydrocarbons. <strong>The</strong> third column (2m × 0.75 mm i.d), packed with Carboxen, allows <strong>the</strong> separation of permanent gases, <strong>in</strong> particular CO and N 2 (Afflalaye et al., 1997). <strong>The</strong> MS subsystem is a quadrupole mass spectrometer with five ion sources: three are dedicated to each of <strong>the</strong> GC columns, one allows direct MS analysis of <strong>the</strong> atmosphere, and <strong>the</strong> fifth is dedicated to direct MS analysis of ACP samples. <strong>The</strong> ACP <strong>in</strong>strument <strong>in</strong>cludes a collector, which will collect Titan’s atmospheric aerosols on a filter, an oven where <strong>the</strong> filter is <strong>in</strong>troduced after <strong>the</strong> collect<strong>in</strong>g phase <strong>for</strong> heat<strong>in</strong>g <strong>the</strong> sample at different temperatures, and a
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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
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I.3 Limits of Life
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temperatures of 95-106ºC. This sug
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(mainly found <str
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cannot exclude fin
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Responses to Cosmic Radiation <stro
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acid to identify the</stron
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Horneck, G. (1993). Responses of Ba
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SP-1231 Fig. I.4.2.2/1. Size distri
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SP-1231 Fig. I.4.2.2/4. Evaporite w
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SP-1231 Fig. I.4.2.3/1A (left). Net
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SP-1231 48 A detailed chemical anal
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SP-1231 Fig. I.4.3.1.1/1. Scheme of
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SP-1231 Fig. I.4.3.1.2/1. A: Compar
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SP-1231 54 I.4.3.2.1 Sedimentary Or
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SP-1231 56 The iso
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SP-1231 Fig. I.4.3.2.3/1. Cholester
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SP-1231 60 References regard to non
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SP-1231 62 Klein,
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SP-1231 64 ities in</strong
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SP-1231 Fig. I.5.1.1/1. A 34×42 km
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SP-1231 Fig. I.5.2.1/1. Titan’s a
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SP-1231 Fig. I.5.2.2/2. Modelled Ti
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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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