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Meteorite group Number Mean carbon content,
analyzed percent by weight
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Pallasites 10 0.08
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Ureilites 2 .69
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Bronzite chondrites 12 .05
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Hypersthene chondrites 8 .04
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Enstatite chondrites 8 .29
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Carbonaceous chondrites 16 2.04
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Most meteorites possess only traces of carbon, and studies of this
carbon indicate that it is composed largely of graphite, cohenite, and
moissanite, with some diamond. However, studies of the carbon in the
carbonaceous chondrites have failed to detect any of these forms. Some
carbonates are present in a minority of the carbonaceous group, but
account for only a small percentage of the total carbon (perhaps about
10 percent of the total C in type I only).
The carbonaceous chondrites contain organic carbon. The word "organic"
is not used in a biological sense, merely as a chemical term to describe
compounds of carbon other than carbonates, bicarbonates, and carbides.
No evidence has been found of any form of carbon other than organic,
except for traces of carbonates.
Various studies have demonstrated possible methods of estimating the
total amount of organic matter present in meteorites. Wiik ([ref.27])
has suggested that organics can be estimated by measuring the loss of
weight on ignition. Unfortunately, this method has several disadvantages
and gives very low values. Corrections must be made for weight gains due
to oxidation of reduced constituents, such as FeO, Fe, Ni, and Co, and
for weight losses due to H₂O, S, etc. The water loss is exceedingly
difficult to estimate, as part comes from the combustion of organic
hydrogen and part comes from the loss of mineral-bound water. The carbon
also proves difficult to combust completely, and high temperatures (over
1000° C) are required for efficient conversion to CO₂.
In one study the major fraction of organic matter removed proved to have
a carbon content of about 47 percent ([ref.28]). Thus, if all the
meteorite carbon is present as organic matter of approximately this
composition, total organics must be approximately double the carbon
content; that is, 2 percent by weight carbon indicates 4 percent by
weight organic matter. This estimate may be too low, for Mueller
([ref.29]) has extracted a major organic fraction containing only 24
percent carbon; however, this work has not been confirmed for other
meteorites.
Briggs and Mamikunian ([ref.26]) have pointed out that only 25 percent
of the organic matter has been extracted, and only about 5 percent of
this has been chemically characterized. Most of this 5 percent is a
complex mixture of hydroxylated aromatic acids together with
hydrocarbons of the aliphatic, napalicyclic, and aromatic series. Small
amounts of amino acids, sugars, and fatty acids are also present.
Thus far, these chemical analyses point to an abiogenic origin for the
organic matter, and no conclusive evidence exists of biological activity
on the meteorite parent body. Microbiological investigations of samples
of the carbonaceous chondrites have yielded only inconclusive evidence
on the problem of "organized elements."
Several of these microstructures from different carbonaceous chondrites
are illustrated in a paper by Mamikunian and Briggs ([ref.30]). It has
been difficult to identify the organized structures, and most do not
have morphologies identical to known terrestrial micro-organisms.
However, they may prove to be a variety of mineral grains, droplets of
organic matter and sulfur, as well as a small amount of contaminating
terrestrial debris.
A comparison between the photographs of the organized elements observed
in the Orgueil and Ivuna meteorites and the synthetic proteinoid
microspheres observed by Fox ([ref.25]) point to similarities between
the two. One inference from this finding is that the organized elements
in carbonaceous chondrites were never alive but, rather, should be
considered as natural experiments in molecular evolution. Also, these
similarities strengthen the belief that the laboratory experiments are
similar to the natural experiments in space.
In cooperation with the Smithsonian Astrophysical Observatory, NASA has
a network to track meteors in the Midwest (South Dakota, Nebraska,
Kansas, Oklahoma, Iowa, Missouri, and Illinois). Photographs of meteor
trails are used for scientific study, and attempts are made to track and
recover meteorites for examination for traces of organic material of
extraterrestrial origin.
Fundamental research in terrestrial organic geochemistry has shown that
ancient sediments and drill core samples subjected to organic analysis
contain certain stable biochemical components of past life. This
preserved record is significant not only in studies of early-life
chemical pathways but also in studies of the interaction of organic
matter with the geological factors. Since life on any planetary body
will interact with the soil, or surface material, it is of interest to
understand the relationship.