Updated Interior Structure of Callisto from a Reanalysis of Galileo Data

The internal structure of Callisto has remained a significant open question since the Galileo mission, particularly its degree of differentiation compared to its neighbor Ganymede. Previous analyses of Galileo gravity data suggested a normalized Moment of Inertia (MOI) of approximately 0.35, implying a largely undifferentiated interior composed of a mixture of ice and rock. In this work, we present a comprehensive reanalysis of Galileo’s Doppler tracking data, including the previously unanalyzed C30 flyby, using modern orbit determination and signal processing techniques.

We provide two solutions for Callisto's gravity field. While our first solution assumes hydrostatic equilibrium and aligns with previous results, our favored solution also accounts for non-hydrostatic contributions arising from mass concentrations (mascons) associated with the Asgard and Valhalla impact basins. This improved model yields a normalized MOI of 0.345 ± 0.005, a value lower than previously reported canonical figures.

When combined with magnetic induction data, this lower MOI indicates that Callisto is more differentiated than previously believed. Our MCMC inversion indicates an interior structure consisting of a 10–120 km thick ice shell, a deep subsurface ocean approximately 300 km thick, and a large rocky core. Notably, the inferred density of the core is low, inconsistent with a pure rock composition. We propose that the core contains a significant mass fraction of organic material mixed with rock, similar to the interior configuration proposed for Saturn’s moon Titan. These findings challenge the traditional view of Callisto as a simple mixture of ice and rock and offer new constraints on the formation of giant icy moons in the outer solar system.