Possible negative value of the Grüneisen coefficient of hydrogen in the pressure range from 40 to 75 GPa and temperature range from 3500 to 7500 K

Experimental data on single and double shock compression of initially liquid and gaseous (compressed by initial pressure) hydrogen isotopes (protium and deuterium) at pressures of $\approx$ 10–180 GPa and temperatures of $\approx$ 3000–20000 K are considered. The mean values of the measured variables (pressure, density, internal energy, and temperature) show that hydrogen at a pressure of $\approx$ 41 GPa in the temperature interval of $\approx$ 3500–5700 K and at a pressure of $\approx$ 74 GPa in the temperature interval of $\approx$ 5000–7500 K is characterized by a negative value of the Grüneisen coefficient. Such an anomaly may play a key role in some processes, including those proceeding in the Jupiter gas shell, which mainly consists of protium ($\approx$ 90%) and helium ($\approx$ 10%). In the range of pressures (depths) of its manifestation, convection in the protium shell is forbidden with an increase in temperature in the shell with increasing pressure. Possibly, a comparatively moderate fraction of helium does not suppress the anomaly, and it serves as a barrier for large-scale convection in the Jupiter shell. Experiments are required to confirm this anomaly.