Amorphous water ice and its ability to trap gases

Abstract

The trapping of argon by amorphous water ice at 19–80 K and its release from the ice were studied experimentally, by flowing gas into ice, or by codepositing a gas-water vapor mixture. Upon warming the gas-laden ice, the trapped argon is released from it in seven temperature ranges: (a) 23 K, (b) 35 K, (c) 44 K, (d) ∼80 K, (e) 136.8 K, (f) 160.0 K, and (g) ∼180 K. The amount of internally trapped argon, in (b)–(g), can be as high as 3.3 times the amount of the ice itself. By using argon to probe the ice’s structure and dynamics, it was found that the highly porous amorphous ice anneals at all temperatures, at a rate which is strongly temperature dependent, and transforms into the cubic form at 136.8±1.6 K and then into the hexagonal form at 160.0±1.0 K. When gas is made to flow into the amorphous ice, it fills open holes and cracks in it with gaseous or frozen argon, depending upon the temperature. The slow creeping of the ice closes some gas-filled holes and squeezes out some of the gas, while locking the rest and letting it escape only during the transformations. The last range, (g), is attributed to the simultaneous evaporation of gas and water from the argon clathrate-hydrate. Gas flow at a pressure exceeding 2.6 dyn ${\mathrm{cm}}^{\mathrm{-}2}$, results in a very fluffy ice, with a density of only 0.17 g ${\mathrm{cm}}^{\mathrm{-}3}$. The release of gas from this kind of ice, or from ice codeposited with gas, is accompanied by massive ejection of 0.1–1-μm ice grains and by argon jets, which propel them. Many of the experimental findings could be important for interpreting observations on comets, icy satellites, icy ring particles, and interstellar grains.