The effect of methanol clathrate-hydrate formation and other gas-trapping mechanisms on the structure and dynamics of cometary ices

Gas trapping by clathrate-hydrate formation in low temperature "cometary" w ater ice has been quoted repeatedly, especially regarding CO. However, of t he plethora of gases detected in Comets Halley, Hyakutake, and Hale-Bopp, o nly methanol (CH3OH) and hydrogen sulfide (H2S) were shown experimentally t o form clathratehydrates under the low temperatures and densities of the so lar nebula or in the collapsing interstellar cloud. We studied experimental ly three physico-chemical processes relevant to the ice grains which agglom erated into cometary nuclei: (1) Clathrate-hydrate formation with methanol does not alter the trapping of other gases such as CH4, Ar, CO, and N-2, wh ich do not form clathrate-hydrates. It does, however, change somewhat their release from the ice upon its warming up, between 120 and 135 K, yet leavi ng intact the major gas release temperature range when the ice itself subli mates. (2) When gas is released from the warmed up upper layers, even a 200 0-molecule-thick laver is impermeable enough to drive some of the gas inwar d and bury it in underlying ice layers, while releasing a large fraction of gas outward. (3) The structural changes in the ice are sluggish, but are a ided by the presence of trapped gas, which prevents the formation of well b ound, tight, structures. These experimental measurements might aid the obse rvers and modelers in understanding comets. (C) 2000 Academic Press.