Ice Giant

Ab Initio Entropy Calculations of Water Predict the Interiors of Uranus and Neptune to be 15-30% Colder than Previous Models

By Keith Cowing

Status Report

astro-ph.EP

August 29, 2025

Filed under astro-ph.EP, exoplanet, Ice Giants, Neptune, sub Neptunes, Uranus

Ab Initio Entropy Calculations of Water Predict the Interiors of Uranus and Neptune to be 15-30% Colder than Previous Models — Two representative interior models for Uranus and Neptune with four layers: 1) hydrogen and helium, 2) H2O and hydrogen, 3) carbon-nitrogen-hydrogen mixture and 4) the rocky core composed of iron and rock. m specifies the fractional layer masses. The temperatures for B and C type models are printed without and with parenthesis. The dashed lines in the second layer mark the conditions for the transition from liquid to superionic in pure H2O. — astro-ph.EP

Ab initio free energy calculations are employed to derive the entropy of liquid and superionic water over a wide range of conditions in the interiors of Uranus and Neptune. The resulting adiabats are much shallower in pressure-temperature space than those adopted for earlier models of Uranus and Neptune.

Our models for their interiors are thus much colder, increasing the likelihood that diamond rain or the recently predicted phase separation of planetary ices has occurred in the mantles of ice giant planets. Based on our ab initio data, we construct interior models for Uranus and Neptune with the Concentric MacLaurin Spheroid method that match the existing gravity measurements.

We compare fully convective models with models that include a convective boundary between liquid and superionic water. We also share a code to characterize giant planet atmospheres where para and ortho hydrogen as well as helium are present.

Burkhard Militzer

Comments: 25 pages, 12 figures, 1table, 1 external dataset, 1 computer code
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Materials Science (cond-mat.mtrl-sci)
Cite as: arXiv:2508.16754 [astro-ph.EP] (or arXiv:2508.16754v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.16754
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Journal reference: Astrophysical Journal 990 (2025) 20
Related DOI:
https://doi.org/10.3847/1538-4357/adef48
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Submission history
From: Burkhard Militzer
[v1] Fri, 22 Aug 2025 19:19:54 UTC (27,557 KB)
https://arxiv.org/abs/2508.16754

Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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