Hydrogenated Amorphous Carbon Grains in Comet Halley?

Abstract

Recent IR observations of Comets Halley, Wilson, and Bradfield have shown the existence of an emission feature falling at around 3.4 microns. In this paper, it is shown that a good fit of both the IR continuum and the band can be obtained by assuming a simple thermal emission model based upon the optical properties of hydrogenated amorphous carbon (HAC) grains, measured in laboratory. The best fits of P/Halley's spectra are obtained for HAC dust amounts which appear consistent with the constraints imposed by the observed production rates. The presence of amorphous carbon solid particles is also supported on the basis of in situ mass spectrometry measurements performed by the Vega 1/2 and Giotto spacecraft in the environment of P/Halley. At present, it appears reasonable to suggest that amorphous carbon grains are able to match the observations and can be considered among likely candidates for cometary materials.