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

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soils. In this approach, <strong>the</strong> sample is exposed <strong>in</strong> an oven to aliquots of oxygen as <strong>the</strong>
oven temperature is <strong>in</strong>creased from ambient. All known <strong>for</strong>ms of carbon are
combustible, organic matter be<strong>in</strong>g <strong>the</strong> most easily changed to carbon dioxide.
Aliphatic-rich material burns at <strong>the</strong> lowest temperatures and highly crossl<strong>in</strong>ked
(aromatic) kerogens at someth<strong>in</strong>g approach<strong>in</strong>g 600ºC. <strong>The</strong> combustion temperature
level is a cont<strong>in</strong>uum right up to where organic matter becomes amorphous carbon
(poorly crystall<strong>in</strong>e graphite), so it is possible to recognise natural kerogens from <strong>the</strong>ir
stability with oxygen. Although organic matter overlaps with <strong>the</strong> decomposition of
carbonates, <strong>the</strong> latter are easily recognised because <strong>the</strong>y are still degradable <strong>in</strong> <strong>the</strong>
absence of oxygen. Stepped combustion is thus a universal method <strong>for</strong> detect<strong>in</strong>g
organic rema<strong>in</strong>s even when <strong>the</strong>y have begun to degrade through geological
process<strong>in</strong>g. It provides valuable <strong>in</strong><strong>for</strong>mation on <strong>the</strong> nature and orig<strong>in</strong> of carbon
compounds. This technique is also <strong>in</strong>cluded on <strong>the</strong> MODULUS experiment.
Mass spectrometers can operate <strong>in</strong> a variety of ways to measure isotopes. All
require <strong>the</strong> elements under study to be presented <strong>in</strong> a gaseous <strong>for</strong>m. <strong>The</strong> procedure
generat<strong>in</strong>g <strong>the</strong> greatest accuracy is <strong>the</strong> ‘dual-<strong>in</strong>let dynamic’ mode, where a sample and
standard are repetitively compared by <strong>in</strong>troduction from separate <strong>in</strong>let systems. <strong>The</strong>
highest sensitivity method is <strong>the</strong> ‘static’ mode, where a standard is <strong>in</strong>serted <strong>in</strong>to a
mass spectrometer closed off from its pump<strong>in</strong>g system after a sample analysis has
been completed and any residual specimen pumped away. A compromise situation is
af<strong>for</strong>ded by GC-IR-MS (gas chromatography-isotope ratio-mass spectrometry). All
<strong>the</strong> specimens are burned <strong>in</strong> an <strong>in</strong>terface reactor as <strong>the</strong>y are eluted from a gas
chromatography column (see below) on <strong>the</strong> fly. Standards are <strong>in</strong>troduced as spikes <strong>in</strong>
appropriate regions of <strong>the</strong> gas chromatogram where no component is present. <strong>The</strong>
disadvantage of GC-IR-MS is that it can be applied only <strong>for</strong> carbon species that are
sufficiently volatile to be chromatographed. It is, however, compatible with pyrolysisgas
chromatography-mass spectrometry (PYR-GC-MS; see below).
Methods of measur<strong>in</strong>g isotopes us<strong>in</strong>g lasers tunable to <strong>the</strong> absorption frequency of
specific carbon bonds, e.g. 12 C-O or 13 C-O, are under consideration but <strong>the</strong>ir
application even <strong>in</strong> terrestrial laboratory situations is still limited by <strong>the</strong> accuracy of
measurement. Although all k<strong>in</strong>ds of mass spectrometers are able to detect and
measure isotopes, <strong>the</strong> accuracy will always be greatest with magnetic-sector <strong>in</strong>struments
because only this <strong>for</strong>mat is capable of simultaneous detection to elim<strong>in</strong>ate
fractionations dur<strong>in</strong>g ion source fluctuations. New developments of m<strong>in</strong>iaturised
magnetic mass spectrometers, based on Mattauch-Herzog geometry double-focus<strong>in</strong>g,
is under study. A breadboard <strong>in</strong>strument, <strong>in</strong>clud<strong>in</strong>g ion source, electric sensors and
magnetic sector with an array detector is already available <strong>in</strong> Europe, with a total
mass of only 1 kg. Its application to an exobiological package <strong>for</strong> <strong>the</strong> martian
environment still requires <strong>the</strong> development of a powerful pump<strong>in</strong>g device.
Laser ablation (LA) <strong>in</strong>ductively coupled plasma (ICP) mass spectrometry (MS)
could be a very attractive technique. It needs only m<strong>in</strong>imum sample preparation and
could be done directly from <strong>the</strong> microscope. If <strong>the</strong> mass spectrometer could be
furnished with an LA-ICP-MS ion source, it could be utilised <strong>for</strong> isotopic analyses on
<strong>in</strong>organic and organic matter such as:
– <strong>the</strong> major rock-<strong>for</strong>m<strong>in</strong>g m<strong>in</strong>erals;
– secondary m<strong>in</strong>erals replac<strong>in</strong>g primary m<strong>in</strong>erals and <strong>for</strong>m<strong>in</strong>g crack fill<strong>in</strong>gs, etc;
– organic fill<strong>in</strong>gs of cracks, etc.
Such <strong>in</strong> situ isotopic analysis of H, C, N and O will reveal conditions of <strong>for</strong>mation
of primary and secondary phases as well as organic matter.
Action needed: a study of <strong>the</strong> feasibility of a low-mass high-per<strong>for</strong>mance pump,
such as a m<strong>in</strong>iaturised turbo molecular pump, <strong>for</strong> space applications, and <strong>the</strong> possible
use of LA-ICP-MS.
Previous <strong>in</strong>vestigations by <strong>the</strong> Vik<strong>in</strong>g missions provided no evidence <strong>for</strong> any
reduced species, <strong>in</strong> pr<strong>in</strong>ciple because of <strong>the</strong> strongly oxidis<strong>in</strong>g surface conditions on
Mars. <strong>The</strong> <strong>in</strong>vestigation of subsurface material represents an important advantage,