Interior models of Uranus and Neptune

"Empirical" models (pressure versus density) of Uranus and Neptune interiors constrained by the gravitational coefficients J₂, J₄, the planetary radii and masses, and Voyager solid-body rotation periods are presented. The empirical pressure-density profiles are then interpreted in terms of physical equations of state of hydrogen, helium, ice (H₂O), and rock (SiO₂) to test the physical plausibility of the models. The compositions of Uranus and Neptune are found to be similar with somewhat different distributions of the high-Z material. The big difference between the two planets is that Neptune requires a non-solar envelope, while Uranus is best matched with a solar composition envelope. Our analysis suggests that the heavier elements in both Uranus' and Neptune's interior might increase gradually toward the planetary centers. Indeed it is possible to fit the gravitational moments without sharp compositional transitions.