Using the newly developed IHDAC, high-pressure melting temperatures of H2O were measured. Two independent melting experiments were performed using rhenium (Re) and platinum-iridium (Pt-Ir) alloy heaters at P = 42 and 44 GPa, respectively. As the sample in a DAC is extremely small (< 100 microns), a micron-sized heater was created from thin foils with initial thickness of 10-25 microns by compressing them until their thickness become less than 5 microns. The starting material was in liquid state (water), thus it required cryogenic loading method in order to suppress water flowing out of a sample chamber. By cooling the DAC below the freezing point of H2O, the sample was maintained inside the sample chamber even at high pressure. The pressure was then increased to corresponding pressures. The temperature was increased by supplying electric current to the heater from a DC power supply. The temperature was measured by using the spectroradiometric optics. The melting of H2O ice was identified based on Raman spectra taken before and after heating using high-resolution Raman microscope. Formation of ReO2 and solid O2 were observe in Raman spectra at similar P-T conditions (P =42 GPa, T = 1500 K & P = 44 GPa, T = 1493 K) in Re and Pt-Ir alloy heater experiments, respectively, suggesting the melting of H2O ice occurred at these conditions.
Uranus and Neptune, or so called “Ice Giants”, have non-dipolar, non-axisymmetric magnetic fields unlike that of the Earth which has an axial dipole. It has been predicted that a high-pressure phase(s) of H2O inside these planets contributes to the formation of such complex magnetic fields. Unravelling the dynamics of these outer planets will not only help us to understand the history of our solar system, but also place valuable constrain on the evolution of our planet Earth. As such, high-pressure phase diagram of H2O has been extensively studied by high-pressure research community since the discovery of high-pressure ice polymorphs in 1930’s. Melting temperature of H2O is one of the most prioritised properties being studied in the interest of seeking the answer to a fundamental question whether the interiors of Ice Giants are solid, melt, or partially molten. The question, however, was left unresolved due to a large temperature gap between the high-pressure melting curves of H2O in each of earlier studies. The melting temperatures measured in the present project are in good agreement with the highest estimates among the earlier studies. This supports the existence of solid H2O (ice) in Ice Giants, which could lead us to unlock the mystery of their complex magnetic fields.
