[meteorite-list] Science Fair Again. Sigh!

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:54:05 2004
Message-ID: <3C6EE9DF.AF0323D1_at_lehrer1.rz.uni-karlsruhe.de>

Mike Tettenborn sighed:

> Once again my budding scientist son has decided to work with meteorites in
> his science fair. I am thrilled at this but I may have to donate a 1.2 gram
> sample of Murchison.

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.
When living things finally did take over the earth, the assembly of proteins was
made possible only by the uptake of entirely right-handed or entirely
left-handed amino acids. The geometry of long-chain carbon compounds allowed no
room for random associations of both right-handed and left-handed components in
their construction. On Earth, it was the left-handed variety that won
acceptance. Hence, terrestrial proteins, whether they be derived from trees or
mosquitoes or men (except for a special class of single-celled organisms, which
utilize right-handed amino acids in their cell walls), are composed entirely of
left-handed amino acids.
Using a beam of plane-polarized light, it is possible to determine the
right-handedness or left-handedness of a set of molecules. A right-handed
molecule will twist or rotate the plane of polarization to the right, whereas
the left-handed variety will twist it to the left. When homogenous mixtures of
amino acids from the Murray and Murchison meteorites were examined in this
manner, no such rotation was observed, indicating that both forms were present
in equal quantities. These findings are reminiscent of ratios presumed to have
existed in Precambrian seas prior to the emergence of cellular enzyme activity,
and strongly suggest an origin held, not in the biology of cells, but in the
chemistry of atoms.
The nature of meteoritic amino acids is different from those on your fingertips,
to be sure. Equal distributions of both molecular configurations would seem to
cast serious doubts on the feasibility of their ever having been generated by
any kind of cellular activity or by life as we know it. Nevertheless, comparison
with other carbonaceous-chondrite meteorites occasionally leads to points of
confusion. A meteorite that in 1864 fell in Orgueil, France, and a 1938 fall in
lvuna, Tanganyika, contain greater traces of right-handed than left-handed amino
acids. Not only does this mixture run counter to amino acids found in earthly
proteins, but their tendency toward one polarity points to a possible origin in
cells.
We can already provide a good explanation for discrepancies of amino-acid ratios
among carbonaceous chondrites. Given irradiation by light, heat, X-ray, or other
energy sources, it is possible to interconvert amino acids from one form to
another. For example, a solitary left-handed molecule of valine, impelled by a
constant input of energy, would eventually flip over to a right-handed
configuration. An entire vial of left-handed valine exposed to the radiations of
the sun would, given enough time, undergo total interconversion. One should not,
however, expect to recover a vial filled only with right-handed molecules since,
once produced, they are as likely to flip back to the left-handed variety as
left-handed molecules become right-handed.
The situation is analogous to laying amillion pennies heads up (to represent
left-handed molecules) in a large tray. By randomly tossing handfuls of them
into the air - our application of energy - more and more of the pennies would
land heads down until the distribution of heads and tails was nearly equal.
Probabilities being what they are, from this point on one would always expect to
find about the same number of heads and tails no matter how many more handfuls
wer tossed.
Thus, a vial of pure left-handed valine suspended in space and irradiated (but
not fried) for a million years or so would ultimately turn up as an optically
inactive mixture of left-handed and right-handed molecules, in spite of its
initially pure form.
The Murray and Murchison meteorites are among the lightest and least densely
packed of the nearly 40 carbonaceous chondrites known today. If, during their
long passage through the solar system, they were ever part of a large asteroidal
body, then surely they resided on or very near its surface. Consequently, their
contents were left naked to the raw energies of space, and amino acids recovered
from these meteorites are presumed to have undergone many "flips of the coin" -
they have become utterly randomized. A meteorite originating in the depths of a
parent body would have received more adequate shielding against such energy.
The lvuna and Orgueil specimens reveal compression of their matrix, suggesting
the operation of mild gravitational forces exerted by overlaying rock in their
respective parent bodies. Mixtures of amino acids extracted from these
meteorites deviate sharply from the half-and-half composition of lighter
specimens (which include the Murray and Murchison meteorites). Skewness among
the denser carbonaceous chondrites infers an initial sample consisting largely
or entirely of right-handed amino acids.
Although the origin of these substances is still a matter of speculation, most
planetary geologists and organic chemists are in agreement that they were
contained in the meteorites prior to any contact with our atmosphere. If we
assume the least glamorous hypothesis, then some manner of preliving chemical
evolution, perhaps advancing in the direction of molecules that would one day be
able to reproduce themselves, appears to be preserved or fossilized in
meteorites.
These celestial vagrants offer the alluring possibility that the universe is not
such a lonely place in which to live. Clouds of formaldehyde (HCHO) spread
across various parts of the galaxy seem to exemplify the trend: wherever carbon
and hydrogen and their associated counterparts lie scattered and heated at the
right temperature, it is a fair bet that they will coalesce into compounds of
higher order. That you are alive and reading these words is evidence that such
reactions can and do occur.
Received on Sat 16 Feb 2002 06:23:11 PM PST