  In this paper, we look at space weathering processes on the icy surface of
Jupiter's moon Europa. The heavy energetic ions of the Jovian plasma (H+, O+,
S+, C+) can erode the surface of Europa via ion sputtering (IS), ejecting up to
1000 H2O molecules per ion. UV Photons impinging the Europa's surface can also
result in neutral atom release via photon-stimulated desorption (PSD) and
chemical change (photolysis). In this work, we study the efficiency of the IS
and PSD processes for ejecting water molecules, simulating the resulting
neutral H2O density. We also estimate the contribution to the total neutral
atom release by the Ion Backscattering (IBS) process. Moreover, we estimate the
possibility of detecting the sputtered high energy atoms, in order to
distinguish the action of the IS process from other surface release mechanisms.
Our main results are: 1) The most significant sputtered-particle flux and the
largest contribution to the neutral H2O-density come from the incident S+ ions;
2) The H2O density produced via PSD is lower than that due to sputtering by
~1.5 orders of magnitude; 3) In the energy range below 1 keV, the IBS can be
considered negligible for the production of neutrals, whereas in the higher
energy range it becomes the dominant neutral emission mechanism; 4) the total
sputtering rate for Europa is 2.0\cdot 1027 H2O s-1; 5) the fraction of
escaping H2O via IS is 22% of the total sputtered population, while the escape
fraction for H2O produced by PSD is 30% of the total PSD population. Since the
PSD exosphere is lower than the IS one, the major agent for Europa's surface
erosion is IS on both the non-illuminated and illuminated side. Lastly, the
exospheric neutral density, estimated from the Galileo electron density
measurements appears to be higher than that calculated for H2O alone; this
favours the scenario of the presence of O2 produced by radiolysis and
photolysis.
