  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 GPa and $\sim$40000 K. Along the
Hugoniot, the temperature measured as a function of pressure shows a subtle
change in slope at $\sim$7000 K and $\sim$90 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.
