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

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2015-01, Vol.245, p.306-319
• Main Authors: Plainaki, Christina, Milillo, Anna, Massetti, Stefano, Mura, Alessandro, Jia, Xianzhe, Orsini, Stefano, Mangano, Valeria, De Angelis, Elisabetta, Rispoli, Rosanna
• Format: Article
• Language: English
• Subjects: Computer simulation; Ganymede; Ices; Jupiter; Jupiter (planet); Jupiter satellites; Magnetospheres; Mathematical models; Precipitation; Satellites, atmospheres
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2014.09.018

•At small altitudes above the moon’s subsolar point the main exosphere component is sublimated H2O.•The sputtered-H2O distribution exhibits close correspondence with the plasma precipitation map.•The energetic O2 distribution depends both on the plasma surface impact and surface temperature.•Assuming full mirroring, surface sputtering at polar cap explains the observed higher albedo.•The estimated H2O and O2 escape rates are ∼1.2⋅1025s−1and ∼4.2⋅1026s−1 respectively. 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. Our results 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 direct