The final way in which the composition of the outer planet atmospheres may evolve is though differentiation, where heavier materials preferentially fall towards the center of the planet decreasing their abundance in the exterior. This is a separate effect to the initial differentiation of the giant planets arising from the different phases of their growth described in Chapter 2. This secondary differentiation is inhibited by convection which tends to make the interior of the planets well-mixed and homogeneous. If differentiation does occur, however, it should be observable both via observed depletions in the outer parts of the atmosphere and through the additional source of internal heat that such gravitational settling would release. To evaluate the degree of internal heating currently active in the interiors of the giant planets we need to compare their current observed bolometric temperatures with those expected were the planets to be in thermal equilibrium with incident solar radiation.
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