The Discovery of Argon in Comet C/1995 O1 (Hale-Bopp)
  On 30.14 March 1997 we observed the EUV spectrum of the bright comet C/1995 O1 (Hale-Bopp) at the time of its perihelion, using our EUVS sounding rocket telescope/spectrometer. The spectra reveal the presence H Ly beta, O+, and, most notably, Argon. Modelling of the retrieved Ar production rates indicates that comet Hale-Bopp is enriched in Ar relative to cosmogonic expectations. This in turn indicates that Hale-Bopp's deep interior has never been exposed to the 35-40 K temperatures necessary to deplete the comet's primordial argon supply. 
S.A. Stern, D.C. Slater, M.C. Festou, J.Wm. Parker, G.R. Gladstone.   M.F. A'Hearn, and E. Wilkinson