CARBON SUBOXIDE IN COMET HALLEY

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%) 1, 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 2,3, particularly H, C, N and O, suggests the presence of a significant organic component. The emission feature near 3.4-mu-m also implies the presence of organic material in the dust 4-6. But the parent species for the primary carbon-containing material that have been identified so far (such as CO, CO2 and CH4) are not present in sufficient quantities to account for all of it. Here we propose that an additional contribution from carbon suboxide (C3O2) in the coma dust and the nucleus material is consistent with the observational data. A production rate in the coma for C3O2 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.