Effect of the Magnetospheric Plasma Interaction and Solar Illumination on Ion Sputtering of Europa's Surface Ice

For the entire ion energy range observed at Europa, we calculate spatially-resolved maps of the surface sputtering rates of H₂O_, O₂, and H₂ from impacts by magnetospheric ions. We use the perturbed electromagnetic fields from a hybrid model of Europa’s plasma interaction, along with a particle-tracing tool, to calculate the trajectories of magnetospheric ions impinging onto the surface and their resultant sputtering yields. We examine how the distribution of the sputtering rates depends on the electromagnetic field perturbations, the angle between the solar radiation and the corotating plasma flow, and the thickness of the oxygen-bearing layer within Europa's surface. Our major findings are: (a) Magnetic field-line draping partially diverts the impinging ions around Europa, reducing the sputtering rates on the upstream hemisphere, but allowing for substantial sputtering from the downstream hemisphere. In contrast, zero sputtering occurs in much of the downstream hemisphere with uniform electromagnetic fields. (b) If the oxygen-bearing surface layer is thin compared to the penetration depth of magnetospheric ions, thermal ions dominate the O₂ sputtering rates, and the region of strongest sputtering is persistently located near the upstream apex. However, if the oxygen-bearing layer is thick compared to the penetration depth, energetic ions sputter the most O₂, and the location of maximum sputtering follows the sub-solar point as Europa orbits Jupiter. (c) The global production rate of O₂ from Europa’s surface varies by a factor of three depending upon the moon's orbital position, with the maximum particle release occurring when Europa's sun-lit and upstream hemispheres coincide.