Shock temperature and reflectivity of precompressed H₂O up to 350 GPa: Approaching the interior of planets

Using a combination of static precompression and laser-driven shock compression, shock temperature and reflectivity of H₂O have been measured up to 350 GPa and 2.1 × 10⁴ K. Here, two calibration standards were applied to enhance temperature measurement reliability. Additionally, in temperature calculations, the discrepancy in reflectivity between active probe beam wavelength and self-emission wavelength has been taken into account to improve the data's precision. Precompressed water's temperature–pressure data are in very good agreement with our quantum molecular dynamics model, suggesting a superionic conductor of H₂O in the icy planets' deep interior. A sluggish slope gradually approaching Dulong–Petit limit at high temperature was found at a specific heat capacity. Also, high reflectivity and conductivity were observed at the same state. By analyzing the temperature–pressure diagram, reflectivity, conductivity and specific heat comprehensively at conditions simulating the interior of planets in this work, we found that as the pressure rises, a change in ionization appears; it is supposedly attributed to energetics of bond-breaking in the H₂O as it transforms from a bonded molecular fluid to an ionic state. Such molecular dissociation in H₂O is associated with the conducting transition because the dissociated hydrogen atoms contribute to electrical properties. (paper)