The Geophysics of the Icy Galilean Satellites: Surface Evolution and Gravity

The icy Galilean satellites of Jupiter (Europa, Ganymede, and Callisto) are considered ocean worlds. Because habitability necessitates an energy source, a key geophysical question is how active or “warm” these worlds are or were in the past. I investigate these moons’ surfaces and interiors, aiming to shed more light on their unique geophysical properties and processes. A common theme across this research is exploring whether the materials composing these moons are warm enough to flow and behave in a ductile manner mechanically. I investigate this through large impact craters for all three satellites and through gravity in the case of Europa. These moons present unique impact crater morphologies whose origins are poorly understood. In Chapters 2 and 3, I use numerical simulations to investigate the formation and evolution of large craters on Ganymede, Callisto, and Europa. The stresses induced by a newly formed crater prompt the ice to redistribute itself over million-year time scales, gradually relaxing and restoring a flatter surface. I find that viscoelastic relaxation of ice, when accounting for impact-generated heat, sufficiently explains the formation of dome craters on Ganymede and Callisto. For Tegid Crater on Europa, which exhibits a seemingly flattened dome, additional mechanisms beyond relaxation are required to explain its shape. In Chapter 4, I present a methodology for retrieving line-of-sight gravity anomalies in preparation for NASA’s Europa Clipper mission. Gravity anomalies stem from deviations in the reference gravity field, such as those caused by seafloor topography. A cold and rigid seafloor can support greater topography, potentially resulting in stronger gravity anomalies. Conversely, a warmer silicate crust allows topography to subside into more ductile mantle material, compensating the topography and producing weaker gravity anomalies. In this way, gravity measurements can indicate the thermal state of Europa’s interior. I implement a data processing method to convert the spacecraft’s velocity changes—observed when flying close to Europa through Doppler shifts in the radio signal—into line-of-sight gravity anomalies. Using synthetic mission data, my results show that these anomalies can indeed be detected above anticipated noise levels.