Loss rates of Europa's tenuous atmosphere

Abstract

Loss processes in Europa's tenuous atmosphere are dominated by plasma-neutral interactions. Based on the updated data of the plasma conditions in the vicinity of Europa (Bagenal et al. 2015), we provide estimations of the atmosphere loss rates for the H<SUB>2</SUB>O, O<SUB>2</SUB> and H<SUB>2</SUB> populations. Due to the high variability of the plasma proprieties, we perform our investigation for three sample plasma environment cases identified by Bagenal et al. as hot/low density, cold/high density, and an intermediate case. The role of charge-exchange interactions between atmospheric neutrals and three different plasma populations, i.e. magnetospheric, pickup, and ionospheric ions, is examined in detail. Our assumptions related to the pickup and to the ionospheric populations are based on the model by Sittler et al. (2013). We find that O<SUB>2</SUB>-O<SUB>2</SUB><SUP>+</SUP> charge-exchange is the fastest loss process for the most abundant atmospheric species O<SUB>2</SUB>, though this loss process has been neglected in previous atmospheric models. Using both the revised O<SUB>2</SUB> column density obtained from the EGEON model (Plainaki et al., 2013) and the current loss rate estimates, we find that the upper limit for the volume integrated loss rate due to O<SUB>2</SUB>-O<SUB>2</SUB><SUP>+</SUP> charge exchange is in the range (13-51)×10<SUP>26</SUP> s<SUP>-1</SUP>, depending on the moon's orbital phase and illumination conditions. The results of the current study are relevant to the investigation of Europa's interaction with Jupiter's magnetospheric plasma.