Hydrogen at high pressure in the fluid state is of great interest for understanding interiors of gas giant planets. We newly obtained Hugoniot data for liquid hydrogen up to 55 GPa under laser-driven shock loading using impedance matching to a quartz standard. The shocked temperature was determined simultaneously from the brightness temperature. The compression and temperature are almost consistent with theoretical models. High reflectivity of hydrogen was observed at 40 GPa, which suggests the fluid becomes conducting.
High-pressure and high-temperature reactions in meteorite impact events were subjected to extensive research in order to resolve origin of organic molecules on the early Earth. In this article, I introduce several researches of impact chemical reactions synthesizing ammonia and organic molecules on the early Earth. Atmosphere of the early Earth is thought to be slightly oxidized from which organic molecules are difficult to be synthesized. Several hypotheses related to impact synthesis of organic molecules have been proposed. Recently, ammonia and several organic molecules were synthesized demonstrating meteoritic impact reactions experimentally and numerically. Progress about the impact-induced chemical reactions will be able to reveal more clearly the origin of biomolecules on the early Earth.
In this article, we review our recent neutron diffraction and quasi- and inelastic neutron scattering works on the following two water systems related to clathrate hydrates. The first one is the aqueous solutions prepared by applying high pressure of non-polar guest gas molecules. The second one is the amorphous solids prepared by depositing mixed gases of water and guest molecules on a cold substrate (10 K) under high vacuum. In the first system, the diffusion of water molecules became excessively slower below the formation temperature of gas hydrates and it depends on the amounts of solved guest molecules and formed hydrate crystals. In the second system, the intensity of low-energy excitation was reduced by increasing the hydrophobic interaction between the water and guest molecules. As increasing the temperature of the amorphous solids, the local cage structure grew up above the glass transition temperature (ca. 130 K) and finally hydrate crystals formed at 165 K. All of the above results indicate that the local structural fluctuation of hydrate cage occurs in the liquid or amorphous states in advance of the crystallization of clathrate hydrates.
In order to analyse the dynamics of molecules at high pressures, we are applying high-resolution nuclear magnetic resonance (NMR) spectroscopy for samples at gigapascals of pressures in a diamond anvil cell. Here we report some results of its application to various phases of hydrogen hydrates. These hydrates are stable only at high pressures and have never been analyzed in situ by NMR. The observed 1H-NMR spectra of filled-ice hydrogen hydrates at pressures 1 to 4 GPa gave anomalously narrow resonances of the H2 guests encapsulated into hydrogen-bonded H2O frameworks. Observed effects of pressure on NMR relaxation times of these H2 guests indicate that molecular rotation and translational diffusion contribute together to their spin relaxation. We determined the two motional correlation times of the H2 guest molecules as a function of pressure. From the diffusion correlation time, liquid-like fast diffusion of the H2 guests within the hydrate, of the order of 10-8 cm2/s, has been deduced. For hydrogen clathrate hydrate stable at much lower pressure, such diffusion is even faster, which was separately confirmed by pulsed-gradient field NMR method using a sapphire gas-pressure cell.
Convection in icy outer shell and icy mantle has great influences on thermal evolution, internal dynamics, and survival of internal ocean in icy bodies. Rheological and kinetic properties in polycrystalline icy materials are key factors controlling the convective current in the interiors of icy bodies. It is indispensable to examine the grain-size sensitive flow law dominant at low stress conditions and kinetic processes affecting the grain-size evolution such as grain growth, dynamic recrystallization, and high-pressure transformation. Results of preliminary experiments on these properties are presented in this article.