The Record in the Meteorites. III. on the Development of Meteorites in Asteroidal Bodies.

Abstract

It is proposed that the meteorites originated in planetesimals of asteroidal dimensions, heated by some transient internal energy source, such as extinct radioactivity. The usual objections to an origin in small bodies are examined and are shown to be unfounded. It is shown that segregation of metal and silicate phases and mineral differentiation by crystal settling will take place on reasonable time scales even in small bodies. Evidence concerning the occurrence of diamonds in meteorites is critically examined and is found to be inconsistent with an origin in large bodies. Instead, it is regarded that diamonds were formed as a metastable phase by decomposition of cohenite (Fe,C), under localized stresses, or else upon impact with the earth (as proposed by Nininger). A detailed study of the thermal evolution of internally heated planetesimals is undertaken with a view to discovering spontaneous processes that might have led to the development of the meteorites. After the onset of melting, the equilibrium configuration of the planetesimal will comprise an inner core of metal and an outer core of silicate; a mantle of chondritic composition, compacted by sintering; and an unconsolidated surface layer. The expected properties of material from each of these strata agree well with those of the known classes of meteorites. Further temperature rise in the planetesimal will result in quasi-volcanic eruptions due to evolution of gases and vapors (e.g., elemental sulfur, carbon monoxide, silicon monosulfide) from the interior. This quasi-volcanic activity will cause extensive recycling of material and can lead to the development of many detailed features of the meteorites. New evidence is presented to show that the capillary veins in stone meteorites were produced by momentary action of hot, sulfur-containing gases, in accord with the proposed model. Another consequence of the model is the operation of a cyclic process that will deplete the chrondritic mantle in some chalcophile elements, such as In, Tl, Pb, and Bi, and might account for the discrepancies between the observed and predicted abundances of these elements in chondrites. Possible energy sources are critically examined. Only extinct radioactivity seems to meet all requirements. In order for this source to have been important, it is necessary for the parent bodies of the meteorites to have accreted within 7 million years after nucleogenesis, in which case 7.3 X 105-year Al26 could have provided an adequate source of heat, or an uncharacterized nuclide with a half4ife of 108 108 years exists and was present in the early solar system.