Michael
H. Engel, School of Geology & Geophysics
100
East Boyd Street
The
University of Oklahoma, Norman, Oklahoma 73019 USA
Randall
S. Perry, Department of Earth and Space Sciences
University
of Washington, Seattle, Washington 98195
On
Earth, organic matter is found in association with sediments and sedimentary
rocks. Sedimentary rocks that have
undergone slight to moderate metamorphism may also contain remnants of organic
matter incorporated prior to deformation.
Igneous rocks, however, may only contain organic matter if it is
introduced via fluid migration into fractures subsequent to
crystallization. The Martian meteorites
collected to date are all igneous rocks.
Thus, the chances that they contain indigenous organic matter, biotic or
abiotic, are remote. Whether the
organic material in fractures of ALH84001 is indigenous or is contamination
resulting from an extended residence time on Earth prior to collection, remains
unresolved. What is clear, however, is
that the current collection of Martian meteorites is far from ideal with
respect to determining the types of organic compounds that may have been
incorporated into sediments on the Martian surface. Carbonaceous meteorites consist of material derived from the
solar nebula 4.5 billion years ago.
They also exhibit varying degrees of aqueous processing thought to have
occurred on a parent body(s) in the region of the asteroid belt during the
early stages of formation of the solar system. Life appears to have existed on Earth for as far back in time as
the rock record extends (~3.8 Ga).
Thus, the only record for the solar system organic inventory that preceded
life’s origin on Earth or Mars are carbonaceous meteorites. In particular, the CI and CM carbonaceous
meteorites contain many of the building blocks for life as we know it. It is interesting to note, however, that of
the twenty protein amino acids common to all organisms, only eight have been
observed in carbonaceous meteorites. A
hypothesis is presented to account for the absence of the remaining twelve
amino acids that are essential for life and how their presence or absence can
be used to determine if life was present in ancient rocks from Mars. In summary, carbonaceous meteorites
collected at the time of or shortly after impact provide the most reliable
record of the solar system’s organic inventory during the early stages of its
formation. They provide the best
analogue for what organic synthesis and aqueous processing may have been like
on planetary surfaces prior to life’s origin.