To investigate the interiors of Uranus and Neptune, the team created a new framework for modeling planetary structure that combines physically motivated equations with a flexible numerical procedure. Earlier interior models either imposed strict, predefined layering or relied on simplified empirical density profiles, which limited the range of possible solutions. Lead author Luca Morf and co author Ravit Helled instead generated many internal density profiles, calculated the resulting gravity fields, compared them with the observed gravitational moments of the planets, and iterated until the models matched the data while remaining thermodynamically and compositionally consistent.
With this agnostic but physical approach, the researchers obtained a wide set of interior solutions for both Uranus and Neptune. These solutions span rock to water mass ratios from about 0.04 to nearly 4 for Uranus and from about 0.20 to just under 2 for Neptune, covering both water dominated and rock dominated cases. Within the uncertainties of current measurements, the gravity data do not require Uranus and Neptune to be ice rich and instead allow structures in which rock provides a substantial share of the interior mass.
The models also include layers of ionic water that play a key role in generating the planets magnetic fields. Unlike Earth, which has a relatively simple dipolar magnetic field with two clear poles, Uranus and Neptune exhibit complex fields with multiple poles and strong tilts. The new work shows that magnetic dynamos operating in ionic water layers away from the planetary centers can reproduce these non dipolar, multipolar field geometries.
According to the modeling, the region where Uranus generates its magnetic field lies deeper inside the planet than the equivalent dynamo region in Neptune. This difference in depth may help explain why the two planets, despite similar sizes and masses, show distinct magnetic configurations. The results link the details of interior structure, including how rock and water are distributed with depth, to the properties of the magnetic fields measured by spacecraft flybys.
The authors caution that key uncertainties remain in how materials behave at the extreme pressures and temperatures inside giant planets. In particular, limited knowledge of the equations of state of rock and water mixtures and of hydrogen helium water systems at these conditions introduces uncertainties in the inferred compositions. Improving laboratory measurements and theoretical calculations for such materials is essential to narrowing the range of allowed interior models.
Even with these uncertainties, the study broadens the spectrum of plausible interior configurations for Uranus and Neptune and challenges the simple classification of both worlds as ice giants. The results suggest that both planets could reasonably be described as rock giants or ice giants, depending on which part of the allowed model space turns out to match reality. The work underscores that current data on mass, radius, and gravity are not sufficient to resolve this compositional degeneracy.
The findings also have implications for understanding planet formation in the outer Solar System and for interpreting exoplanets with similar masses and radii. If Uranus and Neptune can host a large rock fraction, then models of how icy and rocky material accumulated in the protoplanetary disk may need revision. Comparable sized exoplanets could also span a wide range of rock and water ratios, meaning that planets with similar bulk properties might have very different internal makeups.
Research Report:Icy or rocky? Convective or stable? New interior models of Uranus and Neptune.
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