Washington, DC—The interiors of ice giant planets like Uranus and Neptune could be home to a previously unknown state of matter, according to new computational simulations by Carnegie's Cong Liu and Ronald Cohen.
Their work, published in Nature Communications , predicts that a quasi-one-dimensional superionic state of carbon hydride exists under the extreme pressures and temperatures found deep inside these outer Solar System bodies.
More than 6,000 exoplanets have been discovered. As this number grows, astronomers, planetary scientists, and Earth scientists are crossing disciplinary boundaries—combining observation, experimentation, and theory—to define and probe the factors that help us understand the dynamic processes that shape them, including the generation of magnetic fields.
As such, interest has grown in understanding the processes that are occurring deep beneath the surfaces of planets and moons in our own Solar System, which can inform our understanding of planetary dynamics, and even planetary habitability in more-distant neighborhoods.
Measurements of Uranus and Neptune's densities indicate that the interiors of these giant planets contain intermediate layers of unconventional "hot ices," which exist below their hydrogen and helium atmospheric envelopes and above their rocky cores. These layers are believed to be composed of water (H2O), methane (CH4), and ammonia (NH4), but due to the extreme conditions, it is thought that exotic phases would emerge.
The physics in these high-pressure, high-temperature regions can give rise to unconventional states of matter, which is why theorists and experimentalists attempt to predict and recreate what would be found there.
Using high-performance computing and machine-learning, Liu and Cohen performed fundamental quantum physics simulations of carbon hydride (CH) under pressures ranging from nearly 5 million to nearly 30 million times atmospheric pressure (500 to 3,000 gigapascals) and at temperatures ranging from 6,740 to 10,340 degrees Fahrenheit (4,000 to 6,000 Kelvin).
Their tools predicted the emergence of an ordered hexagonal framework in which hydrogen atoms move along spiral pathways, creating a quasi-one-dimensional superionic state.
Superionic materials occupy an unusual middle ground between solids and liquids—one type of atom remains arranged in a crystalline framework and another becomes mobile.
"This newly predicted carbon–hydrogen phase is particularly striking because the atomic motion is not fully three-dimensional," Cohen explained. "Instead, hydrogen moves preferentially along well-defined helical pathways embedded within an ordered carbon structure."
This directionality of this movement has important implications for how heat and electricity move through planetary interiors. Such behavior could influence interior energy redistribution, electrical conductivity, and possibly the interpretation of magnetic-field generation in ice giants.
Their findings also expand our understanding of the behavior of simple compounds under extreme conditions, suggesting that even simple systems can organize into unexpectedly complex phases.
"Carbon and hydrogen are among the most abundant elements in planetary materials, yet their combined behavior at giant-planet conditions remains far from fully understood," Liu concluded.
Beyond planetary interiors, the ability to identify strongly directional emergent phenomena in condensed matter could have ramifications for materials science and engineering.
