Carbon suboxide in comet Halley?

Abstract

OBSERVATIONAL data acquired during the recent appearance of comet Halley pose a puzzle about the nature and distribution of elemental carbon and carbonaceous material in its nucleus and coma. The nucleus is darker even than coal (albedo <4%), suggesting that its volatile ices contain a few per cent of carbonaceous material in the form of graphitic or amorphous carbon. The very high abundance of light elements in the coma dust, particularly H, C, N and O, suggests the presence of a significant organic component. The emission feature near 3.4 μm also implies the presence of organic material in the dust. But the parent species for the primary carbon-containing material that have been identified so far (such as CO, CO_2 and CH_4) are not present in sufficient quantities to account for all of it. Here we propose that an additional contribution from carbon suboxide (C_3O_2) in the coma dust and the nucleus material is consistent with the observational data. A production rate in the coma for C_3O_2 of about 0.03–0.04 times that of water would provide the distributed source of elemental carbon and CO within 10^4 km of the nucleus that is required to explain the data from the Giotto spacecraft and from ground-based observations.