Structure and dynamics of amorphous water ice.
ABSTRACT Further insight into the structure and dynamics of amorphous water ice, at low temperatures, was obtained by trapping in it Ar, Ne, H2, and D2. Ballistic water-vapor deposition results in the growth of smooth, approximately 1 x 0.2 micrometer2, ice needles. The amorphous ice seems to exist in at least two separate forms, at T < 85 K and at 85 < T < 136.8 K, and transform irreversibly from one form to the other through a series of temperature-dependent metastable states. The channels formed by the water hexagons in the ice are wide enough to allow the free penetration of H2 and D2 into the ice matrix even in the relatively compact cubic ice, resulting in H2-(D2-) to-ice ratios (by number) as high as 0.63. The larger Ar atoms can penetrate only into the wider channels of amorphous ice, and Ne is an intermediate case. Dynamic percolation behavior explains the emergence of Ar and Ne (but not H2 and D2) for the ice, upon warming, in small and big gas jets. The big jets, each containing approximately 5 x 10(10) atoms, break and propel the ice needles. Dynamic percolation also explains the collapse of the ice matrix under bombardment by Ar , at a pressure exceeding 2.6 dyn cm-2, and the burial of huge amounts of gas inside the collapsed matrix, up to an Ar-to-ice of 3.3 (by number). The experimental results could be relevant to comets, icy satellites, and icy grain mantles in dense interstellar clouds.
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ABSTRACT: The thermal evolution of a spherical cometary nucleus (initial radius of 2.5 km), composed initially of very cold amorphous ice and moving in comet Halley's orbit, is simulated numerically for 280 revolutions. It is found that the phase transition from amorphous to crystalline ice constitutes a major internal heat source. The transition does not occur continuously, but in five distinct rounds, during the following revolutions: 1, 7, 40-41, 110-112, and 248-252. Due to the (slow) heating of the amorphous ice between crystallization rounds, the phase transition front advances into the nucleus to progressively greater depths: 36 m on the first round, and then 91 m, 193 m, 381 m, and 605 m respectively. Each round of crystallization starts when when the boundary between amorphous and crystalline ice is brought to approximately 15 m below the surface, as the nucleus radius decreases due to sublimation. At the time of crystallization, the temperature of the transformed ice rises to 180 K. According to experimental studies of gas-laden amorphous ice, a large fraction of the gas trapped in the ice at low temperatures is released. Whereas some of the released gas may find its way out through cracks in the crystalline ice layer, the rest is expected to accumulate in gas pockets that may eventually explode, forming "volcanic calderas." The gas-laden amorphous ice thus exposed may be a major source of gas and dust jets into the coma, such as those observed on comet Halley by the Giotto spacecraft. The activity of new comets and, possibly, cometary outbursts and splits may also be explained in terms of explosive gas release following the transition from amorphous to crystalline ice.The Astrophysical Journal 03/1987; 313(2):893-905. · 6.73 Impact Factor
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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 cm-2, results in a very fluffy ice, with a density of only 0.17 g cm-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.Physical review. B, Condensed matter 03/1987; 35(5):2427-2435.