Assessing Io's Libration Using JunoCam Acquisitions

NASA’s Juno mission has been orbiting Jupiter since 2016, aiming to unveil the planet’s origin and evolutionary history. Equipped with a suite of advanced remote sensing instruments, the spacecraft has delivered groundbreaking insights into Jupiter’s atmosphere, magnetic field, and internal structure, enabled by its highly elliptical orbit. In 2021, after completing 35 perijoves, Juno’s prime mission ended, transitioning into its extended mission phase. This extended phase is characterized by altered trajectories, driven by perturbations from Jupiter’s complex gravity field. These new trajectories have provided invaluable opportunities for close approaches to the Galilean satellites, including Io, the most volcanically active body in the Solar System.

The prevailing understanding of Io's intense volcanic activity attributes it to the energy generated by tidal deformations resulting from variations in its distance from Jupiter during its orbit. This energy may lead to varying degrees of interior melting, which in turn determines whether Io's subsurface hosts a global magma ocean. The presence and depth of such a melted subsurface layer significantly influences the dynamic behavior of Io’s crust. Notably, this manifests as diurnal librations, whose amplitude depends on whether a magma ocean is present. However, the phenomenon of longitudinal librations of Io’s surface has yet to be observed directly. Two of the most recent flybys of Juno, I57 and I58, that occurred on December 30, 2023, and February 3, 2024 enabled the acquisition of high-resolution images of the moon using the JunoCam camera. From an altitude of approximately 1500 km, these JunoCam data products contain valuable data to estimate or provide an upper bound for the amplitude of the longitudinal libration of Io.

Incorporating optical observables generated from landmarks and surface features with traditional radiometric measurements can enhance orbit determination procedures and enable the estimation of an upper bound for Io’s diurnal libration, contributing to a more comprehensive understanding of Io's interior structure and evolution. In this work, we focus on the analysis of JunoCam images to obtain the coordinates of notable surface features and the target’s centroid in the camera frame. The uncertainty in the estimation of these and other parameters depends on the number of available features and images, as well as the accuracy of the registration methodology employed. While the high resolution of JunoCam images acquired during the flybys may not be sufficient to clearly determine Io's libration amplitude, we aim to establish a reliable constraint on the models describing the libration of Io.