Simultaneous Evolutionary Fits for Jupiter and Saturn Incorporating Fuzzy Cores

With the recent realization that there likely are stably stratified regions in the interiors of both Jupiter and Saturn, we construct new nonadiabatic, inhomogeneous evolutionary models with the same microphysics for each that result at the present time in respectable fits for all major bulk observables for both planets. These include the effective temperature, radius, atmospheric heavy-element and helium abundances (including helium rain), and the lower-order gravity moments J₂ and J₄. The models preserve from birth most of an extended “fuzzy” heavy-element core. Our predicted atmospheric helium mass fraction for Saturn is ∼0.2, close to some measured estimates but in disagreement with some published predictions. To preserve a fuzzy core from birth, the interiors of both planets must start out at lower entropies than would be used for traditional “hot start” adiabatic models, though the initial exterior mantle entropies can range from hot to warm start values. We do not see a helium ocean in Saturn’s interior, and both models have inner envelopes with significant Brunt-Väisälä frequencies; this region for Saturn at the current epoch is more extended, and in it, the Brunt is larger. The total heavy-element mass fraction in Jupiter and in Saturn is determined to be ∼14% and ∼26%, respectively, though there is some play in these determinations.