Ammonia is predicted to be one of the major components in the depths of the ice giant planets Uranus and Neptune. Their dynamics, evolution, and interior structure are insufficiently understood and models rely imperatively on data for equation of state and transport properties. Despite its great significance, the experimentally accessed region of the ammonia phase diagram today is still very limited in pressure and temperature. Here we push the probed regime to unprecedented conditions, up to $\sim 350\mathrm{GPa}$ and $\sim 40000\mathrm{K}$. Along the Hugoniot, the temperature measured as a function of pressure shows a subtle change in slope at $\sim 7000\mathrm{K}$ and $\sim 90\mathrm{GPa}$, in agreement with ab initio simulations we have performed. This feature coincides with the gradual transition from a molecular liquid to a plasma state. Additionally, we performed reflectivity measurements, providing the first experimental evidence of electronic conduction in high-pressure ammonia. Shock reflectance continuously rises with pressure above 50 GPa and reaches saturation values above 120 GPa. Corresponding electrical conductivity values are up to 1 order of magnitude higher than in water in the 100 GPa regime, with possible significant contributions of the predicted ammonia-rich layers to the generation of magnetic dynamos in ice giant interiors.

• Received 18 August 2020
• Revised 5 November 2020
• Accepted 10 December 2020

DOI:https://doi.org/10.1103/PhysRevLett.126.025003