  Ganymede's atmosphere is produced by radiative interactions with its surface,
sourced by the Sun and the Jovian plasma. The sputtered and thermally desorbed
molecules are tracked in our Exospheric Global Model (EGM), a 3-D parallelized
collisional model. This program was developed to reconstruct the formation of
the upper atmosphere/exosphere of planetary bodies interacting with solar
photon flux and magnetospheric and/or the solar wind plasmas. Here, we describe
the spatial distribution of the H$_2$O and O$_2$ components of Ganymede's
atmosphere, and their temporal variability along Ganymede's rotation around
Jupiter. In particular, we show that Ganymede's O$_2$ atmosphere is
characterized by time scales of the order of Ganymede's rotational period with
Jupiter's gravity being a significant driver of the spatial distribution of the
heaviest exospheric components. Both the sourcing and the Jovian gravity are
needed to explain some of the characteristics of the observed aurora emissions.
As an example, the O$_2$ exosphere should peak at the equator with systematic
maximum at the dusk equator terminator. The sputtering rate of the H$_2$O
exosphere should be maximum on the leading hemisphere because of the shape of
the open/close field lines boundary and displays some significant variability
with longitude.
