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.

The distant activity of the Long Period Comets C/2003 O1 (LINEAR) and C/2004 K1 (Catalina)

“…In contrast, models and simulations performed on the SPCs and the few observed LPCs showed that CO and other volatiles can be present in the cometary nuclei as gases trapped in the cells of amorphous water ice and that a significant release of gas can occur when the irreversible transition from amorphous to crystalline state takes place, at temperatures around 90 K (Prialnik et al 2005), corresponding to r h ∼ 11 AU. In this case, the trapped gas could be of relatively high abundance: water ice has the ability to trap gases up to a gas-to-ice ratio of 3.3, as seen in laboratory experiments (Laufer et al 1987).…”

The dual origin of the terrestrial atmosphere

“…5) and it is unclear whether volatiles degassed from the Earth could have had the low Xe/Kr ratio and unfractionated isotopic ratios required by some models (Pepin 1991, 1992, 1997, Tolstikhin and O'Nions 1994. The noble gas composition of comets can be inferred from laboratory experiments (Laufer et al 1987, Bar-Nun et al 1988, Bar-Nun and Owen 1998, Notesco et al 1999, Notesco et al 2003, Owen et al 1992, Owen and Bar-Nun 1995a Table 1 and Fig. 1).…”

Computer modeling of the thermal conductivity of cometary ice

“…The X-ray results on hda ice also show (Jenniskens and Blake, 1994), and amorphous ice layers from smooth and dense (Jenniskens and Blake, 1994) to extremely porous and "fluffy" (Laufer et al, 1987) …”