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]). Table I.—_Meteorite Carbon_ ----------------------------------------------------------------- Meteorite group Number Mean carbon content, analyzed 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).