NASA/ESA
New research challenges the long-held assumption that Uranus and Neptune are “ice giants,” with both planets may have rockier cores than previously thought.
Although they are technically gas giants, Uranus and Neptune are called “ice giants” due to their composition. This refers to the fact that Uranus and Neptune have more methane, water and other volatiles than their larger counterparts (Jupiter and Saturn). Due to the pressure conditions inside these planets, these elements solidify, essentially becoming “ice”.
However, new research from the University of Zurich (UZH) and the National Center for Research Competence (NCCR) PlanetS is challenging our understanding of these inner regions of planets.
According to the research team’s conclusions, this month in the magazine Astronomy & Astrophysics, Uranus and Neptune may have rockier and less “icy” cores than was thought. Furthermore, research suggests that their interiors may undergo convection, a process in which material is recycled (as on Earth through tectonic activity), rather than remaining stable. These possibilities, the researchers indicate, could explain some of the most mysterious characteristics of the “ice giants”.
Historically, scientists divided the planets in the Solar System into three distinct categories based on their composition, which corresponds to their distance from the Sun. This includes the terrestrial (rocky) planets of the inner Solar System — Mercury, Venus, Earth and Mars — followed by the planets beyond the so-called “Ice Line” (where volatile materials like water freeze). This includes the gas giants (Jupiter and Saturn) and the ice giants (Uranus and Neptune). The new study, led by doctoral candidate Luca Morf and professor Ravit Helled, from the University of Zurich (UZH) and the National Center for Planetary Research (NCCR PlanetS), challenges this structure.
Of all the planets in the Solar System, Uranus and Neptune are the least understood. This is due to the fact that only one mission, the *Voyager 2* probe, has studied them closely (in 1986 and 1989, respectively). Morf and Helled developed a unique process to simulate the interiors of Uranus and Neptune, which considered compositions beyond the water-rich model.
The study consisted of random density profiles, followed by calculations of the resulting planetary gravitational field. The process was then repeated to obtain results consistent with observational data from Uranus and Neptune. As Morf explained in a UZH press release:
The classification of ice giants is too simplistic, since Uranus and Neptune are still poorly understood. Physics-based models were too loaded with assumptions, while empirical models were too simplistic. We combine both approaches to obtain internal models that are at the same time ‘agnostic’ or unbiased and physically consistent.
The results showed that the best description of the internal composition of these planets is not limited to ice (predominantly water) and may instead be composed predominantly of rock. These results are consistent with findings from the Hubble Space Telescope and the New Horizons mission, which indicate that the Pluto’s composition is about 70% rock and metals and 30% water by mass. The study also offers possible explanations for the fact that Uranus and Neptune have such mysterious magnetic fields, characterized by more than two poles. Helled stated:
It’s something we first suggested almost 15 years ago, and now we have the numerical structure to demonstrate it. Our models have so-called ‘ionic water’ layers, which generate magnetic dynamos in locations that explain the observed non-dipolar magnetic fields. We also discovered that Uranus’s magnetic field originates at greater depths than Neptune’s. Both Uranus and Neptune could be rock giants or ice giants, depending on the model’s assumptions. You current data is insufficient to distinguish the two, and therefore we need dedicated missions to Uranus and Neptune that can reveal their true nature.
Naturally, there are uncertainties in this model, which highlights the need for future missions to investigate “ice giants” further. However, the new results present new scenarios and challenge decades-old assumptions about the internal composition of giant planets. They can also guide future materials science studies on planetary conditions and how matter behaves in extreme conditions.
