The latest computational research from the Carnegie Institution for Science shows that in the deep interiors of ice giants such as Uranus and Neptune, common elements carbon and hydrogen may exist in an unprecedented form. This new state of matter is expected to reshape the scientific community's understanding of the internal structure of planets and the formation mechanism of magnetic fields.
The research was led by Cong Liu and Ronald Cohen of the Carnegie Institution for Science, and the relevant results have been published in the journal Nature Communications. Using high-performance computing and starting from the first principles of quantum mechanics, they systematically simulated the behavior of simple hydrocarbons (chemical formula is CH, that is, hydrocarbons) under extreme high-pressure and high-temperature conditions.
Uranus and Neptune are classified as "ice giants." Existing observations and models show that the internal structures of these two planets can be roughly divided into three layers: the outermost layer is a hydrogen-helium atmosphere, sandwiched by a thick layer of "hot ice", and the innermost layer is a dense core composed of rocks and metals. The scientific community generally believes that these "hot ices" are mainly composed of water (H₂O), methane (CH₄) and ammonia (NH₄); but under extreme pressure and temperature, these substances will show completely different structures and properties from normal temperature and pressure.
Cong Liu and Cohen's simulations covered a pressure range of about 500 to 3,000 gigapascals (equivalent to 5 million to 30 million times the Earth's atmospheric pressure) and a temperature range of about 4,000 to 6,000 kelvin (about 6,740 to 10,340 degrees Celsius), conditions comparable to those deep in ice giant planets. The results show that under such planetary interior conditions, hydrocarbons can form a compound with a hexagonal lattice structure: carbon forms spiral chains on the outside, and hydrogen forms spiral chains on the inside and migrates directionally along these spiral paths.
In this structure, the material exhibits a so-called "quasi-one-dimensional superionic state." Superionic substances are a special state between solids and liquids: some atoms in the crystal lattice remain ordered in a solid state, while other atoms can move freely in the crystal lattice like a liquid. Research shows that in this new phase, the carbon skeleton maintains an ordered hexagonal crystal structure, while hydrogen atoms mainly move directionally along predefined spiral channels rather than isotropically diffusing in three-dimensional space.
Cohen pointed out that the reason why this newly predicted carbon-hydrogen phase is "especially eye-catching" is that its atomic motion is not completely three-dimensional, but is strongly biased towards certain specific spiral paths. This highly directional migration feature is very rare in planetary materials. This "quasi-one-dimensional" superionic behavior means that the way heat and charge are transported inside such materials may be very different from the traditional understanding of isotropic high-temperature fluids.
This discovery has multiple potential implications for planetary science. First of all, the directional migration of hydrogen in the crystal lattice will directly affect the thermal conductivity and electrical conductivity of the deep material, thus changing how the internal energy of the planet is transferred from the deep layer to the outer layer. Secondly, this abnormal conductive property may be related to the special magnetic field shape of the ice giant planets, which helps explain the more distorted and eccentric observational characteristics of Uranus and Neptune's magnetic field structures compared to the Earth and gas giant planets (such as Jupiter and Saturn).
In recent years, the number of confirmed exoplanets has exceeded 6,000 and is still growing, driving closer cross-collaboration in astronomy, planetary science and earth science. Through a combination of observations, experiments and theoretical simulations, researchers try to characterize the material state and physical processes inside the planet, including the generation mechanism of the magnetic field and the evolution of deep layered structures. Modeling the "invisible" regions deep within the planets and moons of the solar system will not only help understand the behavior of these celestial bodies themselves, but is also expected to provide clues to issues such as extraterrestrial habitability.
Liu Cong pointed out that carbon and hydrogen are one of the two most common elements in planetary materials, but the behavior of this simple element combination under giant planet-like conditions is far from being fully understood. This work shows that even the most basic chemical systems can evolve complex and unexpected crystal and dynamic structures under extreme pressure and temperature, expanding the boundaries of scientific researchers' understanding of the high-pressure material world.
In addition to its significance in planetary physics, this material with strong directional transport properties may also find application prospects in the broader fields of materials science and engineering. For example, in scenarios that require highly anisotropic electrical or thermal conductivity, this type of superionic material is expected to become a theoretical blueprint for new functional materials, providing new ideas for future energy and electronic device design.