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

SP-1231
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vidual compounds is such that <strong>the</strong> simplest <strong>in</strong> terms of structure are most abundant.
This implies an abiogenic orig<strong>in</strong>, i.e. built up from s<strong>in</strong>gle carbon atoms. <strong>The</strong> same
k<strong>in</strong>d of effects are also seen <strong>in</strong> short-cha<strong>in</strong> aliphatic hydrocarbons, mono and
dicarboxylic acids (Yuen & Kvenvolden, 1973) and polyaromatic hydrocarbons. <strong>The</strong>
above <strong>in</strong>terpretation is confirmed by <strong>the</strong> <strong>in</strong>vestigation of <strong>the</strong> isotopic composition of
homologous series (Yuen et al., 1984; Gilmour & Pill<strong>in</strong>ger, 1994), where isotope
fractionation effects are <strong>in</strong> keep<strong>in</strong>g with <strong>the</strong> greater strength of 12 C- 13 C bonds as might
be expected if <strong>the</strong> compound were constructed from simple precursors by a cha<strong>in</strong>build<strong>in</strong>g
process. Biosyn<strong>the</strong>sis, while produc<strong>in</strong>g isotopic differences between
oxidised and reduced carbon, does not seem to show regular fractionation between
small and large homologues.
Although <strong>the</strong> orig<strong>in</strong>al breakthrough <strong>in</strong> recognis<strong>in</strong>g <strong>in</strong>digenous compounds <strong>in</strong>
Murchison came from <strong>the</strong> discovery of racemic mixtures of am<strong>in</strong>o acids, <strong>the</strong> most
recent activities <strong>in</strong> this area have <strong>in</strong>volved show<strong>in</strong>g that <strong>in</strong> some <strong>in</strong>stances <strong>the</strong>re are
slight excesses (5-10%) of L enantiomers <strong>in</strong> some compounds (Cron<strong>in</strong> & Pizzarello,
1996). <strong>The</strong> great pa<strong>in</strong>s taken with extraction procedures suggest that this is a real
effect ra<strong>the</strong>r than a result of contam<strong>in</strong>ation. One proposed explanation of <strong>the</strong> results
is that UV synchrotron circularly polarised light, emitted by neutron stars, might
<strong>in</strong>troduce an antiomeric bias. If this explanation turns out to be valid, <strong>the</strong>n while it
could account <strong>for</strong> <strong>the</strong> orig<strong>in</strong> of optical activity, it possibly calls <strong>in</strong> to question <strong>the</strong>
whole philosophy of us<strong>in</strong>g chirality as a means of detect<strong>in</strong>g exobiology <strong>in</strong>
extraterrestrial environments <strong>in</strong> <strong>the</strong> early <strong>Solar</strong> <strong>System</strong>.
Although <strong>the</strong> organic compounds, which are solvent-extractable from carbonaceous
chondrites, are treated somewhat warily because of contam<strong>in</strong>ation issues, <strong>the</strong>
macromolecular material that comprises <strong>the</strong> bulk of <strong>the</strong> carbon is usually considered
as <strong>in</strong>digenous. <strong>The</strong>re is no plausible way that such a large accumulation, up to 5wt‰,
could have accrued from contam<strong>in</strong>ation. In any case, even <strong>in</strong> <strong>the</strong> early 19th century,
bulk carbon contents were measured soon after <strong>the</strong> fall of specimens such as <strong>the</strong> CI
chondrite Alais, and high carbon contents were recorded at that time. <strong>The</strong> carbon
isotopic composition of this material is remarkably constant (Smith & Kaplan, 1970;
Kerridge, 1985) and relatively high <strong>for</strong> terrestrial contam<strong>in</strong>ation anyway. Nitrogen
isotopic composition is well outside of <strong>the</strong> terrestrial range (Kerridge, 1985). It should
be noted that when compared to <strong>the</strong> carbonates found <strong>in</strong> <strong>the</strong> same samples, <strong>the</strong>re is a
relatively large fractionation, 50-80‰, <strong>in</strong> <strong>the</strong> same sense as used by Schidlowski as
an <strong>in</strong>dicator of biogenicity (Schidlowski, 1987). Like kerogen <strong>in</strong> <strong>the</strong> Earth’s
sedimentary rocks, meteorite macromolecules are very <strong>in</strong>tractable, yield<strong>in</strong>g
<strong>in</strong><strong>for</strong>mation only about <strong>the</strong>ir general structure when broken down by various pyrolytic
or oxidative techniques; <strong>the</strong> build<strong>in</strong>g blocks appear to be substantially aromatic.
<strong>The</strong>re is a strong likelihood that <strong>the</strong> material constitut<strong>in</strong>g <strong>the</strong> bulk of <strong>the</strong> carbon <strong>in</strong>
carbonaceous chondrites and unequilibrated ord<strong>in</strong>ary chondrites orig<strong>in</strong>ates <strong>in</strong> dark
<strong>in</strong>terstellar clouds. <strong>The</strong> key results <strong>in</strong> this respect are <strong>the</strong> recognition of substantial
deuterium enrichments (Kerridge, 1993; McNaughton et al., 1981). Deuterium to
hydrogen ratios approach<strong>in</strong>g unity are encountered <strong>in</strong> small <strong>in</strong>terstellar molecules
because of ion-molecule reactions, which encourage H to D exchange (Geiss &
Reeves, 1981).
In addition to <strong>the</strong> organic chemistry discussed above, <strong>the</strong>re have been <strong>in</strong>stances
when <strong>the</strong> existence of microfossils were suggested <strong>in</strong> carbonaceous chondrites (Nagy
et al., 1972), but such reports have been strongly criticised, sometimes justifiably, but
perhaps not always. It should not be considered unscientific to <strong>in</strong>dulge <strong>in</strong> detailed
characterisation of <strong>the</strong> organic matter <strong>in</strong> carbonaceous chondrites from <strong>the</strong> po<strong>in</strong>t of
view of exobiology (Nagy et al., 1961) but as already stated this is almost a taboo
area. As can be seen from <strong>the</strong> am<strong>in</strong>o acid studies above, <strong>in</strong>vestigations are almost
always motivated by abiogenic <strong>in</strong>terests, with large quantities of <strong>in</strong><strong>for</strong>mation ignored
because contam<strong>in</strong>ation is perhaps too readily <strong>in</strong>voked. Methods to recognise
contam<strong>in</strong>ation should be fur<strong>the</strong>r <strong>in</strong>vestigated, us<strong>in</strong>g <strong>for</strong> example isotopic order, ra<strong>the</strong>r
than discard<strong>in</strong>g unconsidered large quantities of possibly useful data. Huge amounts
of carbonaceous matter from carbonaceous chondrites have been destroyed <strong>in</strong> <strong>the</strong>