A Direct Simulation Monte Carlo–Driven Photochemical Model of Callisto’s Ionosphere

• Published in: The Planetary Science Journal
• Main Authors: Shane R. Carberry Mogan, Luke E. Moore, Lucas Liuzzo, Andrew R. Poppe
• Format: Article
• Language: English
• Published: IOP Publishing 2025-01-01
• Subjects: Exosphere; Callisto; Galilean satellites; Planetary science; Planetary atmospheres; Planetary ionospheres
• Online Access: https://doi.org/10.3847/PSJ/adc5eb

Here we present a photochemical model of Callisto’s ionosphere with inputs supplied by a direct simulation Monte Carlo model of its neutral atmosphere. We compare a model that considers interactions with photons as the sole external ionization mechanism to models also including magnetospheric electron impacts, where upstream electron densities and temperatures are varied over an order of magnitude to constrain the role that the highly variable electron population near Callisto has on the underlying ionospheric structure. Depending on the implemented upstream electron density, magnetospheric electron impacts dominate the production of Callisto’s ionosphere or induce reactions at rates comparable to or less than those from solar photons. Furthermore, depending on the implemented upstream electron temperature, electron impacts either preferentially dissociate or ionize molecules. We show that ionization of an O _2 component with a surface number density of 10 ^9 cm ^−3 , which is consistent with recent remote observations and models, is capable of producing the electron densities detected during Galileo radio occultations, from which an O _2 surface density of ∼10 ^10 cm ^−3 was initially inferred. The modeled total plasma densities are also compared to those inferred from Galileo plasma-wave measurements and yield a reasonable agreement up to ∼1000 km, presenting the first model capable of simultaneously producing both Galileo radio occultations and plasma-wave observations. Finally, the implications of this work are discussed, highlighting several leads that need to be explored going forward to better constrain Callisto’s atmosphere, ionosphere, and local plasma environment in anticipation of the eventual Jupiter Icy Moons Explorer observations.