Is CO2 responsible for the outbursts of comet Halley?

Outbursts are thought to originate from the nucleus of comet Halley; cameras on the Vega and Giotto spacecraft have recorded the images of jets^1,2, and rapid fluctuations are seen in the visible light curve, detected, for example, by the International Ultraviolet Explorer satellite during the time the spacecraft were making their observations³. This activity does not appear to be limited to when the comet is near perihelion, but has been reported for heliocentric distances >4.6AU pre-perihelion⁴. The outbursts are probably triggered by the heating of relatively volatile material within the ice. We report here spectroscopic data from the IUE showing a strong correlation between a visible outburst in the near-nucleus region on 18.8 March 1986 and the sudden appearance of a large column abundance of CO⁺₂ ions in the tail. These data suggest that CO² may play a significant role in the outburst process. This outburst may have been the source of the detached clumps of CO⁺ emission recorded by ground-based observers on 19 and 20 March 1986^5,6