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

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protons of discrete energies. <strong>The</strong> proton spectra can be measured <strong>in</strong>dependently of
<strong>the</strong> alpha spectra because of <strong>the</strong> different ranges of alphas and protons. <strong>The</strong> proton
mode is of <strong>in</strong>terest <strong>for</strong> <strong>the</strong> determ<strong>in</strong>ation of Na, Mg, Al, Si and S.
<strong>The</strong> X-ray mode. <strong>The</strong> alpha particles from <strong>the</strong> radiation sources are also used as a very
efficient excitation source <strong>for</strong> <strong>the</strong> production of characteristic X-rays. Charged
particle excitations are, <strong>in</strong> fact, highly superior to any o<strong>the</strong>r k<strong>in</strong>d of X-ray
excitation as <strong>the</strong>y give by far <strong>the</strong> best signal-to-noise ratio.
<strong>The</strong> comb<strong>in</strong>ation of all three modes allows determ<strong>in</strong>ation of all elements heavier
than helium present <strong>in</strong> concentration levels greater than 1%. In order to achieve <strong>the</strong>
highest accuracy and sensitivity, a measur<strong>in</strong>g time per sample of up to 10 h is
required. That will tend to def<strong>in</strong>e <strong>the</strong> rate of sample presentation from <strong>the</strong> drill<strong>in</strong>g
systems. While <strong>the</strong> alpha mode has a good resolution <strong>for</strong> <strong>the</strong> light elements C, N and
O, <strong>the</strong> resolution decreases <strong>for</strong> heavier elements. On <strong>the</strong> o<strong>the</strong>r hand, <strong>the</strong> resolution of
<strong>the</strong> X-ray mode <strong>in</strong>creases <strong>for</strong> heavier elements.
<strong>The</strong> <strong>in</strong>strument is well developed, has flown on several missions and is accepted <strong>for</strong>
future Mars missions. <strong>The</strong>re was an APXS onboard Vik<strong>in</strong>g, <strong>the</strong> Mars Pathf<strong>in</strong>der rover
Sojourner (Rieder et al., 1997) and on <strong>the</strong> Russian Mars-96 mission. Sojourner’s
version conta<strong>in</strong>ed n<strong>in</strong>e 224 Cm sources with a total activity of about 50 mCi (1.85×10 9
decays/s). <strong>The</strong>re will be a similar <strong>in</strong>strument on NASA’s Mars Surveyor 2001 mission
as part of <strong>the</strong> ATHENA payload, and ano<strong>the</strong>r APXS on ESA’s Rosetta comet lander <strong>for</strong>
launch <strong>in</strong> 2003. This demonstrates <strong>the</strong> good heritage of <strong>the</strong> <strong>in</strong>strument.
<strong>The</strong> total APXS mass, cover<strong>in</strong>g <strong>the</strong> sensor head and electronics box, is 570 g.
Power consumption is 340 mW and data output is 16 Kbytes per sample analysis.
However, <strong>in</strong> <strong>the</strong> case of Mars <strong>the</strong>re is a potential problem concern<strong>in</strong>g <strong>the</strong>
quantitative analysis of C and N. <strong>The</strong> atmospheric CO 2 and N 2 signals can <strong>in</strong>terfere
with those from <strong>the</strong> soil and rocks. To m<strong>in</strong>imise <strong>the</strong> atmospheric signals, <strong>the</strong> orig<strong>in</strong>al
Pathf<strong>in</strong>der <strong>in</strong>strument design has been modified us<strong>in</strong>g additional collimators <strong>for</strong> <strong>the</strong>
alpha particles. F<strong>in</strong>ally, <strong>the</strong> <strong>in</strong>strument needs to be calibrated under real
environmental conditions to account <strong>for</strong> <strong>the</strong> rema<strong>in</strong><strong>in</strong>g atmospheric carbon and
nitrogen contribution. Such a modified APXS, which will be part of <strong>the</strong> Mars
Surveyor 2001 mission, is able to analyse carbon down to <strong>the</strong> 0.1wt% level. <strong>The</strong>
sensitivity <strong>for</strong> nitrogen is less because of <strong>the</strong> proximity of <strong>the</strong> nitrogen and oxygen
signals (similar mass numbers).
II.5.7.4 Mössbauer Spectrometer
Mössbauer spectroscopy (Kl<strong>in</strong>gelhöfer et al., 1996) is a powerful tool <strong>for</strong> analys<strong>in</strong>g
Fe-bear<strong>in</strong>g m<strong>in</strong>erals and iron compounds. In particular, <strong>the</strong> oxidation state of <strong>the</strong> iron
and <strong>the</strong> Fe 2+ /Fe 3+ ratio <strong>in</strong> <strong>the</strong> soil and rocks will be determ<strong>in</strong>ed directly. With
Mössbauer spectroscopy, <strong>the</strong> Fe 2+ /Fe 3+ ratio can be determ<strong>in</strong>ed <strong>for</strong> samples taken at
different depths <strong>in</strong> <strong>the</strong> soil, which provides complementary <strong>in</strong><strong>for</strong>mation with respect
to <strong>the</strong> H 2O 2 oxidant depth profile determ<strong>in</strong>ation.
Biogenic magnetite very often (<strong>in</strong> most cases) shows up <strong>in</strong> Mössbauer spectra as a
superparamagnetic component hav<strong>in</strong>g strongly temperature-dependent Mössbauer
parameters. F<strong>in</strong>d<strong>in</strong>g such signatures <strong>in</strong> <strong>the</strong> Mössbauer spectra will be an <strong>in</strong>dication of
previous biological activity. However, <strong>the</strong>re are <strong>in</strong>organic processes that also produce
such nanophase magnetic particles. So, we must be careful <strong>in</strong> <strong>the</strong> <strong>in</strong>terpretation of <strong>the</strong>
data.
<strong>The</strong> general scientific objectives of <strong>in</strong> situ Mössbauer spectroscopy are:
– determ<strong>in</strong>ation of <strong>the</strong> oxidation state of iron-bear<strong>in</strong>g m<strong>in</strong>erals;
– identification of Fe carbonates, sulphates, nitrates, etc that would provide
<strong>in</strong><strong>for</strong>mation on early martian environmental conditions;
– identification of iron-bear<strong>in</strong>g phases with low detection limits;
– determ<strong>in</strong>ation of <strong>the</strong> iron oxides and <strong>the</strong> magnetic phase <strong>in</strong> <strong>the</strong> martian soil;
– detection of nanophase and amorphous hydro<strong>the</strong>rmal Fe m<strong>in</strong>erals that could
preserve biological materials.