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  • 1
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    Clathrate type 2 hydrate formation in vacuo under astrophysical conditions (1991)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The properties of clathrate hydrates were used to explain the complex and poorly understood physical processes taking place within cometary nuclei and other icy solar system bodies. Most of all the experiments previously conducted used starting compositions which would yield clathrate types I hydrates. The main criterion for type I vs. type II clathrate hydrate formation is the size of the guest molecule. The stoichiometry of the two structure types is also quite different. In addition, the larger molecules which would form type II clathrate hydrates typically have lower vapor pressures. The result of these considerations is that at temperatures where we identified clathrate formation (120-130 K), it is more likely that type II clathrate hydrates will form. We also formed clathrate II hydrates of methanol by direct vapor deposition in the temperature range 125-135 K.
    Keywords: ASTROPHYSICS
    Type: Lunar and Planetary Inst., Abstracts for the 54th Annual Meeting of the Meteoritical Society; p 2
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    Low-pressure clathrate-hydrate formation in amorphous astrophysical ice analogs (1991)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: In modeling cometary ice, the properties of clathrate hydrates were used to explain anomalous gas release at large radial distances from the Sun, and the retention of particular gas inventories at elevated temperatures. Clathrates may also have been important early in solar system history. However, there has never been a reasonable mechanism proposed for clathrate formation under the low pressures typical of these environments. For the first time, it was shown that clathrate hydrates can be formed by warming and annealing amorphous mixed molecular ices at low pressures. The complex microstructures which occur as a result of clathrate formation from the solid state may provide an explanation for a variety of unexplained phenomena. The vacuum and imaging systems of an Hitachi H-500H Analytical Electron Microscope was modified to study mixed molecular ices at temperatures between 12 and 373 K. The resulting ices are characterized by low-electron dose Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED). The implications of these results for the mechanical and gas release properties of comets are discussed. Laboratory IR data from similar ices are presented which suggest the possibility of remotely observing and identifying clathrates in astrophysical objects.
    Keywords: ASTROPHYSICS
    Type: Lunar and Planetary Inst., Abstracts for the International Conference on Asteroids, Comets, Meteors 1991; p 23
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    NASA TECHNICAL REPORTS
  • 3
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    Direct determination of the morphology, structure and composition of cometary and interstellar ice analogs in the laboratory (1989)
    In:  Other Sources
    Details
    Publication Date: 2019-01-25
    Description: Ices of various kinds (H2O, CO, CO2, CH4, NH3, CH3OH, etc.) comprise a volumetrically significant proportion of objects in the solar system (comets, outer planets, planetary rings, satellites) as well as in interstellar space (astrophysical ices). The refinement of analytical electron microscopy (AEM) procedures for storing, preparing and analyzing ices and other materials at cryogenic temperatures is discussed. These procedures will be essential to the successful analysis of returned comet nucleus samples.
    Keywords: ASTROPHYSICS
    Type: Lunar and Planetary Inst., Workshop on Analysis of Returned Comet Nucleus Samples; p 9-10
    Format: text
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    NASA TECHNICAL REPORTS
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