Comet Halley's Gas Composition and Extended Sources: Results from the Neutral Mass Spectrometer on Giotto

The Neutral Mass Spectrometer on the Giotto spacecraft established that H₂O is the dominant species in Comet Halley's volatiles and determined the abundance of more than 10 parent species. The instrument discovered strong extended H₂CO and CO sources in the coma of Comet Halley. Polymerized H₂CO associated with the cometary dust and evaporating slowly as the monomer is most likely the extended H₂CO source. Photodissociation of the H₂CO into CO fully accounts for the extended CO source. This revised version was published online in June 2006 with corrections to the Cover Date.