The interior of Neptune

Abstract

Neptune, together with Uranus, belongs to the ice-rich class of Jovian planets. Although molecular hydrogen is the dominant species in its atmosphere, all available evidence points toward an interior composition dominated by the planetary ices, primarily water. In some models, small amounts of metallic hydrogen may also be present in deeper layers. If a rock component (magnesium-silicates and iron) is present, it is either underabundant with respect to solar composition, or is not strongly differentiated from the ice component. Neptune interior models which fit constraints imposed by atmospheric chemistry, mean density, and gravitational harmonics, strongly resemble Uranus models. Yet Neptune and Uranus differ substantially in their interior heat flow budgets. While Neptune's intrinsic heat flow is reliably detected, only an upper limit is available for the intrinsic heat flow of Uranus. Possible important influences on interior heat flow include solar heating at the upper boundary, and compositional gradients in the interior. The potential for hydromagnetic dynamo action in the interior depends on the vigor of convection in electrically conducting regions. Electrical conductivity in the icy material is adequate for dynamo action if temperatures within Neptune are comparable to those achieved in the shock experiments. Although the zonal gravitational harmonics J_2 and J_4 provide important clues to interior structure, for a given pressure-density relation the J_4 harmonic can be substantially reduced in absolute value by the observed atmospheric equatorial subrotation, if the subrotation involves significant mass.

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