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: In our experimental study we observed massive ice grain ejection driven by CO2, during several temperature ranges when the ice is warmed up. A'Hearn et al., 2011. Science 332, 1396–1400 and Meech et al., 2011. The Astrophysical Journal Letters 734, L1 among others, found that CO2 is the major driving force of the activity of Comet 103P/Hartley 2. In our experimental studies we found that CO2 is trapped in low temperature "cometary" amorphous water ice 3–4 orders of magnitude more efficiently than gases such as CO, CH4 and Ar. Thus, the ice grains which agglomerated to form the nuclei of comets where CO dominates CO2 in the coma seem to have formed in regions highly depleted in CO2. In the experiments we observed ice grains ranging from submicron to hundreds of microns, accompanied by jets of CO2. The experimentally observed cm size fragments, which were formed by cracking the overlying ice layer, could also be ejected under the cometary microgravity. These size ranges were also observed on Comet 103P/Hartley 2. The smooth terrain observed on Hartley 2's "waist" is similar to the smooth terrain observed on comet Temple 1 and could have been formed by settling of ice grains ejected nearby, as observed in our experiments.Planetary and Space Science 09/2013; · 1.63 Impact Factor
- Journal of Geophysical Research Atmospheres 06/1999; 104(E6):14179-14182. · 3.44 Impact Factor
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ABSTRACT: In Part 2 of this paper we comment on the modeling of the complex interactions which take place in cometary comae and tails between parent molecules, radicals, ions, dust grains and the solar electromagnetic and corpuscular radiation, and we summarize some of the current thoughts about the nature of the elusive cometary nucleus. Comets are ephemeral phenomena whose lifetimes are short on the cosmic scale; their evolution, statistically and as individual objects, is a main theme in contemporary research. Although their origins are still not well known, comets undoubtedly carry important clues to the early history and evolution of the solar system. Finally, we mention the main questions now being asked by cometary studies and illustrate some of the future observational possibilities which may provide crucial data for the next steps forward.Astronomy and Astrophysics Review 01/1993; · 13.31 Impact Factor