Infrared emission by organic grains in the coma of comet Halley

Spacecraft¹ and ground-based^2-5 observations of comet Halley in the near-infrared reveal a triple-peaked emission feature near 3.4 μm, characteristic of C-H stretching in hydrocarbons (-CH³ and -CH²- alkanes). Here we discuss transmission spectra of organic residues produced by laboratory irradiation of candidate cometary ices, which serve as the basis for a model of emitting cometary dust. The laboratory synthesis of solid organic residue from irradiated low-occupancy methane ice clathrate simulates the radiation processing experienced by comet Halley. The transmission spectrum of this residue, convolved with a simple two-component thermal emission model based on the spacecraft-determined dust distribution in the Halley coma, fits the 3.4 μm feature, provides optical depths in excellent agreement with those observationally determined, and accounts for the absence of features at longer wavelengths (despite their presence in transmission spectra of typical ice irradiation residues).