Effect of Radiative Transport on the Evolution of Jupiter and Saturn

Astronomy and Astrophysics – Astronomy

Scientific paper

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Convection, Planets And Satellites: Individual Jupiter, Planets And Satellites: Individual Saturn, Radiative Transfer

Scientific paper

Conventional evolutionary models for Jupiter and Saturn, which assume convection throughout the entire planet interior, yield ages of 5.1 Gyr 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 fully 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 fully convective, but retain a stable radiative window near the surface. We present new evolutionary models for these two planets, which do include the aforementioned possibility of radiative transport in the molecular hydrogen-helium envelope. These calculations yield ages of 4.2 Gyr for Jupiter and 2.4 Gyr for Saturn. We show that the importance of the radiative window was larger in the past than now, so that the ratio of the radiative to the adiabatic gradient in the radiative region increases with time. This speeds up the cooling with respect to a fully 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 atmospheres and the fact that the ages inferred from homogeneous evolution models of these two planets are smaller than the age of the solar system.

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