Planetary Original Diagnostics at Extreme Conditions with Raman Spectroscopy

Jupiter is a planet comprised of more than 87% H2 and He. The deep interior of Jupiter has extremely high temperatures (Ts) and pressures (Ps). Additionally, Uranus and Neptune are known as the "ice giants", because their mantle holds large amounts of H2O, NH3 and CH4. These planets have dynamos which generate magnetic fields. In Jupiter, the dynamo can be explained by the immiscibility of H2 and He under conditions where H2 becomes a metallic fluid, while in Uranus and Neptune, the conduction results from the movement of free H2 ions, liberated through the disintegration of molecules into a state known as superionic H2O. These phenomena are caused by the immense P and T in the core of the planets. High P leads to extraordinary changes in the properties of matter: under high P, metals can become insulators, H2O transforms into H3O and H2 gas transitions into a metallic and even superconducting state. Quantum chemistry has revealed that our understanding of planetary materials cannot depend solely on observations made under Earth's conditions. Thus, it is crucial to measure the fundamental chemical changes under extreme conditions.

Standard models in the astrophysical literature often use extremely simplistic descriptions of condensed matter, typically relying on linear extrapolations from ambient conditions. This approach is understandable in the absence of data, but with the experimental data provided by this project, these models will require a more sophisticated treatment. Communication via open literature can be inefficient; so we are participating in conferences, meetings and establishing collaborations with members of planetary research groups to ensure that our findings are adopted by the astrophysics community.

In this project, we have non-destructively created the conditions of these planetary outer layers in our laboratory. We use the diamond anvil cell (DAC), a device capable of generating P exceeding 1,000,000 atm by compressing a tiny sample between two diamonds, combined with resistive heating or pulsed laser to raise the sample T to thousands of Kelvin. The DAC is a portable device, which allows to exploit many different probes appropriate for the study of metallic and superionic hot fluids.

The aim of this research is to provide a window into the world of planetary interiors, addressing questions such as:

-Are H2 and He immiscible above the metallic H2 transition temperature?
-Are the magnetic properties of Uranus and Neptune exclusively due to superionic H2O?
-Do CH4, H2O and NH3 decompose and react to form superionic compounds?
-Can H2 diffuse into and/or react with the ice layers of the giant planets?

To achieve these goals, we are studying planetary-relevant systems in the DAC at high T, individually and in mixtures. These systems include H2, H2-He mixtures, H2O, NH3, H2O-NH3 mixtures, CH4 and CH4/H2O/NH3 mixtures (H2-H2O, H2-NH3 and H2-CH4). Our experimental setup incorporates resistive, and laser (CO2 or IR) heating methods coupled with the DAC and the dynamic-DAC (d-DAC), which allows compression rates between those of classical static and laser-driven shock compression.

A significant innovation is the implementation of the novel d-DAC coupled with resistive heating, which helps to overcome the time scales of sample diffusion out of the chamber. We have integrated IR pulse laser heating and d-DAC with time-resolved Raman spectroscopy. We have also performed transport measurements at P up to 200 GPa to investigate how P and T affect the conductivity of planetary ices and the resistance changes in fluid metallic H2. These experiments are extremely challenging and come with a very steep learning curve. To maximise their potential, we are implementing transport measurements to characterise materials synthesised at lower pressures in solid state that are candidates for superconductivity.