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The Metastable Persistence of Vapor-Deposited Amorphous Ice at Anomalously High TemperaturesStudies of the gas release, vaporization behavior and infrared (IR) spectral properties of amorphous and crystalline water ice have direct application to cometary and planetary outgassing phenomena and contribute to an understanding of the physical properties of astrophysical ices. Several investigators report anomalous phenomena related to the warming of vapor-deposited astrophysical ice analogs. However gas release, ice volatilization and IR spectral features are secondary or tertiary manifestations of ice structure or morphology. These observations are useful in mimicking the bulk physical and chemical phenomena taking place in cometary and other extraterrestrial ices but do not directly reveal the structural changes which are their root cause. The phenomenological interpretation of spectral and gas release data is probably the cause of somewhat contradictory explanations invoked to account for differences in water ice behavior in similar temperature regimes. It is the microstructure, micromorphology and microchemical heterogeneity of astrophysical ices which must be characterized if the mechanisms underlying the observed phenomena are to be understood. We have been using a modified Transmission Electron Microscope to characterize the structure of vapor-deposited astrophysical ice analogs as a function of their deposition, temperature history and composition. For the present experiments, pure water vapor is deposited at high vacuum onto a 15 K amorphous carbon film inside an Hitachi H-500H TEM. The resulting ice film (approx. 0.05 micrometers thick) is warmed at the rate of 1 K per minute and diffraction patterns are collected at 1 K intervals. These patterns are converted into radial intensity distributions which are calibrated using patterns of crystalline gold deposited on a small part of the carbon substrate. The small intensity contributed by the amorphous substrate is removed by background subtraction. The proportions of amorphous and crystalline material in each pattern are determined by subtracting a percentage of crystalline component relative to amorphous and pure crystalline endmembers. Vapor-deposited water ice undergoes two amorphous to amorphous structural transformations in the temperature range 15-130 K with important astrophysical implications. The onset of cubic crystallization occurs at 142-160 K (at 1K per minute heating rates) during which the 220 and 311 diffraction maxima appear and 0.1 micrometer crystallites can be seen in bright field images. This transition is time dependent.
Document ID
20020034881
Acquisition Source
Ames Research Center
Document Type
Conference Paper
Authors
Blake, David F.
(NASA Ames Research Center Moffett Field, CA United States)
Jenniskens, Peter
(NASA Ames Research Center Moffett Field, CA United States)
DeVincenzi, Donald L.
Date Acquired
August 20, 2013
Publication Date
January 1, 1995
Subject Category
Astrophysics
Meeting Information
Meeting: Lunar and Planetary Science Conference
Location: Houston, TX
Country: United States
Start Date: March 13, 1995
End Date: March 17, 1995
Funding Number(s)
PROJECT: RTOP 152-13-60
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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