Published online by Cambridge University Press: 27 February 2018
Comet dust consists primarily of silicate and carbonaceous material. The carbonaceous material is not yet well-characterized. The 3.36 μm emission feature arises primarily from gas phase molecules, while a small feature at 3.29 μm may be due to aromatic hydrocarbons in the grains. Olivine, a high temperature condensate, is present in some comets. Polarization and albedo maps have shown that the coma is not uniform; an inner halo of large particles may be present.