[meteorite-list] Murchison and S & T (Part 1 of 2)

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:56 2004
Message-ID: <3DA00598.F4B5612B_at_lehrer.uni-karlsruhe.de>

Organic Clues in Carbonaceous Meteorites
(April, 1979, Sky & Telescope, pp. 330-332)
C.R. Pellegrino and J.A. Stoff, Rockville Centre, New York

On September 28, 1969, an ancient rock mass slammed into the upper
atmosphere somewhere above Australia. It slid, danced, and leaped
through the air, then exploded over the town of Murchison. For several
days thereafter residents and scientists recovered curious shards of
grayish matter from fields, roadsides, and rooftops. The pieces
resembled dried carbon-rich clay and crumbled with similar ease.
Upon closer examination, their matrix appeared to be studded with tiny
glasslike spheres. When these were sectioned and viewed under a
microscope, concentric layers of material, not unlike those distinctive
patterns recognized in pearls, became visible. Further analysis revealed
unexpected traces of water (as high as 10 percent by weight) locked
inside the stony fragments. The 20th specimen then known of that most
puzzling and sought after of all meteorite types, the carbonaceous
chondrite, had arrived.
Nearly three years later, scientists at NASA's Ames Research Center in
California confirmed the presence of 17 different fatty acids and 18
amino acids in fragments of the Murchison meteorite. These highly
complex substances are composed of organic elements and, when woven
properly together, comprise the foundations of cellular life. But one
very important question soon arose: were these substances truly
indigenous to the meteorite, or did the meteorite, upon its penetration
into our atmosphere, begin to "breathe in" earthly contaminants? After
all, a mere fingerprint on its surface would have contributed most of
the common amino acids known here on Earth.
During the three-year investigation that followed its arrival, the
Murchison meteorite was examined and compared closely with another
carbonaceous chondrite that had fallen near Murray, Kentucky, 19 years
earlier. The results were impressively similar. Of the 18 amino acids
detected in the two meteorites, the 12 most abundant are seldom if ever
associated with the living tissues of terrestrial plants and animals.
The remaining six (valine, alanine, glycine, proline, aspartic acid, and
glutamic acid) are prominent in earthly proteins, but relatively scarce
in carbonecous chondrites. The first of a long series of paradoxes had
begun to emerge.
The meteorites may have originated in an age when the "dust" of the
solar nebula was falling together into little bodies that became
celestial vacuum cleaners, ever increasing in girth as they continued to
sweep up debris in their path. Some, like our own earth, accumulated
great mass. Their interiors began to heat up. Gases, steam, and
vaporized rock held fast to their shifting skin: the primordial
atmospheres were born.
Whether the result of a cataclysm involving the collision of ancient
worlds or simply a collection of discarded planetary scraps left hanging
about the sun, a thin belt of solar driftwood - the asteroids -spreads
wide between Mars and Jupiter. It is from this belt that most meteorites
seem to originate.
The presence of organized elements and hydrocarbons in some of these
meteorites leaves several unanswered questions. These substances seem to
have no business being out there in the first place. If they are native
to the meteorites, then we are faced with a perplexing fact: these
carbon compounds were somehow lifted, against entropy, to a highly
ordered state from vast numbers of random dissociated, inanimate atoms,
and gathered up and arranged in their present condition of seemingly
improbable symmetry. Given only the extreme temperatures, damaging
radiation, and near emptiness of outer space, it is not likely that this
kind of clustering could have proceeded in objects so small as stones,
boulders, or even asteroids (nor that it should be reproduced so
agreeably among individual samples).
Detailed comparisons with earthly tissues seem only to sharpen the
contrasts between terrestrial proteins and the kinds of molecular
ornamentation typically recovered from carbonaceous chondrites. That
the history of these compounds differs from our own is underscored by
important eccentricities in their molecular structure.
It is generally believed by organic chemists that when the earth was
still in its infancy, when its vapors had condensed into newly formed
seas and its shroud of air lacked destructive oxidizing agents, the
first organic acids were probably assembled in two very distinct
varieties. Valine, for example, possibly occurred as mirror images of
itself, much in the same way as your right and left hands are mirror
images, or isomers, of each other. In those days before the dawn of
living self-replicating matter, both "right-handed" and "left-handed"
molecules might have drifted about the Precambrian seas in equal or
near-equal quantities.
Received on Sun 06 Oct 2002 05:42:48 AM PDT