Interstellar graphite in meteorites

Abstract

IN a continuation of our search for interstellar 'needles' in the meteoritic 'haystack'1,2, we have now identified graphite grains, 1–4 µm in diameter, in the Murchison C2 chondrite. The interstellar origin of these grains is demonstrated by their 12C/13C ratio, which ranges from 0.09 to 16 times the Solar System value, and by the presence of the noble-gas component 'Ne–E(L)', nearly monoisotopic 22Ne from the decay of 22Na (with a half-life of 2.6 yr). The grains apparently formed in the outflows of novae and red giants, and demonstrate that graphite can form as a circumstellar condensate. Curiously, interstellar graphite is much rarer than interstellar microdiamonds (<2 p.p.m. compared to 400 p.p.m.) or even SiC (6–9 p.p.m.), although diamond is thermodynamically unstable relative to graphite. The main reason may be preferential destruction. Graphite is the third type of circumstellar grain that has become available for laboratory study.