The Influence of Non-Thermal Collisions in Europa's Atmosphere

In this study, we show that non-thermal collisions can play a significant role in shaping Europa's exospheric structure. Collisions between radiolytically produced O2 ${\mathrm{O}}_{2}$ and the O produced via electron impact dissociation of O2 ${\mathrm{O}}_{2}$ affect the exospheric structure and escape rates. Specifically, O + O2 ${\mathrm{O}}_{2}$ collisions lead to the production of a non-thermal O2 ${\mathrm{O}}_{2}$ population, and increase the O2 ${\mathrm{O}}_{2}$ escape while decreasing the O escape. These collisions are dependent on three specific physical parameters: (a) the density of O2 ${\mathrm{O}}_{2}$, (b) the electron impact dissociation rate of O2 ${\mathrm{O}}_{2}$, and (c) the O + O2 ${\mathrm{O}}_{2}$ collision cross section. We demonstrate here that O + O2 ${\mathrm{O}}_{2}$ collisions affect Europa's atmosphere even in the lowest limits considered. Thus, to more accurately determine the influence O + O2 ${\mathrm{O}}_{2}$ collisions have on Europa's atmosphere in preparation for the forthcoming spacecraft missions, Europa Clipper and the JUpiter ICy moons Explorer (JUICE), these physical parameters need to be better constrained. This study investigates the effects of collisions in Europa's atmosphere, specifically focusing on those involving oxygen atoms (O) and molecules (O2 ${\mathrm{O}}_{2}$). We find that collisions between O2 ${\mathrm{O}}_{2}$ produced within Europa's icy surface and the O it produces via dissociative processes while aloft in the atmosphere can affect the structure of Europa's atmosphere as well as the rates at which neutral particles escape to the local space environment. The study highlights three important factors that influence these collisions: the density of O2 ${\mathrm{O}}_{2}$ near Europa's surface, how often O2 ${\mathrm{O}}_{2}$ molecules are broken apart by electrons producing O, and the size of the O + O2 ${\mathrm{O}}_{2}$ collision cross section. Constraining these values is crucial for accurately modeling Europa's atmosphere in preparation for the forthcoming spacecraft missions, Europa Clipper and the JUpiter ICy moons Explorer (JUICE). DSMC modeling demonstrates that collisions cannot be neglected in modeling or analysis studies of Europa's atmosphere Collisions between radiolytically produced O2 and its dissociated product O produce a non-thermal O2 population and enhances O2 escape The O2 non-thermal densities and escape rates produced via O + O2 collisions can far exceed those due to sputtering