Ice Giants Revisited: Uranus and Neptune as Magma Ocean Worlds

Uranus and Neptune are commonly interpreted as volatile-rich "ice giants", an assumption that underpins most interior models. Here we show that their observed radii, bulk densities, gravitational harmonics, normalized moments of inertia, intrinsic luminosities, and key features of their atmospheric compositions are consistent with interiors comprising supercritical, hydrogen-rich magma oceans overlain by H2-rich envelopes. Our results provide a parsimonious explanation for the structure, thermal states, and atmospheric chemistry of Uranus and Neptune. We find that the Solar System's ice giants are better understood as magma-ocean giants, with origins parallel to those of sub-Neptune gas-dwarf planets.