The Effect of Methanol Clathrate–Hydrate Formation and Other Gas-Trapping Mechanisms on the Structure and Dynamics of Cometary Ices
✍ Scribed by G. Notesco; A. Bar-Nun
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 171 KB
- Volume
- 148
- Category
- Article
- ISSN
- 0019-1035
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✦ Synopsis
Gas trapping by clathrate-hydrate formation in low temperature "cometary" water ice has been quoted repeatedly, especially regarding CO. However, of the plethora of gases detected in Comets Halley, Hyakutake, and Hale-Bopp, only methanol (CH 3 OH) and hydrogen sulfide (H 2 S) were shown experimentally to form clathratehydrates under the low temperatures and densities of the solar nebula or in the collapsing interstellar cloud. We studied experimentally three physico-chemical processes relevant to the ice grains which agglomerated into cometary nuclei: (1) Clathrate-hydrate formation with methanol does not alter the trapping of other gases such as CH 4 , Ar, CO, and N 2 , which 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 leaving intact the major gas release temperature range when the ice itself sublimates.
(2) When gas is released from the warmed up upper layers, even a 2000-molecule-thick layer is impermeable enough to drive some of the gas inward 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 aided by the presence of trapped gas, which prevents the formation of well bound, tight, structures. These experimental measurements might aid the observers and modelers in understanding comets.