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

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iii. specific oxygen electrode (O 2 produced by H 2O 2 decomposition). This last method uses <strong>the</strong> enzymatic reduction of H 2O 2 with catalase: H 2O 2 → H 2O + 1/ 2 O 2 followed by <strong>the</strong> quantitative measurement of <strong>the</strong> produced oxygen by an O 2 electrode (diffusion of O 2 through <strong>the</strong> Teflon membrane of <strong>the</strong> electrode and its reduction on <strong>the</strong> plat<strong>in</strong>um electrode). This last technique can easily be m<strong>in</strong>iaturised (microcathode), has a high selectivity to H 2O 2 (enzyme) and <strong>the</strong>n to O 2 (selective permeability of a Teflon membrane). Although its space qualification is probably not easy (liquid handl<strong>in</strong>g, temperature dependence, need <strong>for</strong> <strong>in</strong>ternal calibration with standard solutions, cathode passivation, enzyme utilisation), it seems to be a possible way <strong>for</strong> space application and should be explored. New technological developments are under study at NASA’s Ames Research Center, based on specific detectors, to analyse different k<strong>in</strong>d of oxidants, <strong>in</strong>clud<strong>in</strong>g H 2O 2. It is recommended that ESA should make sure <strong>the</strong> technology is available from European sources. II.5.10.1 Bulk Chirality Measurements A measurement of bulk optical rotation could be provided by <strong>the</strong> ‘SETH Cigar’, a m<strong>in</strong>iaturised space polarimeter (MacDermott et al., 1996). Mov<strong>in</strong>g parts are avoided by replac<strong>in</strong>g <strong>the</strong> normal rotat<strong>in</strong>g analys<strong>in</strong>g polarisers by multiple fixed polarisers at different angles, used with a diode array detector. Measurements at several different wavelengths, us<strong>in</strong>g three or four different diode sources, could provide clues to <strong>the</strong> team II: <strong>the</strong> search <strong>for</strong> chemical <strong>in</strong>dicators of life/II.5 Fig. II.5.10.1/1. Overall view chromatogram <strong>for</strong> some diols, one alcohol and three derivatised am<strong>in</strong>es by <strong>the</strong> use of a G-TA Cyclodextr<strong>in</strong> phase, 10 m × 0.25 mm <strong>in</strong>ternal diameter, (T = 3 m<strong>in</strong> at 85ºC, 15ºC/m<strong>in</strong> to 115ºC, 5 m<strong>in</strong> at 115ºC, p = 60 kPa). (Courtesy W. Thiemann and U. Meierhenrich) II.5.10 Chirality Measurements 151
SP-1231 Fig. II.5.10.2/2. Enantiomers of trifluoracetylated am<strong>in</strong>o acids (alan<strong>in</strong>e, leuc<strong>in</strong>e, aspartic acid and methion<strong>in</strong>e) are basel<strong>in</strong>e separated by <strong>the</strong> use of a Chirasil-L-Val phase, 25 m × 0.25 mm <strong>in</strong>ternal diameter, Chrompack. (Courtesy W. Thiemann and U. Meierhenrich) 152 identity of <strong>the</strong> soil constituent responsible <strong>for</strong> <strong>the</strong> optical activity. Complex<strong>in</strong>g agents could be used <strong>in</strong> <strong>the</strong> solution to enhance <strong>the</strong> optical rotation of important target molecules such as am<strong>in</strong>o acids. II.5.10.2 Enantiomeric Separations Enantiomers can be separated us<strong>in</strong>g chiral GC columns, such as <strong>the</strong> COSAC GC-MS system, which will use pairs of columns: one has a homo chiral coat<strong>in</strong>g, while <strong>the</strong> o<strong>the</strong>r has a racemic coat<strong>in</strong>g of <strong>the</strong> same substance. If two peaks are obta<strong>in</strong>ed through <strong>the</strong> homo chiral column but only one through <strong>the</strong> racemic column, <strong>the</strong>y are very likely enantiomers, and <strong>the</strong> D/L ratio can be obta<strong>in</strong>ed from <strong>the</strong> peak areas. However, different columns are required to separate different enantiomers, and am<strong>in</strong>o acids require derivatisation, which is difficult <strong>in</strong> space. A GC system cannot be directly coupled to a polarimetric detector because GC samples are too small. But <strong>the</strong> SETH Cigar polarimeter could be used separately to complement <strong>the</strong> chiral GC (e.g. to detect am<strong>in</strong>o acids). However, HPLC is ideal with polarimetric detection: <strong>the</strong> output from chiral HPLC columns could go directly <strong>in</strong>to a SETH Cigar-type polarimeter to give a more powerful system, able to separate a wider range of enantiomeric compounds, <strong>in</strong>clud<strong>in</strong>g am<strong>in</strong>o acids, without prior derivatisation. Development of space HPLC <strong>in</strong>strumentation should become possible <strong>in</strong> <strong>the</strong> com<strong>in</strong>g years with <strong>in</strong>creas<strong>in</strong>g m<strong>in</strong>iaturisation and improved handl<strong>in</strong>g of liquids. However, such a development is still very tentative. On <strong>the</strong> contrary, <strong>the</strong> use of GC <strong>in</strong> space has been fully demonstrated. GC (chiral columns) coupled to optical rotation measurements or/and MS should provide <strong>the</strong> separation of enantiomers, <strong>the</strong>ir quantitative analysis and <strong>the</strong> determ<strong>in</strong>ation of <strong>the</strong> R/S isomers ratios. One or several chiral column(s) <strong>in</strong> parallel on a PYR-GC-MS system can provide such a measurement. However, this technique (except if a chemical derivatisation system is used) is not compatible with <strong>the</strong> analysis of am<strong>in</strong>o acid enantiomers. It should be noted, however, that diastereoisomeric separation can be carried out on an
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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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II.3 The Martian M
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principal atmosphe
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Page 133 and 134: SP-1231 138 optical microscope. Bot
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Page 141 and 142: SP-1231 146 soils. In this approach
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Page 161 and 162: SCIENTIFIC METHOD AND REQUIREMENTS
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