The H2O and O2 exospheres of Ganymede: The result of a complex interaction between the jovian magnetospheric ions and the icy moon

The H2O and O2 exospheres of Jupiter’s moon Ganymede are simulated through the application of a 3D Monte Carlo modeling technique that takes into consideration the combined effect on the exosphere generation of the main surface release processes (i.e. sputtering, sublimation and radiolysis) and the surface precipitation of the energetic ions of Jupiter’s magnetosphere. In order to model the magnetospheric ion precipitation to Ganymede’s surface, we used as an input the electric and magnetic fields from the global MHD model of Ganymede’s magnetosphere (Jia, X., Walker, R.J., Kivelson, M.G., Khurana, K.K., Linker, J.A. [2009]. J. Geophys. Res. 114, A09209). The exospheric model described in this paper is based on EGEON, a single-particle Monte Carlo model already applied for a Galilean satellite (Plainaki, C., Milillo, A., Mura, A., Orsini, S., Cassidy, T. [2010]. Icarus 210, 385–395; Plainaki, C., Milillo, A., Mura, A., Orsini, S., Massetti, S., Cassidy, T. [2012]. Icarus 218 (2), 956–966; Plainaki, C., Milillo, A., Mura, A., Orsini, S., Saur [2013]. Planet. Space Sci. 88, 42–52); nevertheless, significant modifications have been implemented in the current work in order to include the effect on the exosphere generation of the ion precipitation geometry determined strongly by Ganymede’s intrinsic magnetic field (Kivelson, M.G. et al. [1996]. Nature 384, 537–541). The current simulation refers to a specific configuration between Jupiter, Ganymede and the Sun in which the Galilean moon is located close to the center of Jupiter’s Plasma Sheet (JPS) with its leading hemisphere illuminated. sults are summarized as follows: (a) at small altitudes above the moon’s subsolar point the main contribution to the neutral environment comes from sublimated H2O; (b) plasma precipitation occurs in a region related to the open-closed magnetic field lines boundary and its extent depends on the assumption used to mimic the plasma mirroring in Jupiter’s magnetosphere; (c) the spatial distribution of the directly sputtered-H2O molecules exhibits a close correspondence with the plasma precipitation region and extends at high altitudes, being, therefore, well differentiated from the sublimated water; (d) the O2 exosphere comprises two different regions: the first one is an homogeneous, relatively dense, close to the surface thermal-O2 region (extending to some 100s of km above the surface) whereas the second one is less homogeneous and consists of more energetic O2 molecules sputtered directly from the surface after water-dissociation by ions has taken place; the spatial distribution of the energetic surface-released O2 molecules depends both on the impacting plasma properties and the moon’s surface temperature distribution (that determine the actual efficiency of the radiolysis process).