Updated Interior Structure of Callisto from a Reanalysis of Galileo Data and Perspectives for Geodetic Detection of the Ocean with Upcoming Missions

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 (Anderson et al., 2001). 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.

The inference that Callisto has a deep ocean is entirely based on the recent analysis by Cochrane et al. (2025), who reanalyzed Galileo magnetometer data and showed that they are best explained by the combination of induction in a subsurface ocean and in an ionosphere, rather than an ionosphere alone as suggested by previous work (Hartkorn and Saur, 2017). However, a stronger confirmation or rejection of the existence of an ocean will rely on the measurements that will be acquired by Europa Clipper and Juice. Based on the gravity field solutions and interior models derived in our analysis, we computed the geodetic parameters that can be used for ocean detection, including tidal response, librations and obliquity. We identified confidence intervals for ocean detection that will be useful for future studies based on measurements that will be acquired by future missions.

 

 

References

Anderson, J., Jacobson, R. A. & McElrath, T. P. Shape, Mean Radius, Gravity Field, and Interior Structure of Callisto. Icarus 153, 157–161 (2001).

Cochrane, C. J., Vance, S. D., Castillo‐Rogez, J. C., Styczinski, M. J. & Liuzzo, L. Stronger Evidence of a Subsurface Ocean Within Callisto From a Multifrequency Investigation of Its Induced Magnetic Field. AGU Advances 6, e2024AV001237 (2025).

Hartkorn, O. & Saur, J. Induction signals from Callisto’s ionosphere and their implications on a possible subsurface ocean. Journal of Geophysical Research: Space Physics 122, 11, 611–677, 697 (2017).