The seven mysteries of Callisto: A science case for a next-generation orbiter and lander mission

This is the authors' abstract. We don't add key points for this paper.

Callisto, the outermost and least evolved Galilean moon, is a primordial witness to the Jupiter System’s formation that has recorded its evolution and exhibits dynamic present-day processes. In this article, we identify seven key mysteries that Callisto can illuminate: (1) the accretionary origin of the Galilean moons; (2) the early outer Solar System impact record; (3) the nature and maintenance of Callisto’s putative subsurface ocean; (4) its tidal coupling mechanisms with Jupiter; (5) the billions of years of its surface chemical evolution; (6) the dynamics of its tenuous atmosphere-ionosphere system; and (7) its unique location and potential role as a probe of Jovian magnetospheric processes. While the JUICE and Europa Clipper missions, currently in the cruise phase, will provide critical flyby data, fully resolving these interconnected puzzles demands a dedicated mission. We detail a science traceability matrix for a mission combining a long-duration, low-altitude (200 km or less) orbiter for global geophysical and compositional mapping with a geophysics/geochemistry-focused lander. An orbiter will measure the gravity field (for moment of inertia), internal structure, tidal deformation, and global surface properties, and offer the best possible location for a magnetospheric observatory. A lander will enable such transformative investigations as very broad-band seismology, precise geodetic tracking, and direct surface chemistry measurements. This synergistic orbiter-lander architecture is the definitive path to decoding Callisto’s frozen record, answering fundamental questions about icy moon formation and degree of internal differentiation, internal oceans, and moon-magnetosphere interactions. It will ideally complement the on-going inventory of Galilean icy moons and set a new benchmark for a deeper understanding of satellite systems across the outer Solar System.