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

tation at shorter wavelengths can swamp <strong>the</strong> weaker Raman scatter<strong>in</strong>g from organic
molecules.
In <strong>the</strong> Raman work carried out at <strong>the</strong> University of Brad<strong>for</strong>d, (Edwards et al.,
1999), blue, green and red excitation have been used and, most recently, 1064 nm
excitation <strong>in</strong> <strong>the</strong> near-IR from a Nd 3+ /YAG laser has been used <strong>in</strong> conjunction with
Fourier Trans<strong>for</strong>m techniques <strong>for</strong> data process<strong>in</strong>g and accumulation.
Sampl<strong>in</strong>g <strong>for</strong> Raman spectroscopy is much easier than <strong>for</strong> IR absorption and this
has enabled its rapid application to geological problems. Raman spectroscopy is
essentially a surface technique and sample spots from 100 mm diameter down to less
than 1 mm are now be<strong>in</strong>g achieved us<strong>in</strong>g microscope systems and remote sens<strong>in</strong>g
probes with optical fibres of 1-50 m length.
A big advantage of Raman over IR spectroscopy <strong>for</strong> geological studies is that <strong>the</strong>
whole spectrum from about 100-4000 cm -1 can be obta<strong>in</strong>ed simultaneously us<strong>in</strong>g
ei<strong>the</strong>r FT-techniques or CCD detectors. <strong>The</strong> vibrations and characteristic signatures
of m<strong>in</strong>erals, crystals and <strong>in</strong>organic species are generally with<strong>in</strong> <strong>the</strong> region 100-1200
cm -1 , a region which is not fully covered by non-specialist IR <strong>in</strong>struments.
A vital factor <strong>in</strong> <strong>the</strong> assessment of <strong>the</strong> capability of Raman spectroscopy to provide
<strong>in</strong><strong>for</strong>mation about extraterrestrial molecules of life is eng<strong>in</strong>eer<strong>in</strong>g restrictions on
<strong>in</strong>strument size and weight. This is already be<strong>in</strong>g addressed <strong>in</strong> o<strong>the</strong>r quarters and <strong>the</strong>re
are claims of ultra-small, micro-Raman spectrometers that could be employed <strong>in</strong> a
Lander payload. However, a critical test must be <strong>the</strong> use of <strong>the</strong>se systems on terrestrial
problems <strong>in</strong> order to evaluate <strong>the</strong>ir potential aga<strong>in</strong>st <strong>the</strong>ir bigger, heavier research
laboratory-grade <strong>in</strong>struments.
M<strong>in</strong>iaturised Raman spectrometers are under development <strong>in</strong> <strong>the</strong> US, and <strong>for</strong> <strong>the</strong>
Mars Surveyor 2001 rover a Raman spectrometer is expected to be <strong>in</strong>cluded <strong>in</strong> <strong>the</strong>
A<strong>the</strong>na payload. This <strong>in</strong>strument is currently under development at JPL and
Wash<strong>in</strong>gton University, St. Louis (Wang et al., 1997). It will operate at a wavelength
of about 685 nm, where <strong>the</strong>re is high Raman scatter<strong>in</strong>g effect, excitation of relatively
little fluorescence and high CDD detection efficiency.
It is an essential part of <strong>the</strong> molecular structural analysis that <strong>the</strong> development of a
m<strong>in</strong>iaturised Raman spectrometer is undertaken <strong>for</strong> both m<strong>in</strong>eralogical and biological
organic materials <strong>in</strong> situ analysis. This will be a novel piece of <strong>in</strong>strumentation <strong>for</strong> a
European lander programme – it is not be<strong>in</strong>g envisaged fully elsewhere – <strong>in</strong> that <strong>the</strong>
exam<strong>in</strong>ation of material <strong>in</strong> <strong>the</strong> martian subsurface is perceived as an <strong>in</strong>tegral part of
<strong>the</strong> molecular structural <strong>in</strong>vestigation.
<strong>The</strong> major problem concern<strong>in</strong>g isotopic analysis <strong>in</strong> space is that of isobaric <strong>in</strong>terferences
if <strong>the</strong> mass-resolution of <strong>the</strong> <strong>in</strong>strument is not high enough. In <strong>the</strong> simplest
case, this is <strong>the</strong> effect of protons attached to light isotopes be<strong>in</strong>g confused with heavy
ones. For example, 12 CH has <strong>the</strong> same <strong>in</strong>teger mass as 13 C. Thus, unless <strong>the</strong> two
species can be dist<strong>in</strong>guished, an <strong>in</strong>correct assessment is made. In practice, on Earth,
<strong>the</strong> separation of 12 CH and 13 C can be achieved by a high-resolution mass spectrometer.
However, higher resolution almost always means a heavier <strong>in</strong>strument
package. Ef<strong>for</strong>ts are underway to improve resolution of <strong>the</strong> folded time-of-flight mass
spectrometer, under development <strong>for</strong> <strong>the</strong> COSAC <strong>in</strong>strument on <strong>the</strong> Rosetta lander.
<strong>The</strong> alternative approach is to process chemically species of <strong>in</strong>terest down to
molecules that do not give isobaric problems with a low-resolution mass spectrometer.
On Earth, <strong>for</strong> example, carbon <strong>in</strong> a complex organic macromolecule such as
kerogen has to be combusted. Combustion reactions are feasible under a chemical
pre-treatment scheme called MODULUS (Methods Of Determ<strong>in</strong><strong>in</strong>g and Understand<strong>in</strong>g
Light elements from Unequivocal Stable isotope measurements). <strong>The</strong>
pr<strong>in</strong>ciple also applies to o<strong>the</strong>r process<strong>in</strong>g activities needed <strong>for</strong> species related to
organic analysis, e.g. <strong>the</strong> determ<strong>in</strong>ation of nitrogen <strong>in</strong> organics or oxygen and
hydrogen composition of water.
By us<strong>in</strong>g <strong>the</strong> stepped combustion method, <strong>in</strong><strong>for</strong>mation is yielded on <strong>the</strong> nature of
<strong>the</strong> carbon-bear<strong>in</strong>g samples, and detection is possible of organic matter <strong>in</strong> rocks and
team II: <strong>the</strong> search <strong>for</strong> chemical <strong>in</strong>dicators of life/II.5
II.5.8 Isotopic Analysis
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