Hydrogen isocyanide in comet 73P/Schwassmann-Wachmann (fragment B)

We present a sensitive 3 sigma upper limit of 1.1% for the HNC/HCN abundance ratio in comet 73P/SchwassmannWachmann (fragment B), obtained on 2006 May 10-11, using Caltech Submillimeter Observatory (CSO). This limit is a factor of similar to 7 lower than the values measured previously in moderately active comets at 1 AU from the Sun. Comet 73P/Schwassmann-Wachmann was depleted in most volatile species, except of HCN. The low HNC/HCN ratio thus argues against HNC production from polymers produced from HCN. However, thermal degradation of macromolecules, or polymers, produced from ammonia and carbon compounds, such as acetylene, methane, or ethane appears a plausible explanation for the observed variations of the HNC/HCN ratio in moderately active comets, including the very low ratio in comet 73P/Schwassmann-Wachmann reported here. Similar polymers have been invoked previously to explain anomalous N-14/N-15 ratios measured in cometary CN.