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M.L. Marconi (Fresh Pond Research)
Recent observations of Ganymede indicate that Ganymede has a substantial atmosphere. Hall et al. (ApJ 499, 475, 1998) interpreted HST UV observations of O 1356 emissions from Ganymede as demonstrating the presence of an O2 column between 1 and 10 x 1014 cm-2 concentrated near the poles of Ganymede, suggesting a sputtering source. Barth et al. (GRL 24, 2147, 1997) detected H with the Galileo UV spectrometer and estimated a column of ~ 5 x 1012 cm-2. In addition, Galileo photopolarimeter- radiometer observations of the surface have been interpreted as indicating a maximum surface temperature of 152 K (Orton et al.; Science 274, 389, 1996). Taking this temperature at face value together with the high surface coverage of water-ice implies the presence of a significant H2O sublimation atmosphere near the subsolar point. There is also an energetic charged particle flux (Williams et al.; Science, 274, 401, 1996) that due to the presence of an intrinsic shielding magnetic field (Kivelson et al.; Nature 284, 537, 1996) is expected to produce a sputtered atmosphere of water group species near the poles.
A multispecies axisymmetric hybrid fluid/kinetic model for the species H2O, OH, O, H, O2, and H2 has been developed to calculate the quasi-collisional neutral atmosphere of Ganymede. The model also includes photodissociation, neutral-neutral chemistry, and electron impact dissociation. Sublimation of surface water-ice and polar sputtering are assumed to be the sources for the atmosphere. For nominal sputtering rates (R. E. Johnson private communication, 2001) reasonable agreement with the foregoing column densities is obtained. In addition, it appears that sputtering is the primary source of O2 while H is primarily from the electron impact dissociation of H2O. These and other results will be presented.
This work was sponsored by the NASA Planetary Atmospheres Program