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

SP-1231
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much higher (about three times) C/O ratios reflect<strong>in</strong>g higher carbon contents; ii) a
very strong depletion, by at least a factor of 20, of chondrules that constitute 20%
of <strong>the</strong> mass of CM chondrites; iii) a pyroxene/oliv<strong>in</strong>e ratio about 10 times higher.
Thus, large micrometeorites seem to represent a population of <strong>Solar</strong> <strong>System</strong>
objects probably related to comets and not yet represented <strong>in</strong> <strong>the</strong> meteorite
collections.
Important clues about <strong>the</strong> carbon chemistry of micrometeorites were recently
obta<strong>in</strong>ed:
• a detailed analysis of <strong>the</strong> carbon content of <strong>the</strong> various groups of micrometeorites
(Engrand, 1995) <strong>in</strong>dicates that <strong>the</strong> flux of total carbon delivered to <strong>the</strong> whole Earth
can be estimated to be 500 t per year. This value is 50 000 times higher than <strong>the</strong>
correspond<strong>in</strong>g value estimated <strong>for</strong> meteorites;
• Clemett et al. (1998) have used <strong>the</strong> technique of Microscope Double Laser Mass
Spectrometry to study <strong>the</strong> polycyclic aromatic hydrocarbons (PAHs) <strong>in</strong> both
micrometeorites and major groups of carbonaceous chondrites. <strong>The</strong> PAH content
of micrometeorites is ra<strong>the</strong>r similar to that of CM chondrites. This suggests that <strong>the</strong>
amount of PAHs delivered to <strong>the</strong> Earth by <strong>the</strong> micrometeorite flux is about 50 000
times higher than <strong>the</strong> correspond<strong>in</strong>g figure <strong>for</strong> meteorites. Micrometeorites also
show a much larger variety of PAHs, <strong>in</strong>clud<strong>in</strong>g reactive v<strong>in</strong>yl-PAHs not yet found
<strong>in</strong> meteorites;
• Br<strong>in</strong>ton et al. (1998) detected am<strong>in</strong>o acids, a-am<strong>in</strong>o isobutyric acid and isoval<strong>in</strong>e
<strong>in</strong> micrometeorites by high-per<strong>for</strong>mance liquid chromatography. In CM chondrites,
<strong>the</strong>se two am<strong>in</strong>o acids are found <strong>in</strong> similar abundances and may have,
<strong>the</strong>re<strong>for</strong>e, been <strong>for</strong>med via a Strecker-type syn<strong>the</strong>sis. In micrometeorites, a-am<strong>in</strong>o
isobutyric acid is <strong>the</strong> most abundant am<strong>in</strong>o acid. Its average content is about 10
times higher than <strong>in</strong> CM chondrites. Thus, <strong>the</strong> delivery of am<strong>in</strong>o acids to <strong>the</strong> Earth
would be 500 000 times higher than that expected from meteorites. Moreover, <strong>the</strong>
a-am<strong>in</strong>o isobutyric/isoval<strong>in</strong>e ratio is 20, suggest<strong>in</strong>g that <strong>the</strong>ir syn<strong>the</strong>sis is derived
from HCN, a compound known to be present <strong>in</strong> comets.
Micrometeorites <strong>the</strong>re<strong>for</strong>e appear to be <strong>the</strong> major source of <strong>the</strong> extraterrestrial
organic molecules delivered to <strong>the</strong> primitive Earth which may have contributed to <strong>the</strong>
appearance of life. Studies of micrometeorites by analytical electron microscopy
br<strong>in</strong>g additional support to this scenario. About 80% of <strong>the</strong> micrometeorites appear as
complex aggregates. For <strong>in</strong>stance, a s<strong>in</strong>gle 100 mm micrometeorite is composed of
millions of t<strong>in</strong>y m<strong>in</strong>eral gra<strong>in</strong>s imbedded <strong>in</strong> <strong>the</strong> abundant organic compound
amount<strong>in</strong>g to 7% of carbon. <strong>The</strong>se gra<strong>in</strong>s conta<strong>in</strong> a high proportion of metallic
sulphides, oxides and clay m<strong>in</strong>erals belong<strong>in</strong>g to various classes of catalysts. In
addition to <strong>the</strong> carbonaceous matter, <strong>the</strong> micrometeorites may <strong>the</strong>re<strong>for</strong>e also have
delivered a rich variety of catalysts, hav<strong>in</strong>g perhaps acquired specific crystallographic
properties dur<strong>in</strong>g <strong>the</strong>ir syn<strong>the</strong>sis <strong>in</strong> <strong>the</strong> microgravity environment of <strong>the</strong> early solar
nebula. Electron microscope observations also shows that micrometeorites are tightly
encapsuled <strong>in</strong> a th<strong>in</strong> crust of magnetite, probably ga<strong>in</strong>ed dur<strong>in</strong>g atmospheric entry.
In summary, micrometeorites were delivered at an average rate of about 50 000/m 2
per year all over <strong>the</strong> Earth, between 4.2-3.9 Gyr years ago. <strong>The</strong>y may have functioned
as <strong>in</strong>dividual t<strong>in</strong>y chemical reactors when reach<strong>in</strong>g terrestrial liquid water. <strong>The</strong>y
probably conta<strong>in</strong>ed all <strong>the</strong> <strong>in</strong>gredients required <strong>for</strong> <strong>the</strong> catalyzed process<strong>in</strong>g of <strong>the</strong>ir
complex carbonaceous matter by liquid water. <strong>The</strong>ir magnetic crusts would have
drastically <strong>in</strong>creased <strong>the</strong>ir mechanical stability and conf<strong>in</strong>ed <strong>the</strong> reactants with<strong>in</strong> <strong>the</strong>
gra<strong>in</strong>s. In addition, it probably reduced <strong>the</strong> access of water molecules <strong>in</strong>side <strong>the</strong>
gra<strong>in</strong>s.