Inside the depths of planets, where extreme pressures and temperatures reign, matter takes on some truly bizarre forms. While iron atoms are believed to dance within Earth’s solid inner core, the gas giants Uranus and Neptune likely host an unusual type of ice, known as superionic ice, which exists as both a solid and a liquid simultaneously. This extraordinary form of ice was first recreated in laboratory experiments five years ago and its existence and crystalline structure were confirmed four years later.
Just last year, a group of researchers from various universities in the United States, along with scientists at the Stanford Linear Accelerator Center (SLAC) in California, made a groundbreaking discovery – a new phase of superionic ice. This finding has profound implications for our understanding of the peculiar magnetic fields exhibited by Uranus and Neptune.
FOUND IN EXOPLANETS
On Earth, we often think of water as a simple molecule with one oxygen atom and two hydrogen atoms that freeze into a fixed structure. However, superionic ice is profoundly different and might be one of the most prevalent forms of water in the Universe, found not only within Uranus and Neptune but also in similar exoplanets.
These gas giants boast extreme pressures, up to 2 million times greater than Earth’s atmospheric pressure, and interior temperatures rivaling the surface of the Sun. It’s under these extreme conditions that water takes on its extraordinary properties.
UNIQUE ARRANGEMENT IN ICE
In 2019, scientists confirmed a prediction made by physicists in 1988: superionic ice features a structure in which oxygen atoms are locked within a solid cubic lattice, while ionized hydrogen atoms move freely within this lattice, akin to electrons flowing through metals. This unique arrangement grants superionic ice its remarkable conductive properties and raises its melting point, keeping it solid even at scorching temperatures.
In the most recent study, physicist Arianna Gleason from Stanford University and her colleagues subjected thin layers of water, sandwiched between two diamond layers, to powerful laser shocks. These successive shockwaves pushed the pressure to an astounding 200 gigapascals (equivalent to 2 million atmospheres) and temperatures soared to about 5,000 Kelvin (or 8,500 degrees Fahrenheit) – surpassing the conditions of the 2019 experiments, though at lower pressures.
THE CRYSTAL STRUCTURE
Their paper, from January 2022, underscores the significance of understanding water’s phase diagram under pressure and temperature conditions relevant to Neptune-like exoplanets.
Through X-ray diffraction, the researchers unveiled the crystal structure of the hot, dense ice, despite the brief maintenance of high-pressure and high-temperature conditions. The resulting diffraction patterns unveiled that the ice crystals represented a new phase, distinct from the superionic ice observed in 2019. This newly discovered superionic ice, termed Ice XIX, displays a body-centered cubic structure and enhanced conductivity compared to its predecessor, Ice XVIII.
Conductivity plays a crucial role here, as it’s responsible for generating magnetic fields when charged particles move. This is the foundation of dynamo theory, which explains how conductive fluids, such as the mantle of Earth or the interior of celestial bodies, give rise to magnetic fields.
If more of an ice giant’s interior resembles a semi-solid state, as opposed to a swirling liquid, it could significantly impact the type of magnetic field generated. Gleason and her team suggest that Neptune might contain two superionic layers with different conductivities toward its core. In this scenario, the magnetic field produced by the outer liquid layer would interact differently with each of these layers, leading to even more complex magnetic fields, similar to those observed around Uranus and Neptune.
This discovery could provide a satisfying explanation for the enigmatic magnetic fields detected by NASA’s Voyager II space probe over 30 years ago when it explored our Solar System’s ice giants.
The findings from this study were published in Scientific Reports, adding another piece to the puzzle of understanding the intriguing properties of superionic ice and its role in shaping the magnetic fields of distant ice giants.