Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p23e..03b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P23E-03
Physics
[6020] Planetary Sciences: Comets And Small Bodies / Ices, [6045] Planetary Sciences: Comets And Small Bodies / Physics And Chemistry Of Materials
Scientific paper
Amorphous solid water (ASW) is ubiquitous in space and has been identified as a major component in many astrophysical ices, including cometary nuclei, planetary rings, on the surfaces satellites and planets and frozen out on the surface of interstellar dust grains. The H2O content within these ices typically ranges between 60-70%, hence H2O plays a significant role in interstellar chemistry. It has been shown to catalyze reactions ranging from simple diatomics (such as H2) up to large complex organic molecules thought to be the precursors of life on earth. Furthermore, numerous laboratory studies have shown that ASW can trap a wide range of astrophysically relevant molecules within its porous structure which are released into the gas phase when ASW undergoes a phase transition to cubic crystalline ice (CI). Consequently the physical transformation of ASW to CI has attracted considerable interest in the study of model interstellar ices. Several groups have studied bulk and surface crystallization of ASW on a variety of different substrates in order to determine the mechanism of crystallization. However the exact mechanism of the ASW-CI transition is still not fully understood. Furthermore, a majority of these studies have been performed using ASW films of thicknesses < 50 nm which do not fully reflect the thicknesses of astrophysical ices. With this in mind the crystallization kinetics of 0.3 μm thick vapour deposited ASW films over a temperature range of 130-140 K have been investigated using Fourier Transform Infrared Spectroscopy. The isothermal phase transformation of bulk ASW to CI is well described by the Avrami equation, which shows that crystallization proceeds via continuous nucleation and three dimensional growth of crystallites throughout the ice, with overall activation energy of 59.4 ± 2.2 kJ mol-1. Parameters derived from fitting the crystallization data are used to predict the ASW transformation on astrophysically relevant timescales and at temperatures typically experienced in interstellar space (T < 120 K). The data from this study provides further insight to elucidating the thermal history of astrophysical ices in addition to the timescales and temperatures which trapped molecules within these ices can be released into the gas phase and undergo further chemistry. These processes are particularly important in warmer regions of the interstellar medium, such as molecular hot cores, where temperatures are in excess of 100 K.
Baragiola Raúl A.
Burke Daren J.
Fama Marcelo A.
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