Organic Compounds in Meteorites

There is little doubt that the bulk of
the organic matter in meteorites is in-
digenous, judging from isotopic mea-
surements on C, H, and S (20, 21).
Such proof is not available for indi-
vidual compounds, however, and it is
therefore necessary to resort to less
direct arguments to rule out terrestrial
contamination. We shall briefly review
the most reliably established features
of organic matter in meteorites, and
see. how well FIT reactions can ac-
count for them. Owing to space limi-
tations, we can only give a concise
summary of the most pertinent data.
The interested reader may wish to con-
sult the original sources, as well as the
reviews of Hayes (22), Vdovykin (23),
and Oré and co-workers (24).

Heavy alkanes.’ The same few com-
pounds dominate in meteoritic and FIT
hydrocarbons (/8, 25, 26). Normal
(straight-chain) hydrocarbons are most
prominent (Fig. 2), followed by five
slightly branched (monomethyl and di-
methyl) isomers (27). This resem-
blance is highly significant if one con-
siders that some 10+ structural isomers
exist for saturated hydrocarbons with
16 carbon atoms. Apparently the me-
teoritic hydrocarbons were made by
FTT reactions, or a process of the
same extraordinary selectivity. The
Miller-Urey _ reaction,
shows no such selectivity. Gas chro-
matograms of spark discharge hydro-
carbons show no structure (/3). Ap-
parently all 104 possible isomers are
made in comparable yield, as expected
for random recombination of free radi-
cals,

The indigenous nature of the alkanes
in at least the Murray, Murchison, and
Orgueil meteorites (18) is supported
by four lines of evidence: absence of
the isoprenoids pristane and phytane
(18), which occur in nearly all ter-
restrial hydrocarbons (/3, 26, 28);
characteristic light hydrocarbon pat-
tern (J8); consistently low abundance
of alkanes in the C3 chondrite Allende
* (18,29), which is metamorphosed and

hence may be regarded as a blank;
and carbon isotope data (2/, 30).
Most of the remaining compounds
‘in the meteoritic chromatogram (ace-
naphthene, alkylbenzenes, olefins) can
be made by FIT syntheses under ap-
_ propriate conditions (78). Kvenvolden
et al. (30) and Ord et al. (31) had
reported alicyclic hydrocarbons in the
Murchison meteorite on the basis of
low-resolution chromatograms. These

23 NOVEMBER 1973

incidentally, -

compounds were not confirmed at
higher resolution (1/8), however, but
can, in any event, be made in the FIT
synthesis. The authors have found
large amounts of cycloalkanes in a
commercial Fischer-Tropsch sample.
Carbon isotope fractionations, Me-
teorites show a very large difference in
12C/12C ratio between carbonate and
organic carbon: 60 to 80 per mil (20,
21, 32). This trend remained unex-
plained for a number of years, be-
cause coexisting carbonate and organic
matter on the earth shows a much
smaller difference, typically 25 to 30

400°K
Memo from To: Q { U 4 , father
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JOSHUA LEDERBERG qe ed on
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proposed that the two types of carbon
came from two unrelated reservoirs
while Arrhenius and Alfvén (33) sug-
gested fractionation during carbonate
growth from the gas phase, involving
multiple desorption or metastable mole-
cules,

It turns out, however, that the Fisch-
er-Tropsch reaction gives an isotopic
fractionation of just the right sign and
magnitude, owing to a kinetic isotopic
effect (34, 35). From the temperature
dependence of the fractionation be-
tween 375° and 550°K, the observed
fractionations in C1 and C2 chondrites

PAY

 

 

 

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PROFESSOR JOSHUA LEDERBERG six of
Department of Genetics nt im ,
School of Medicine oples. -
Stanford University from
Stanford, California 94305 . Ab

 

 

 

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