EFFECT OF RADIATIVE TRANSPORT ON THE EVOLUTION OF JUPITER AND SATURN

Conventional evolutionary models for Jupiter and Saturn, which assume convection throughout the entire planet interior, yield ages of 5.1 Gy r for Jupiter and 2.6 Gyr for Saturn. Even though the discrepancy for Saturn can be explained by the additional energy source due to a phase separation of helium, it seems difficult to reconcile the age of full y convective Jovian models with the age of the solar system, i.e., 4.5 Gyr. It has been recently shown that these planets are probably not f ully convective, but retain a stable radiative window near the surface . We present new evolutionary models for these two planets, which do i nclude the aforementioned possibility of radiative transport in the mo lecular hydrogen-helium envelope. These calculations yield ages of 4.2 Gyr for Jupiter and 2.4 Gyr for Saturn. We show that the importance o f the radiative window was larger in the past than now, so that the ra tio of the radiative to the adiabatic gradient in the radiative region increases with time. This speeds up the cooling with respect to a ful ly adiabatic planet. Since the interiors of the new Jupiter and Saturn models are significantly cooler than the adiabatic ones, it is likely that immiscibility of helium occurs in both planets. That provides a natural explanation for the observed helium depletion in their atmosph eres and the fact that the ages inferred from homogeneous evolution mo dels of these two planets are smaller than the age of the solar system .