We examine the comparative thermal evolution of Jupiter and Saturn applying recent theoretical results for helium ’ s immiscibility in fluid metallic hydrogen .
The redistribution of helium in their interiors proceeds very differently for the two planets .
We confirm that based on Jupiter ’ s atmospheric helium depletion as observed in situ by the Galileo entry probe , Jupiter ’ s interior helium has differentiated modestly , and we present models reconciling Jupiter ’ s helium depletion , radius , and heat flow at the solar age .
Jupiter ’ s recently revised Bond albedo implies a lower intrinsic flux for the planet , accommodating more luminosity from helium differentiation such that mildly superadiabatic interiors can satisfy all constraints .
The same phase diagram applied to the less massive Saturn predicts dramatic helium differentiation to the degree that Saturn inevitably forms a helium-rich shell or core , an outcome previously proposed by Stevenson & Salpeter and others .
The luminosity from Saturn ’ s helium differentiation is sufficient to extend its cooling time to the solar age , even for adiabatic interiors .
This model predicts Saturn ’ s atmospheric helium to be depleted to Y = 0.07 \pm 0.01 , corresponding to a He/H _ { 2 } mixing ratio 0.036 \pm 0.006 .
We also show that neon differentiation may have contributed to both planets ’ luminosity in the past .
These results demonstrate that Jupiter and Saturn ’ s thermal evolution can be explained self-consistently with a single physical model , and emphasize that nontrivial helium distributions should be considered in future models for Saturn ’ s internal structure and dynamo .