Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p51b0451g&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #P51B-0451
Physics
0317 Chemical Kinetic And Photochemical Properties, 1010 Chemical Evolution, 1045 Low-Temperature Geochemistry
Scientific paper
The signature of condensed O2 has been reported in optical reflectance measurements of the Jovian moon Ganymede, and a tenuous oxygen atmosphere has been observed surrounding Europa. The surfaces of these moons contain large amounts of water ice, and it is thought that O2 is formed by sputtering of ice by energetic particles from the Jovian magnetosphere. Knowledge of how O2 is produced in low-temperature ice is crucial for accurate theoretical and experimental simulations of the surfaces and atmospheres of icy solar system bodies. However, the role of electronic excitations and the mechanistic details are poorly understood. This talk will present an overview of the non-thermal processes involved in the production and build up of oxygen and related species in icy surfaces like that of Europa. Discussion will include laboratory measurements of the electron energy threshold, flux and fluence dependence, temperature dependence, and cross-section for O2 production during low-energy (5-100 eV) electron bombardment of thin (~40 bilayer) amorphous and crystalline D2O ice films in vacuum that are suitable surrogates of the conditions found on Europa. Comparison of the fluence dependence with kinetic models shows that O2 is formed by direct excitation and dissociation of a stable precursor molecule, and not by diffusion and chemical recombination of radicals. The O2 yield is also strongly dependent on the temperature of the ice, is different for crystalline and amorphous ice films, and shows structure indicative of bulk structural transitions. We also report a previously unobserved hysteresis with temperature cycling. The results point to a novel explanation of thermal effects in ice sputtering, based on a temperature dependence of the dissociative excited state lifetime of water.
Grieves G.
Johnson Robert E.
Orlando T.
Sieger M.
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