Evidence for interstellar material in comets is deduced from IR spectra, in situ measurements of Comet Halley, and chondritic porous interplanetary dust particles (CP IDPs). IR spectra of comets reveal the spectrally active minerals: amorphous carbon, amorphous silicates, and (in some comets) crystalline silicates. Evidence suggests amorphous silicates are of interstellar origin while crystalline silicates are of nebular origin.
10 μm spectra of comets and sub-micron amorphous silicate spherules in CP IDPs have shapes similar to absorption spectra through lines-of-sight in the ISM. Thermal emission models of cometary IR spectra require Fe-bearing amorphous silicates. Fe-bearing amorphous silicates may be Fe-bearing crystalline silicates formed in AGB outflows that are amorphized through He+ ion bombardment in supernova shocks in the ISM.
Crystalline silicates in comets, as revealed by IR spectra, and their apparent absence in the ISM, argues for their nebular origin. The high temperatures (>1000 K) at which crystals form or are annealed occur in the inner nebula or in nebular shocks in the 5 – 10 AU region. Oxygen isotope studies of CP IDPs show only 1% by mass of the silicate crystals are of AGB origin. Together this suggests crystalline silicates in comets are probably primitive grains from the early solar nebula.