Arecent study has investigated this further. An international team led by Dr Andreas Hermann of the University of Edinburgh's School of Physics and Astronomy and Centre for Science at Extreme Conditions simulated water and ammonia mixtures at low temperature conditions. The team discovered that the mixture allowed a compound called ammonia hemihydrate to remain stable as it went through ionic phases at increasingly high pressures.
"There's an interesting state of matter - called superionic - where, due to heat, protons become diffuse, while the heavy atoms of carbon, nitrogen and oxygen remain in a fixed lattice," says Hermann. "It's a partially molten state that still carries signatures from the underlying crystal structures." What this means is that molecules of superionic matter start off as a 'sea' of free-floating ions of their original selves, but then, under increasing pressure, they crystallise to become a strange liquid-solid hybrid. No superionic matter has ever actually been observed, but it's thought to exist inside giant planets.
In their research paper the team say that ammonia hemihydrate will likely precipitate out of ammonia-water mixtures at high enough pressures. The reason this study is particularly important is because this result emerged from modelling a mixture of ices.
instead of individual compounds, which has been the case for studies in the past. But what else did the team's study show? "We calculated that ammonia hemihydrate has a lower density than pure water ice at the same pressure It would then form a well-defined layer above an icy sea," explains Hermann. Imagine a solid-liquid ammonia layer above a slushy, frozen ocean at 3 million Earth atmospheres. However, he also says that truly understanding what layers would actually form, if any, would also require adding methane and excess hydrogen to the simulations.
In the meantime, the team is working on simulations of superionic ammonia hemihydrate. Though physically recreating the interiors of the ice giants may be challenging, it's not completely impossible, as has been discovered. An international team led by Dr Dominik Kraus of the University of Rostock and Helmholtz-Zentrum Dresden- Rossendorf fired a powerful X-ray laser at pieces of polystyrene at the SLAC National Accelerator Laboratory.
The polystyrene was meant to be a stand- in for methane inside ice-giant mantles - both made of carbon and hydrogen and the laser created two shock waves within it. Under those conditions, the shock waves overlapped, creating pressures of 1.5 million atmospheres and temperatures of 5,000 degrees Kelvin for fractions of a second-briefly mimicking the conditions inside an ice-giant mantle. The team was surprised to discover that diamond was created, albeit nanometres in size.
They theorise that in the more sustained conditions of an ice-giant mantle - around 10,000 kilometres (6,214 miles) down-the diamonds will grow to a larger size as the methane breaks down into hydrogen and carbon and precipitate down to the core. Previous teams have used methane inside laboratory diamond anvil cells to create diamond, but under lower temperatures and pressures. However, the results were always inconclusive. In another study, Lawrence Livermore National Laboratory scientists subjected a diamond to 1.1 million Kelvin and 40 million atmospheres to recreate the conditions inside giant planets. Results suggested that at the bottom of the ice giants' mantles could lie a liquid-carbon layer with chunks of floating diamond.
Kraus' team are now working on follow- up experiments. "Our efforts have now turned to looking at what happens when we reduce the carbon concentration in our samples and add other light elements that are also present inside Neptune and Uranus, such as oxygen or nitrogen," he explains. They're also figuring out ways to safely capture the nanodiamond particles, which travel at incredibly high speeds and are currently only detected via spectroscopy.
From the Sun compared to all the other giant planets. Even Neptune, which is farther from the Sun, radiates 2.6 times as much heat as it receives. This heat energy may be what drives Neptune's storm systems and gives it a more dynamic climate than Uranus. It's been suggested that somewhere inside Uranus is a thermal boundary layer that stops heat from escaping. Hermann says that if the planet's layers are more stratified than thought,
"it's not inconceivable that ammonia hemihydrate, or similar strongly bound ionic phases, could form such a layer." Such work could ultimately help in understanding not only the ice giants, but all the giant planets. As Fortney says: "The deep cores of Jupiter and Saturn may actually resemble Neptune, but at much higher pressures and temperatures."
