Kinetic Modeling of the Neutral Gas, Ions, and Charged Dust in Europa's Exosphere

Abstract:

The interaction of the Jovian magnetosphere with Europa has been a subject of active research during the last few decades both through in-situ and remote sensing observations as well as theoretical considerations. Linking the magnetosphere and the moon’s surface and interior, Europa’s exosphere has become one of the primary objects of study in the field. Understanding the physical processes occurring in the exosphere and its chemical composition is required for the understanding of the interaction between Europa and Jupiter.

Europa's surface-bound exosphere originates mostly from ion sputtering of the water ice surface. Minor neutral species and ions of exospheric origin are produced via photolytic and electron impact reactions. The interaction of the Jovian magnetosphere and Europa affects the exospheric population of both neutrals and ions via source and loss processes. Moreover, the Lorentz force causes the newly created exospheric ions to move preferably aligned with the magnetic field lines. Contrary to the ions, heavier and slow-moving charged dust grains are mostly affected by gravity and the electric field component of the Lorentz force. As a result, escaping dust forms a narrow tail aligned in the direction of the convection electric field.

Here we present results of a kinetic model of the neutral species (H₂O, OH, O₂, O, and H), ions (O⁺, O₂⁺, H⁺, H₂⁺, H₂O⁺, and OH⁺), and neutral and charged dust in Europa’s exosphere. In our model H₂O and O₂ are produced via sputtering and other exospheric neutral and ions species are produced via photolytic and electron impact reactions. For the charged dust we compute the equilibrium grain charge by balancing the electron and ion collecting currents according to the local plasma flow conditions at the grain’s location. For the tracking of the ions, charged dust, and the calculation of the grains’ charge we use plasma density and velocity, and the magnetic field derived from our multi-fluid MHD model of Europa’s interaction with the Jovian magnetosphere.

Support for this work was provided by grant NNX13AI66G from the NASA Planetary Atmospheres Program and RSA 1508179 from the Jet Propulsion Laboratory.