Icy or rocky? Convective or stable? New interior models of Uranus and Neptune

TL;DR

This work tackles the poorly constrained interiors of Uranus and Neptune by introducing an agnostic yet physically consistent interior modelling framework that blends random density profiles with a compositional-EoS routine. The global algorithm iteratively refines density, temperature, and composition profiles via the Theory of Figures and an EoS-based compositional solver to satisfy hydrostatic balance and match observed gravity data, yielding a wide family of self-consistent models. The results show both rock-dominated and water-dominated interiors are compatible with current data, and they reveal systematic differences in outer H–He content and dynamo-region depths between Uranus and Neptune, while all models feature convective ionic-water layers capable of driving their multipolar dynamos. The findings challenge the conventional ice-giant classification, illustrate substantial compositional degeneracy, and provide a flexible, data-driven framework for interpreting future observations or missions aimed at constraining planetary interiors.

Abstract

We present a new framework for constructing agnostic and yet physical models for planetary interiors and apply it to Uranus and Neptune. Unlike previous research that either impose rigid assumptions or rely on simplified empirical profiles, our approach bridges both paradigms. Starting from randomly generated density profiles, we applied an iterative algorithm that converges towards models that simultaneously satisfy hydrostatic equilibrium, match the observed gravitational moments, and remain thermodynamically and compositionally consistent. The inferred interior models for Uranus and Neptune span a wide range of possible interior structures, in particular encompassing both water-dominated and rock-dominated configurations (rock-to-water mass ratios between 0.04-3.92 for Uranus and 0.20-1.78 for Neptune). All models contain convective regions with ionic water and have temperature-pressure profiles that remain above the demixing curves for hydrogen-helium-water mixtures. This offers both a plausible explanation for the observed non-dipolar magnetic fields and indicates that no hydrogen-helium-water demixing occurs. We find a higher H-He mass fraction in the outermost convection zones for Uranus (0.62-0.73) compared to Neptune (0.25-0.49) and that Uranus' magnetic field is likely generated deeper in the interior compared to Neptune. We infer upper limits of 0.69-0.74 (Uranus) versus 0.78-0.92 (Neptune) for the outer edges of the dynamo regions in units of normalised radii. Overall, our findings challenge the conventional classification of Uranus and Neptune as 'ice giants' and underscore the need for improved observational data or formation constraints to break compositional degeneracy.

Figures (15)

• Figure 1: Overview of our global algorithm to infer agnostic and self-consistent planetary interior models. The difference between $\tilde{\rho}$ used by the compositional algorithm and $\rho$ compatible with the interior pressure and planetary mass decreases until it vanishes using our iterative process. Traditionally, empirical models only use a single iteration, leading to inconsistencies. Figure \ref{['fig:global_algorithm']} contains more details.
• Figure 2: Inferred composition and convective and radiative structures for the four Uranus interior models. The legends summarise the total mass fractions for each component. The uppermost small arc of each slice is empty because we do not infer a composition for pressures below 100 bars.
• Figure 3: Density, pressure, and temperature profiles for the four Uranus models. Top: Density versus normalised radius. For comparison, also shown are empirical solutions from Neuenschwander2022 (grey area). The panel in the top right shows Uranus' measured French2024 and calculated (coloured) gravitational moments. We also show Uranus' gravitational moments according to Jacobson2025 (grey) who find different results (COR vs COO) corresponding to different ring centre definitions. The coloured uncertainties depict estimated errors from the Theory of Figures. Bottom: Temperature versus pressure. Also shown is the predicted region of ionic water inferred by Redmer2011. The dashed lines show the demixing line, below which hydrogen, helium, and water are expected to become immiscible Howard2025.
• Figure 4: Same as Figure \ref{['fig:Uranus_composition_slices']}, but for Neptune.
• Figure 5: Same as Figure \ref{['fig:Uranus_densities_TvsP']}, but for Neptune.