Neutral particle release from Europa’s surface

Abstract

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 H 2O 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 H 2O 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 H 2O density come from the incident S + ions; (2) the H 2O 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 × 10 27 H 2O s −1; and (5) the fraction of escaping H 2O via IS is 22% of the total sputtered population, while the escape fraction for H 2O 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 total net 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 H 2O alone; this favors the scenario of the presence of O 2 produced by radiolysis and photolysis.