Structure and density of Callisto’s atmosphere from a fluid-kinetic model of its ionosphere: Comparison with Hubble Space Telescope and Galileo observations

Abstract

We develop a model of the ionospheric electron population of Jupiter’s moon Callisto using a prescribed neutral atmosphere composed of O2, CO2 and H2O. A kinetic description of ionospheric suprathermal electrons coupled with a fluid description of ionospheric thermal electrons is well suited to jointly analyze and interpret observations of electron density and atmospheric UV emission. Accordingly, we calculate the electron energy distribution function at each point in the ionosphere by solving a coupled set of equations consisting of the Boltzmann equation for suprathermal electrons and the continuity and energy equation for thermal electrons. We assume a stationary balance between local sources and sinks of electrons and electron energy. Electron transport within the ionosphere is neglected, since collision time scales are shorter than transport time scales in the region of Callisto’s ionosphere where the major concentrations of electrons is located and the major part of the atmospheric UV emission is generated. We consider photoionization, which is the dominant ionospheric electron source, and secondary ionization from collisions of photoelectrons with neutrals. Our calculations yield electron densities and electron impact generated UV emissions from Callisto’s atmosphere. Comparing our modeled UV emission intensities with the Hubble Space Telescope observation of Cunningham et al. (2015) , we find that Callisto’s atmosphere has a mean O2 column density of 2.1−1.1+1.1×1019 m−2. A joint comparison with this HST observation and radio occultation observations of Kliore et al. (2002) shows that Callisto’s atmosphere possesses a day night asymmetry. We derive terminator O2 column densities of ∼ 0.4 × 1019 m−2, for which we find subsolar O2 column densities in the range of 2.4−9.8×1019 m−2. Our calculations also show that the electron density is very sensitive to the relative abundance of H2O due to the thermal electron cooling by rotational state excitation of H2O. For the efficiency of Callisto’s atmospheric UV emission we find that on average one photon is emitted at OI λ135.6 nm per every 170 electron ion pairs generated and per every 60 electron ion pairs produced by secondary electron impact ionization.