Uranus and Neptune are the least-known planets in the solar system. Apart from a flyby by the Voyager 2 probe in the 1980s, no space mission has studied them closely. Nevertheless, scientists believe that beneath an atmosphere of hydrogen and helium, these ice giants have a mantle of water, methane and ammonia. Under the very high pressures and temperatures prevailing inside these planets, water and ammonia could be in a state known as "superionic" ice. However, current models of Uranus and Neptune often consider water as the only component of their mantle, which doesn’t seem sufficient to describe their structure and explain, for example, their unusual magnetic field, thought to be due to the movements of conductive fluids in the mantle. Experiments conducted by an international collaboration at the Laboratory for the Use of Intense Lasers (LULI*), combined with theoretical calculations, have succeeded in studying ammonia under these extreme conditions.
Static and dynamic compression
This superionic phase of ammonia had previously been observed at the Institute of Mineralogy, Materials Physics and Cosmochemistry (IMPMC), but its properties under the conditions of planetary interiors remained unexplored until now. Reaching these pressures and temperatures (several million times higher than Earth’s atmospheric pressure and several thousand degrees) is indeed a challenge. Traditional methods, which involve either compressing ammonia between two small "anvils" made of a highly resistant material such as diamond or sapphire - static compression - or creating a shock wave in the sample using a laser - dynamic compression - are not sufficient on their own. The experiment therefore combines both approaches: ammonia is first inserted and pre-compressed between two anvils at the IMPMC, until it is transformed into a liquid or even solid state. Then, the LULI2000’s intense laser beam is sent through one of the anvils, creating a shock wave that heats and further compresses the sample. The data collected can be used to determine changes in temperature as a function of pressure, as well as other parameters such as the reflectivity of ammonia, which can be traced back to its electrical conductivity. "These experiments, which combine LULI’s expertise in dynamic compression and IMPMC’s expertise in static compression, are conducted in close synergy with ab initio theoretical calculations carried out in collaboration with the Lyon Geology Laboratory and the University of Rostock", explains Alessandra Ravasio, a CNRS researcher at LULI. These theoretical calculations consist in determining, on the basis of the first principles of physics, how the ammonia sample behaves under these conditions. First, the first experiments test the validity of the calculations, then the latter help to interpret the subsequent experiments.The results of these new experiments are published in an article in the journal Nature Physics, with Jean-Alexis Hernandez (a post-doctoral fellow at LULI at the time of the experiment) as first author. They allow us to explore new states of ammonia, in particular its "superionic ice" phase. In this phase, ammonia (a molecule composed of one nitrogen atom and three hydrogen atoms) possesses properties common to both solids and liquids. The nitrogen atoms form an organized network (as in a solid), while the hydrogen atoms move rapidly and more chaotically (as in a liquid). For certain initial conditions associated with pre-compressions of a few gigapascals (1 gigapascal is equivalent to 10,000 times atmospheric pressure), the researchers observed a rather peculiar evolution of temperature as a function of pressure: temperature increases very little, while pressure continues to rise. Above a certain pressure, the rate of temperature change increases again. These "slope breaks" are typical of a phase transformation in which the energy supplied to the ammonia does not cause the temperature to rise, but fuels the change of state. "In our case, this would be the transition from the superionic phase to the plasma fluid phase present at higher temperatures", explains Alessandra Ravasio. This interpretation of the results as showing the fusion of superionic ice was validated by theoretical calculations.
Melting of superionic ammonia
These results, combined with the team’s previous work, plot the melting curve of superionic ammonia over a wide range of pressures, and show that above 100 gigapascal, superionic ammonia melts at a lower temperature than superionic water ice. "This could lead to the melting of ammonia-rich regions in the mantles of Uranus and Neptune, compared with models that only take water into account," says Alessandra Ravasio. What’s more, the results show that ammonia has a higher electrical conductivity than water under the conditions of the Uranus and Neptune interior models.All these results provide crucial information for revising the internal structure of ice giants, and for estimating the extent of the region in which their unusual magnetic field can be generated. Scientists hope to continue in this direction, for example by studying a mixture of ammonia and water, or even adding methane to get as close as possible to real-life conditions. New challenges ahead.
*LULI: a joint research unit of CEA, CNRS, Sorbonne University, École Polytechnique and Institut Polytechnique de Paris.
This study is the result of a collaboration between researchers from LULI, the Laboratoire de géologie de Lyon (LGL-TPE, CNRS/Université Claude Bernard/ENS Lyon), the Institut de minéralogie, de physique des matériaux et de cosmochimie (IMPMC, CNRS/MNHN/Sorbonne Université), the University of Rostock (Germany) and CEA.
