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.
"No gas is released even during several days at this constant temperature. However, an increase by a mere 1–2 K results in the resumption of the gas flow from the ice back to its previous level (Laufer et al., 1987). While we cannot extrapolate from a lab experiment that runs for a few days to the age of the Solar System, the physics of the process of annealing of the ice shows a stepwise behavior. "
[Show abstract][Hide abstract] ABSTRACT: We present observational data for two long-period and three dynamically new comets observed at heliocentric distances between 5.8 to 14.0 AU. All of the comets exhibited activity beyond the distance at which water ice sublimation can be significant. We have conducted experiments on gas-laden amorphous ice samples and show that considerable gas emission occurs when the ice is heated below the temperature of the amorphous–crystalline ice phase transition (T∼137 K). We propose that annealing of amorphous water ice is the driver of activity in comets as they first enter the inner Solar System. Experimental data show that large grains can be ejected at low velocity during annealing and that the rate of brightening of the comet should decrease as the heliocentric distance decreases. These results are consistent with both historical observations of distant comet activity and with the data presented here. If observations of the onset of activity in a dynamically new comet are ever made, the distance at which this occurs would be a sensitive indicator of the temperature at which the comet had formed or represents the maximum temperature that it has experienced. New surveys such as Pan STARRS, may be able to detect these comets while they are still inactive.
"A median velocity of ~100 m s –1 was obtained. This result is not far from the value (≥167 m s –1 ) observed experimentally by Laufer et al. (1987) for ice grains ejected from a thin ice sample, when a large flux of gas was released. "
[Show abstract][Hide abstract] ABSTRACT: Laboratory experiments to simulate cometary materials and their processing contribute to the investigation of the properties and evolution of comets. Experimental methods can produce both refractory materials and frozen volatiles with chemical, structural, and morphological char- acteristics that reproduce those of materials observed and/or expected in comets. Systematic analyses of such samples, before and after energetic processing by various agents effective in the solar system, provide a wealth of useful quantitative information. Such data permit a more complete interpretation of observations, performed remotely or in situ, and suggest ideas about the chemical and physical evolution of cometary dust and ice. Finally, laboratory results help to predict the environmental conditions that future space missions, such as the European Space Agency's Rosetta mission, will experience, and thus aid in properly planning mission and in- strument development.
"As seen in Fig. 1, very large amounts of frozen Ar, much more than the amount of water ice, accumulate at 27 and 22 K at high deposition rates. Previously (Laufer et al., 1987), we attributed this high gas/ice ratio to the ejection of ice grains from the ice, propelled by gas jets, both of which were observed by the mass spectrometer. However, this explanation is not valid for very thin ice layers, where no grains or jets were observed. "
[Show abstract][Hide abstract] ABSTRACT: The effect of water ice formation temperature and rate of ice deposition on a cold plate on the amount of trapped argon (equivalent to CO), and the ratios of Ar/Kr/Xe trapped in the water ice were studied at 50, 27 and 22 K and at ice formation rates ranging over four orders of magnitude, from 10−1 to 10−5 μm min−1. Contrary to our previous conclusions that cometary ices were formed at 50–60 K, we now conclude that these ices were formed at about 25 K. At 25 K the enrichment ratios for Ar, Kr, and Xe remained the same as those at 50 K, reinforcing our suggestion of cometary contribution of these noble gases to the atmospheres of Earth and Mars.
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