Reassessing planetary composition: Evidence of rock-dominated envelopes in Uranus and Neptune

Abstract

Although Uranus and Neptune are commonly classified as ice giants, their exact compositions remain poorly constrained. Recent studies of outer Solar System bodies challenge the traditional view that these planets are primarily ice-dominated, suggesting that refractory material plays a more significant role. Determining the proportions of ice and rock within Uranus and Neptune is essential for understanding their formation and the evolutionary history of the Solar System. In this work we computed interior structure models for both planets and explored, within a Bayesian framework, the range of compositions that satisfy the available observational constraints. We quantified the resulting ice and rock fractions and analyzed their impact on the inferred internal structure. Our results suggest that the envelopes of both Uranus and Neptune are systematically enriched in refractory material, with median rock fractions of approximately 60% within the heavy-element component, similar to Pluto, Kuiper belt objects, and comets. In contrast, the deep interiors of the two planets exhibit distinct compositions: Neptune is best fit by relatively rock-rich mantles (median rock fraction of ~ 55%), whereas Uranus is inferred to have more ice-rich mantles (median rock fraction of ~ 41%), consistent with a more strongly stratified structure. These results point to compositional differences between Uranus and Neptune that may reflect divergent formation and evolutionary pathways.

21 pages, 6 figures, 2 tables, accepted for publication in A&A

References in corpus (15)

• Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core
• New models of Jupiter in the context of Juno and Galileo
• Jupiter internal structure: the effect of different equations of state
• Jupiter's inhomogeneous envelope
• The Formation of Uranus & Neptune: Challenges and Implications For Intermediate-Mass Exoplanets
• Evidence for a Dichotomy in the Interior Structures of Jupiter and Saturn from Helium Phase Separation
• Jupiter's interior from Juno: Equation-of-state uncertainties and dilute core extent
• Thermal evolution of Uranus and Neptune I: adiabatic models
• Evolution of Jupiter and Saturn with helium rain
• Could Uranus and Neptune form by collisions of planetary embryos?
• The interior of Uranus: Thermal profile, bulk composition and the distribution of rock, water and hydrogen and helium
• H${}_2$-H${}_2$O demixing in Uranus and Neptune: Adiabatic structure models
• Zonal winds of Uranus and Neptune: Gravitational harmonics, dynamic self-gravity, shape, and rotation
• The Possibility of Hydrogen-Water Demixing in Uranus, Neptune, K2-18b and TOI-270d
• Towards an agnostic algorithm for sampling empirical structure models: The case of Uranus and Neptune