Ice Giants Revisited: Uranus and Neptune as Magma Ocean Worlds

Abstract

Uranus and Neptune are commonly interpreted as volatile-rich "ice giants", an assumption that underpins most interior models. Here we show that their observed radii, bulk densities, gravitational harmonics, normalized moments of inertia, intrinsic luminosities, and key features of their atmospheric compositions are consistent with interiors comprising supercritical, hydrogen-rich magma oceans overlain by H2-rich envelopes. Our results, based on three fit parameters for each planet, provide a parsimonious explanation for the structures, thermal states, and atmospheric chemistries of Uranus and Neptune. We find that the Solar System's ice giants are better understood as magma-ocean giants, with origins parallel to those of sub-Neptune gas-dwarf planets. A continuum among gas dwarf planets permits Neptune and Uranus to serve as accessible, data-driven test cases for structure models and material properties used to understand sub-Neptunes.

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BibTeX

@misc{young2026giants,
  title = {{Ice Giants Revisited: Uranus and Neptune as Magma Ocean Worlds}},
  author = {Edward D. Young and Sarah P. Marcum and Aaron Werlen and Paula N. Wulff},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2606.18219},
  eprint = {2606.18219},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2606.18219},
}

RIS

TY  - UNPB
TI  - Ice Giants Revisited: Uranus and Neptune as Magma Ocean Worlds
AU  - Edward D. Young
AU  - Sarah P. Marcum
AU  - Aaron Werlen
AU  - Paula N. Wulff
PY  - 2026
DA  - 2026/09/04
PB  - arXiv
DO  - 10.48550/arxiv.2606.18219
UR  - https://arxiv.org/abs/2606.18219
AB  - Uranus and Neptune are commonly interpreted as volatile-rich "ice giants", an assumption that underpins most interior models. Here we show that their observed radii, bulk densities, gravitational harmonics, normalized moments of inertia, intrinsic luminosities, and key features of their atmospheric compositions are consistent with interiors comprising supercritical, hydrogen-rich magma oceans overlain by H2-rich envelopes. Our results, based on three fit parameters for each planet, provide a parsimonious explanation for the structures, thermal states, and atmospheric chemistries of Uranus and Neptune. We find that the Solar System's ice giants are better understood as magma-ocean giants, with origins parallel to those of sub-Neptune gas-dwarf planets. A continuum among gas dwarf planets permits Neptune and Uranus to serve as accessible, data-driven test cases for structure models and material properties used to understand sub-Neptunes.
ER  -

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