Carbon suboxide in Halley

SIR - Crovisier et al. conclude from an examination of the VEGA 1 IKS infrared spectra that carbon suboxide (C_3O_2) is not present in the comet Halley ices at the abundance level suggested by us. They show the measured spectrum near 4 μm and note the absence of the strong C_3O_2 line emission, relative to the CO_2 emission, that should be present if the C_3O_2 production rate was comparable to that of CO_2. Key assumptions in this analysis are that C_3O_2 is released from the comet nucleus and has a scale length similar to the CO_2 scale length.