Metallization of Shock-Compressed Liquid Ammonia

Area of Science:

• Planetary Science
• High-Pressure Physics
• Quantum Chemistry

Background:

• Ammonia is a key component in ice giant interiors.
• Understanding ammonia's properties is crucial for planetary models.
• Experimental data on ammonia at extreme conditions is limited.

Purpose of the Study:

• To experimentally probe the ammonia phase diagram at unprecedented pressures and temperatures.
• To investigate the transition of ammonia from a molecular liquid to a plasma state.
• To measure the electrical conductivity of ammonia under extreme conditions.

Main Methods:

• Dynamic compression experiments (shock compression) up to ~350 GPa and ~40,000 K.
• In situ temperature measurements along the Hugoniot.
• Reflectivity measurements to probe electronic conduction.

Main Results:

• Observed a subtle change in the temperature-pressure Hugoniot slope around 90 GPa and 7000 K, indicating a molecular liquid-to-plasma transition.
• Provided the first experimental evidence of electronic conduction in high-pressure ammonia.
• Measured continuously rising shock reflectance above 50 GPa, saturating above 120 GPa.

Conclusions:

• Ammonia undergoes a significant phase transition under extreme pressures relevant to ice giants.
• High-pressure ammonia exhibits significant electrical conductivity, exceeding that of water.
• These findings have implications for understanding the magnetic dynamos of Uranus and Neptune.