On the orbital variability of Ganymede's atmosphere

TL;DR

This study models Ganymede's atmosphere, revealing its spatial distribution and temporal variability driven by surface interactions, solar radiation, and Jupiter's gravity, with implications for aurora emissions.

Contribution

Developed a 3-D collisional exospheric model to analyze Ganymede's atmospheric composition and variability, highlighting the role of Jupiter's gravity and surface processes.

Findings

O$_2$ atmosphere peaks at the equator, especially at dusk.

H$_2$O sputtering rate varies with longitude and hemisphere.

Jupiter's gravity influences the spatial distribution of exospheric components.

Abstract

Ganymede's atmosphere is produced by radiative interactions with its surface, sourced by the Sun and the Jovian plasma. The sputtered and thermally desorbed molecules are tracked in our Exospheric Global Model (EGM), a 3-D parallelized collisional model. This program was developed to reconstruct the formation of the upper atmosphere/exosphere of planetary bodies interacting with solar photon flux and magnetospheric and/or the solar wind plasmas. Here, we describe the spatial distribution of the H$_2$O and O$_2$ components of Ganymede's atmosphere, and their temporal variability along Ganymede's rotation around Jupiter. In particular, we show that Ganymede's O$_2$ atmosphere is characterized by time scales of the order of Ganymede's rotational period with Jupiter's gravity being a significant driver of the spatial distribution of the heaviest exospheric components. Both the sourcing and the Jovian gravity are needed to explain some of the characteristics of the observed aurora emissions. As an example, the O$_2$ exosphere should peak at the equator with systematic maximum at the dusk equator terminator. The sputtering rate of the H$_2$O exosphere should be maximum on the leading hemisphere because of the shape of the open/close field lines boundary and displays some significant variability with longitude.