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

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