  Uranus and Neptune have intrinsic magnetic fields generated via convection in
a molten H2O layer, where the field strength is determined by its electrical
conductivity (EC) along with convection size and velocity. Previous shock
experiments reported that the EC of molten H2O is high enough to generate
magnetic fields of these ice giant planets with adiabatic thermal structures.
Here we measured the EC of ionic H2O fluid for the first time by static
compression experiments up to 45 GPa and 2,750 K. The EC determined is lower by
a few orders of magnitude than earlier data by shock compression measurements
and not capable of generating a magnetic field with the conventional interior
thermal structures. Our results necessitate recently-suggested fewfold hotter
interiors of Uranus and Neptune to explain their magnetic fields.
