Enrichment of CO over N-2, by their trapping in amorphous ice and implications to Comet P/Halley

Observations of Comet P/Halley showed a cosmic abundance ratio of carbon and oxygen but a depletion of nitrogen. We checked the composition of amorphous water ice samples deposited on a cold plate from gaseous mixtures with various CO:N-2:H2O-vapor ratios at several temperatures. From measuring the amounts of N-2 and CO which are trapped together in the ice, we found an obvious enrichment of carbon monoxide over molecular nitrogen. We therefore propose a possible explanation for the depletion of N-2 in the volatile inventory of Comet P/Halley. The alternative possibility, that intact ice-coated interstellar grains were coagulated into cometary nuclei, is also discussed and arguments against it are brought up. (C) 1996 Academic Press, Inc.