Periodic Reporting for period 1 - NEWDIA4Planet (Development of the new internally-heated diamond-anvil cell for planetary mineral physics: Application to high-pressure melting of H2O ice)
Reporting period: 2017-09-01 to 2019-08-31
The basic design of the new heating system was adopted from a so-called internally heated DAC (IHDAC). The IHDAC is the most advanced high P-T generating system, in which a metallic foil inside a sample chamber is heated through supplied electricity. The internal heating benefits from steady and uniform heating due to resistive heating, resulting in smaller temperature uncertainty than laser heating (ca.±5%). The Fellow improved the IHDAC by developing a micron-sized heater made of chemically inert metals that heat up an adjacent sample. This was significant improvement to make the new IHDAC highly versatile i.e. applicable to diverse samples, because the conventional IHDAC can heat only metals. This was due to the fact that the sample also served as a heater in the conventional system to allow passage of enough electricity necessary for resistive heating.
Another important aspect of the new IHDAC is its applicability to H2O. H2O is a major constituent of planetary bodies in the outer solar system, such as Uranus and Neptune. However, the high-pressure melting temperatures of H2O differs significantly between previous studies (±300 degC at P = 40 GPa), hence there is no clear-cut answer to a basic question whether H2O is in liquid or solid state in these planets. Therefore, as the second part of this project, the Fellow re-examined the high-pressure melting curve of H2O using the newly developed IHDAC. H2O, or water, is particularly challenging material for the DAC experiments because of its liquid state and high chemical activity at the ambient condition. In addition to the inert metal heater, the Fellow employed a unique sample-loading method for water using liquid nitrogen. This further delivered variations to the sample heatable in the IHDAC. By using the newly developed IHDAC system, the high-pressure melting temperature of H2O was measured up to P = 44 GPa in this project.
The summary of the exploitable results are as follow.
• Experimental methods developed for the new IHDAC
• High-pressure melting temperatures of H2O determined using the new IHDAC
The followings are the summary of exploitation and dissemination activities carried out during the project.
• Presenting the project’s overview and results at a seminar
• Giving a poster presentation at an international conference
• Managing project website
• Demonstrating solidification and melting of H2O at high pressure during outreaching events and eplaining implication of high-pressure ice to planetary sciences
In addition to above activities, two research papers on the experimental methods of the new IHDAC and on the results of H2O melting experiments with its planetary implications are in preparation, intended for submission to international peer-reviewed journals.
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.