INTERSTELLAR GRAINS WITHIN INTERSTELLAR GRAINS

We report ion microprobe and transmission electron microscope studies of five interstellar graphitic spherules extracted from the Murchison carbonaceous meteorite. The spherules have C-12/C-13 ratios (7-1330) significantly different from the solar system average (89) as well as anomalous N isotopic compositions. Concentrations of H, N, O, and Si appear to increase with decreasing C-12/C-13 ratios. Two morphological types were observed: three spherules consist of concentric shells of fairly well graphitized carbon, while the other two consist of concentric scales of poorly graphitized carbon. In ultrathin (70 nm) sections of one spherule of the latter type we discovered 7-21 nm crystals of TiC. Because they were protected in the spherule, these crystals are stellar condensates whose surfaces have not been altered by subsequent chemical processing, e.g., in the interstellar medium. Equilibrium condensation calculations predict that C will condense before TiC, thus the occurrence of TiC within the graphitic spherule points to inhibited nucleation and growth of graphite, or mixing of previously formed TiC grains into regions of the stellar atmosphere where graphite was condensing. If the low abundance of TiC (tens of ppm) in the graphitic hosts is similar to its abundance in stellar atmospheres in which such crystals form, TiC may be astronomically unobservable.