Future Directions for the Investigation of Surface-Bounded Exospheres in the Inner Solar System

Abstract

Surface-bounded exospheres result from complex interactions between the planetary environment and the rocky body’s surface. Different drivers including photons, ion, electrons, and the meteoroid populations impacting the surfaces of different bodies must be considered when investigating the generation of such an exosphere. Exospheric observations of different kinds of species, i.e., volatiles or refractories, alkali metals, or water group species, provide clues to the processes at work, to the drivers, to the surface properties, and to the release efficiencies. This information allows the investigation on how the bodies evolved and will evolve; moreover, it allows us to infer which processes are dominating in different environments. In this review we focus on unanswered questions and measurements needed to gain insights into surface release processes, drivers, and exosphere characterizations. Future opportunities offered by upcoming space missions, ground-based observations, and new directions for modelling are also discussed.

How Does the Thermal Environment Affect the Exosphere/Surface Interface at Mercury?

“…Volatiles have also been observed by the NASA MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission, which performed several years of exospheric observations around Mercury (Cassidy et al 2016;Vervack et al 2016). Several mechanisms of ejection of these volatiles from the surface have been described and reproduced in the laboratory (Milillo et al 2023). The efficiency of these mechanisms (in particular, photon-stimulated desorption, PSD, and thermal desorption) is species-dependent but also depends on the surface characteristics (Wurz et al 2023) and the surface temperature.…”