Meteorites provide a more representative sample of average planetary matter than the highly differentiated crust of the Earth. Although it is known that the meteorite parent bodies ceased to be geochemically active shortly after their formation, some 4½ billion years ago, there is no consensus on the nature of the meteorite parent bodies, not even on such basic properties as size, location, and multiplicity. This is not surprising because the meteorite samples commonly available for study represent only about 10-23 to 10-26 of the parent body.
Carbonaceous Meteorites
Analysis and characterization of the chemical constituents (organic) of carbonaceous chondrites, including the possible mechanism of their formation, may be expected to improve methods of analyzing samples from the Moon and planets and of interpreting remote automated biological analyses on the planets' surfaces.
Carbon has been detected in all meteorites analyzed; however, both the amount and forms present vary considerably. Among the forms of meteorite carbon are diamond, graphite, cohenite (Fe,Ni,Co)3C, moissanite SiC, calcite CaCO3, dolomite (Ca,Mg)CO3, bruennerite (Mg,Fe)CO3. A summary of the results of carbon analyses on large numbers of meteorites is given in [table I] ([ref.26]).
| Meteorite group | Number analyzed | Mean carbon content, percent by weight |
|---|---|---|
| Pallasites | 10 | 0.08 |
| Ureilites | 2 | .69 |
| Bronzite chondrites | 12 | .05 |
| Hypersthene chondrites | 8 | .04 |
| Enstatite chondrites | 8 | .29 |
| Carbonaceous chondrites | 16 | 2.04 |
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.