O_2/O_3 MICROATMOSPHERES IN THE SURFACE OF GANYMEDE

摘要

Radiation-induced bubble formation, a process known to cause the deterioration of reactor materials, produces O_2/O_3 microatmospheres in the icy surface of Ganymede. Energetic ions in Jupiter's magnetosphere bombard this surface, producing vacancies and radicals in the ice. At the equatorial temperatures on Ganymede, the radicals migrate and react efficiently, forming new volatile species H_2 and O_2, and the vacancies aggregate to form voids. Whereas the H_2 is lost readily, the O_2 accumulates, permeating the regolith, producing a weak atmosphere, and becoming trapped in the voids, forming gas-filled bubbles. Such bubbles (microatmospheres) form efficiently above ～ 0.3-0.5 times the melting temperature of the material and can attain a high density of O_2, consistent with the observation of "condensed" O_2 on Ganymede. Dissociation of O_2 in a microatmosphere by UV photons or the incident ions leads to formation of O_3. Including the O(~1D) interactions in the Chapman equations, a ratio O_3/O_2 ～ 2 x 10~(-4) is obtained, close to the estimate based on observations of Ganymede.