HR: 0800h
AN: P11D-0804    [Abstracts]
TI: Laboratory Studies of Ammonia-Water Ice--Relevance to Outer Solar system Surfaces
AU: * Moore, M H
EM: Marla.H.Moore@nasa.gov
AF: NASA Goddard Space Flight Center, Astrochemistry Laboratory, Greenbelt, MD 20771,
AU: Ferrante, R F
EM: ferrante@usna.edu
AF: US Naval Academy, Department of Chemistry, Annapolis, MD 21402,
AU: Hudson, R L
EM: hudsonrl@eckerd.edu
AF: Eckerd College, Department of Chemistry, St Petersburg, FL 33733,
AU: Hudson, R L
EM: hudsonrl@eckerd.edu
AF: NASA Goddard Space Flight Center, Goddard Center for Astrobiology, Greenbelt, MD 20771,
AB: Water- and ammonia-ices have been observed or postulated as important components of the icy surfaces of planetary satellites and KBO objects in the outer Solar System. A feature in the near-infrared, near 2.2 microns, is attributed to NH₃ in the spectra of Charon, Quaoar, and Miranda, a possible source being cryomagma brought up to cold surfaces where it is quenched. Models used to fit the data include the presence of ammonia hydrates. These observations, along with some evidence for NH₃ ice on Enceladus, lead to the prediction that NH₃ is one of the more abundant species incorporated into outer Solar System bodies. Significant gaps exist in our knowledge of the spectra of solid water and ammonia mixtures, the formation of NH₃-hydrates, and the stability of these ices for relevant physical conditions. The role of thermal processing and exposure to different particle radiation environments has not been investigated. Therefore our laboratory study has examined the influences of composition, formation temperature, thermal- and radiation-processing, on the spectra of both pure NH₃ ice and various H₂O- NH₃ icy mixtures. We have completed low-temperature spectroscopic studies (1 to 20 microns) of H₂O -rich ices containing NH₃, with an emphasis on features in the near-IR region, which is accessible to ground-based observers. Conditions for the formation and thermal stability of the ammonia hemihydrate and the ammonia monohydrate have been examined. The former undergoes a slow loss of NH₃ in a vacuum environment, to give the monohydrate. Additional warming removes the remaining NH₃ to give the spectrum of H₂O -ice. IR band positions of NH₃ in different H₂O -ices have been tabulated, and compared to the positions for NH₃ hydrates over a large temperature range. We report spectral shifts that depend on both concentration and temperature. The radiation-induced amorphization of hydrates was observed and the radiolytic destruction of NH₃ in H₂O -ices was measured. We also have tabulated the near-IR spectral position of the ammonium ion which is observed to form in irradiated water and ammonia ices. Implications of these results for the formation, stability, and detection NH₃ on Charon and other icy satellite surfaces will be discussed.
UR: http://www- 691.gsfc.nasa.gov/cosmic.ice.lab/
DE: 6020 Ices
DE: 6060 Radiation and chemistry
DE: 6224 Kuiper belt objects
DE: 6270 Pluto and satellites
DE: 6280 Saturnian satellites
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting