Europa's atmosphere, gas tori, and magnetospheric implications

Abstract

A two-dimensional kinetic model calculation for the water group species (H 2O, H 2, O 2, OH, O, H) in Europa's atmosphere is undertaken to determine its basic compositional structure, gas escape rates, and velocity distribution information to initialize neutral cloud model calculations for the most important gas tori. The dominant atmospheric species is O 2 at low altitudes and H 2 at higher altitudes with average day–night column densities of 4.5 × 10 14 and 7.7 × 10 13 cm −2 , respectively. H 2 forms the most important gas torus with an escape rate of ∼ 2 × 10 27 s −1 followed by O with an escape rate of ∼ 5 × 10 26 s −1 , created primarily as exothermic O products from O 2 dissociation by magnetospheric electrons. The circumplanetary distributions of H 2 and O are highly peaked about the satellite location and asymmetrically distributed near Europa's orbit about Jupiter, have substantial forward clouds extending radially inward to Io's orbit, and have spatially integrated cloud populations of 4.2 × 10 33 molecules for H 2 and 4.0 × 10 32 atoms for O that are larger than their corresponding populations in Europa's local atmosphere by a factor of ∼200 and ∼1000, respectively. The cloud population for H 2 is a factor of ∼3 times larger than that for the combined cloud population of Io's O and S neutral clouds and provides the dominant neutral population beyond the so-called ramp region at 7.4–7.8 R J in the plasma torus. The calculated brightness of Europa's O cloud on the sky plane is very dim at the sub-Rayleigh level. The H 2 and O tori provide a new source of europagenic molecular and atomic pickup ions for the thermal plasma and introduce a neutral barrier in which new plasma sinks are created for the cooler iogenic plasma as it is transported radially outward and in which new sinks are created to alter the population and pitch angle distribution of the energetic plasma as it is transported radially inward. The europagenic instantaneous pickup ion rates are peaked at Europa's orbit, dominate the iogenic pickup ion rates beyond the ramp region, and introduce new secondary plasma source peaks in the solution of the plasma transport problem. The H 2 torus is identified as the unknown Europa gas torus that creates both the observed loss of energetic H + ions at Europa's orbit and the corresponding measured ENA production rate for H.