The study reports the melting curve of superionic ammonia under the interior conditions of icy planets

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LULI2000 Experiment Room 1. Credit: LULI, Ecole Polytechnique.

Icy planets such as Uranus (U) and Neptune (N) are found in our solar system and other solar systems throughout the universe. Nevertheless, these planets, characterized by a thick atmosphere and a mantle made of volatile substances (eg, hydrogen water, ammonia, etc.), are the least explored class of planets; Little is known about their origin, internal structure and composition.

The Voyager probes, two robotic probes launched by NASA on a quest to explore the outer solar system, recorded interesting measurements indicating that the icy planets have strange magnetic fields. These measurements indicate that unlike other types of planets such as terrestrial planets and gas giants, icy planets do not have a dipole magnetic field and thus lack clear north and south magnetic poles.

Researchers from Ecole Polytechnique, Sorbonne Université and other institutions in Europe recently conducted a study aimed at better understanding what form material could exist within these largely unexplored planets. Their article, published Natural PhysicsSpecifically astrophysics reports the melting curve of superionic ammonia under conditions one would expect to find within U and N.

„The atmospheric composition of U and N is a complex mixture of C, H, N and O atoms in their mantle, which can also be expressed in the composition of water (H2O), ammonia (NH3) and methane (CH4), so-called 'planetary ices,'” Jean-Alexis Hernandez, one of the researchers who led the study, told

„However, the lack of thermodynamic data on these compounds and their compositions at extreme levels of U and N (several million times Earth’s atmospheric pressure and several thousands of Kelvin) hampers current geophysical models of these planets. Most current models assume that the mantles are composed of pure water and The effect of other compounds is not yet known.”

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Theoretical studies have predicted that water and ammonia can form superionic phases at extreme conditions, similar to those found inside icy planets. In these states or phases, the H atoms are more diffuse and move in ways similar to the motion of liquids, while the remaining atoms (ie, O and N), are still in a crystal lattice.

„Recently, Millett et al We tested the presence of superionic water and determined its melting curve,” Hernandez said. „Our work is a similar experiment but for ammonia. Our experiment was performed at the LULI2000 laser facility in France using a technique called laser-driven shock compression.”

Laser-driven shock compression, the technique used by Hernandez and his colleagues in their experiments, essentially uses a high-power laser pulse to generate a shock wave inside the sample. As this shock wave moves through the sample, it increases both its pressure and temperature.

The transport of the shock wave lasts for a few nanoseconds (i.e. a few billionths of a second). During this short period, the researchers measured both the shock velocity and temperature (T) of the sample. They then correlated the speed of the shock with the pressure inside the sample, specifically using quartz.

„The PT conditions in a shocked sample depend on its phase (its structure),” Hernandez said. „For a given phase, all possible PT conditions that can be reached by a shock lie on a single line called the Hugoniot curve. Therefore, when a phase transition occurs during shock propagation, the PT conditions first follow the Hugoniot. The first phase, then follows the PT boundary between the two phases, finally reach the Hugoniot of the second phase.”

Image of the sample before the laser shot. It is pre-compressed in a diamond anvil cell at 2.45 GPa. Solid ammonia-III grains are visible. Credit: Nature Physics, 2023. (Figure S1 in Supplementary Material).

During the team’s experiment, a shock wave sent through their sample turned the ammonia into a dense liquid. The researchers also looked at the evolution of pressure and temperature during the propagation of the shock.

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„The PT conditions in this kink model correspond to the moment when the liquid phase stops following the Hugoniot and starts following the boundary with the superionic phase,” Hernandez said.

„These results were supported by atomistic simulations performed by our co-authors. These simulations based on quantum mechanics reproduced the behavior of ammonia under experimental conditions. The simulations were able to accurately reproduce the experimental observations. We used them based on this agreement. A fine-grained understanding of how N and H atoms behave in such conditions Gain insights and determine the conductivity of the liquid and the superionic state of ammonia at conditions relative to Uranus and Neptune.”

Creating shock waves at observable levels within U and N was a challenging task. This is primarily because a sample must be solid or liquid in its initial state (ie, before the shock wave enters it) to produce a strong shock, but ammonia is a gas at ambient temperature.

As a result, researchers must first liquefy or solidify the ammonia. To complicate matters further, even after the ammonia is liquefied (by cooling or compressing it), the shock inside the sample raises the temperature significantly, reaching much more extreme conditions than inside U and N.

„To overcome these two challenges, we first had to pre-compress ammonia to 3 GPa (30000 bars) in a device called a diamond anvil cell,” Hernandez said.

„This device is typically used to generate high pressure. In our experiment, we had to combine this constant pre-compression with our main laser-driven shock compression, maintaining the sample in a small press. In the shock, we compressed ammonia into a crystalline phase called ammonia-III, and then we shocked the ammonia-III. , resulting in PT conditions in the interiors of Uranus and Neptune.”

Through their experiments, Hernández and his colleagues were able to outline the melting curve of superionic ammonia up to 300 GPa, where they expect to find it inside icy planets. This could have interesting implications for future work, while shedding new light on the possible properties of these widely unexplored planets.

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„The main conclusion of this study is that dense liquid ammonia has a higher electrical conductivity than pure water, which means that in the interiors of U and N, liquid regions with high concentrations of ammonia have significantly higher conductivity than the surroundings,” explained Hernandez. . „These variations in electrical conductivity influence the formation or propagation of these planets’ peculiar magnetic fields.”

A better understanding of the behavior of pure systems such as water and ammonia at high pressure and high temperature is an important first step in understanding what happens inside icy planets. So far, Hernández and his colleagues have focused on the melting curve of superionic ammonia under these extreme conditions, although mantles of U,N complex mixtures of carbon, hydrogen, oxygen and nitrogen, which they plan to study in further studies.

„The determination of the melting curve is a key finding as we observe a crossover with the melting curve of superionic water between 70 and 100 GPa, meaning that ammonia melts at a lower temperature than water above this pressure range,” Hernández added. „This is an important input for determining the extent (if any) of the solid or superionic regions within the mantles of Uranus and Neptune. In future experiments, we will try to gradually explore more complex systems such as water and ammonia mixtures.”

More information:
J.-A. The melting curve of superionic ammonia at interplanetary conditions, Hernandez et al. Natural Physics (2023) DOI: 10.1038/s41567-023-02074-8.

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