Fe embedded in ice: The impacts of sublimation and energetic particle bombardment. (May 2015)

• Record Type: Journal Article
• Authors: Frankland, Victoria L.; Plane, John M.C.
• Is Part Of: Journal of atmospheric and solar-terrestrial physics. Volume 127(2015:May)
• Page Start: 103
• Page End: 110
• Publication Date: 2015-05
• Subjects: Noctilucent clouds -- Meteoric metals -- Meteoric smoke particles -- Energetic particle precipitation -- Ion sputtering; Geophysics -- Periodicals; Atmospheric physics -- Periodicals; Géophysique -- Périodiques; Météorologie physique -- Périodiques; Electronic journals; 551.51
• DOI: 10.1016/j.jastp.2014.12.004
• Languages: English
• ISSNs: 1364-6826

Abstract

Abstract: Icy particles containing a variety of Fe compounds are present in the upper atmospheres of planets such as the Earth and Saturn. In order to explore the role of ice sublimation and energetic ion bombardment in releasing Fe species into the gas phase, Fe-dosed ice films were prepared under UHV conditions in the laboratory. Temperature-programmed desorption studies of Fe/H2 O films revealed that no Fe atoms or Fe-containing species co-desorbed along with the H2 O molecules. This implies that when noctilucent ice cloud particles sublimate in the terrestrial mesosphere, the metallic species embedded in them will coalesce to form residual particles. Sputtering of the Fe-ice films by energetic Ar + ions was shown to be an efficient mechanism for releasing Fe into the gas phase, with a yield of 0.08 (Ar + energy=600 eV). Extrapolating with a semi-empirical sputtering model to the conditions of a proton aurora indicates that sputtering by energetic protons (>100 keV) should also be efficient. However, the proton flux in even an intense aurora will be too low for the resulting injection of Fe species into the gas phase to compete with that from meteoric ablation. In contrast, sputtering of the icy particles in the main rings of Saturn by energetic O + ions may be the source of recently observed Fe + in the Saturnian magnetosphere. Electron sputtering (9.5 keV) produced no detectable Fe atoms or Fe-containing species. Finally, it was observed that Fe(OH)2 was produced when Fe was dosed onto an ice film at 140 K (but not at 95 K). Electronic structure theory shows that the reaction which forms this hydroxide from adsorbed Fe has a large barrier of about 0.7 eV, from which we conclude that the reaction requires both translationally hot Fe atoms and mobile H2 O molecules on the ice surface. Highlights: When ice containing Fe atoms evaporates, Fe does not co-desorb. Auroral sputtering of Fe from ice particles is ineffective. Sputtering from the rings of Saturn is a possible source of magnetospheric Fe + .