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Hydrogen bond symmetrisation is the phenomenon where a hydrogen atom is located at the centre of a hydrogen bond. Theoretical studies predict that hydrogen bonds in ice VII eventually undergo symmetrisation upon increasing pressure, involving nuclear quantum effect with significant isotope effect and drastic changes in the elastic properties through several intermediate states with varying hydrogen distribution. Despite numerous experimental studies conducted, the location of hydrogen and hence the transition pressures reported up to date remain inconsistent. Here we report the atomic distribution of deuterium in D2O ice using neutron diffraction above 100 GPa and observe the transition from a bimodal to a unimodal distribution of deuterium at around 80 GPa. At the transition pressure, a significant narrowing of the peak widths of 110 is also observed, attributed to the structural relaxation by the change of elastic properties. Atomic distribution in high-pressure water ice are directly observed, via neutron powder structure analysis, found hydrogen bonds become symmetric at pressures about 80 GPa, marking the transition from ice VII to ice X. [ABSTRACT FROM AUTHOR]

Raman spectra of zircon have recently been used as a pressure scale for studies of geological fluids at high temperatures and high pressures using diamond anvil cells (DACs). The zircon scale is advantageous in high chemical stability and the large pressure response of the B1g mode. Despite its excellent applicability, the calibration of the scale has been carried out only in a narrow pressure–temperature range, especially under limited high‐temperature and high‐pressure conditions. In this study, the pressure and temperature dependence of the Raman modes of synthetic zircon was investigated up to 9.5 GPa and from room temperature to 776 K using an externally heated diamond anvil cell. Ruby and gold were used as the reference pressure scales. The Raman shift of the B1g mode for the antisymmetric stretching of the SiO4 structure in zircon showed a linear pressure dependence of 5.48(4) cm−1/GPa up to 8 GPa at room temperature, in agreement with the previous studies. Measurements under high‐pressure and high‐temperature conditions confirmed that the pressure dependence up to 9.5 GPa along the isotherms from 373 to 675 K was consistent with the room‐temperature value; the wavenumbers can be well deduced from the sum of the individual effects of pressure and temperature, obtained at ambient temperature and pressure, respectively. A comparison of the zircon scale with the c‐BN Raman spectroscopic scale confirmed that the pressures determined with these scales were in reasonable agreement. The present results provide a confident use of the zircon Raman spectroscopic scale in a wider pressure–temperature range than previous studies for the internally consistent pressure determination. [ABSTRACT FROM AUTHOR]

The structure of a recently found hyperhydrated form of sodium chloride (NaCl·13H2O and NaCl·13D2O) has been determined by insitu single‐crystal neutron diffraction at 1.7 GPa and 298 K. It has large hydrogen‐bond networks and some water molecules have distorted bonding features such as bifurcated hydrogen bonds and five‐coordinated water molecules. The hydrogen‐bond network has similarities to ice VI in terms of network topology and disordered hydrogen bonds. Assuming the equivalence of network components connected by pseudo‐symmetries, the overall network structure of this hydrate can be expressed by breaking it down into smaller structural units which correspond to the ice VI network structure. This hydrogen‐bond network contains orientational disorder of water molecules in contrast to the known salt hydrates. An example is presented here for further insights into a hydrogen‐bond network containing ionic species. [ABSTRACT FROM AUTHOR]

Complex organic aerosols in the upper atmosphere of Saturn's moon Titan reach the troposphere and surface, where a methane (CH4)‐based hydrological cycle occur. Previous studies have assumed no interactions between organic aerosols and liquid CH4, although the dissolution of low‐molecular‐weight photochemical products in liquid CH4 has been considered. Here we report experimental results of soaking a laboratory analog (so‐called tholin) of Titan's organic aerosols in liquid CH4 at 93–98 K for several hours and then evaporating the liquid, simulating wet–dry cycling on Titan. After wet–dry cycling, residual tholin particles form aggregates through cementation. Solid evaporitic deposits formed by evaporation of interacted liquid contain nitrogen‐bearing aromatics, suggesting selective dissolution of aromatics. Our results suggest that organic aerosols or high‐molecular‐weight compounds adsorbed on them partly dissolve in liquid CH4 on Titan, even during short‐term wetting events, promoting the growth of aerosols to dune particles via aggregation and providing aromatics to evaporites. Plain Language Summary: Saturn's largest moon, Titan has a dense atmosphere composed of nitrogen and methane. Active organic chemistry induced by sunlight occurs in the atmosphere to form complex organic aerosols. These organic aerosols eventually settle on the surface, where rivers, lakes, and seas of liquid methane are present. Upon seasonal changes in precipitation and evaporation, wet–dry cycling of liquid methane is believed to occur on Titan, similar to Earth's water cycles. Can organic aerosols settled on the surface be dissolved into liquid methane? If so, what components of aerosol can be dissolved, and how could this affect Titan's geology? Here we perform laboratory experiments to address these questions. Unlike predictions by previous studies, we find that organic aerosols or aromatics adsorbed on them would partly dissolve into liquid methane efficiently. These processes could play key roles in the formation of unique landforms, such as dunes and evaporites, on Titan. Key Points: We experimentally simulate interactions between laboratory analogs (tholin) of Titan's aerosols and liquid methane at low temperaturesComplex aromatics are dissolved from tholin, forming evaporites after drying; while, residual tholin form aggregatesWet–dry cycling of aerosols on Titan may provide aromatics to evaporites and could explain size growth of aerosols to dune particles [ABSTRACT FROM AUTHOR]

The density of the Earth's core is several percent lower than that of iron-nickel alloy under conditions of pressure and temperature equivalent to the Earth's core. Hydrogen is one of the most promising constituents accounting for the density deficit, but hydrogen occupation sites and density decrease of iron-nickel alloy caused by hydrogenation have never been investigated. In this study, the phase relation and crystal structure of Fe0.9Ni0.1Hx(Dx) at high pressures and temperatures up to 12 GPa and 1000 K were clarified by in situ X-ray diffraction and neutron diffraction measurements. Under the P-T conditions of the present study, no deuterium atoms occupied tetragonal (T) sites of face-centered cubic (fcc) Fe0.9Ni0.1Dx, although the T-site occupation was previously reported for fcc FeHx(Dx). The deuterium-induced volume expansion per deuterium vD was determined to be 2.45(4) and 3.31(6) Å3 for fcc and hcp Fe0.9Ni0.1Dx, respectively. These vD values are significantly larger than the corresponding values for FeDx. The vD value for fcc Fe0.9Ni0.1Dx slightly increases with increasing temperature. This study suggests that only 10% of nickel in iron drastically changes the behaviors of hydrogen in metal. Assuming that vD is constant regardless of pressure, the maximum hydrogen content in the Earth's inner core is estimated to be one to two times the amount of hydrogen in the oceans. [ABSTRACT FROM AUTHOR]

Basaltic rocks occur widely on the terrestrial planets and differentiated asteroids, including the asteroid 4 Vesta. We conducted a shock recovery experiment with decaying compressive pulses on a terrestrial basalt at the Chiba Institute of Technology, Japan. The sample recorded a range of pressures, and shock physics modeling was conducted to add a pressure scale to the observed shock features. The shocked sample was examined by optical and electron microscopy, electron back‐scattered diffractometry, and Raman spectroscopy. We found that localized melting occurs at a lower pressure (∼10 GPa) than previously thought (>20 GPa). The shocked basalt near the epicenter represents "shock degree C" of a recently proposed classification scheme for basaltic eucrites and, as such, our results provide a pressure scale for the classification scheme. Finally, we estimated the total fraction of the basaltic eucrites classified as shock degree C to be ∼15% by assuming the impact velocity distribution onto Vesta. Plain Language Summary: Basaltic rocks occur on numerous planetary bodies, including Mars, the Moon, and the asteroid Vesta. Shock metamorphic features in meteorites from such bodies are the ancient imprints of past impact events. We can extract information about the bombardment histories experienced by such bodies if we have an accurate method to link the degree of metamorphism to the impact conditions. Although two such methods for basaltic rocks have been published, one of these does not have a scale that relates the shock features and peak pressures. In this study, we designed an impact experiment with a terrestrial basalt sample to add a pressure scale to one of these methods. We found that basaltic materials are more easily melted than previously expected. The shock features of our shocked sample match "shock degree C." The required pressure for producing the materials classified into this shock degree is 1–2 × 105 times greater than atmospheric pressure. Our results may provide insights into impact processes on Vesta. We estimate that the total fraction of meteorites from Vesta classified into shock degree C is ∼15%. Key Points: We investigated the shock effects in basaltic rocks with impact experiments and shock physics modelingWe added a pressure scale to the shock degree classification for basaltic eucrites, allowing us to link our results with the Stöffler tableLocalized melting occurs from 10 GPa rather than 20 GPa as previously thought [ABSTRACT FROM AUTHOR]

The Earth's iron-rich core contains light elements. Studying the interaction of multiple light elements with iron and silicates during core–mantle segregation process in early Earth evolution has become important. In-situ X-ray diffraction and imaging observations of the iron–silicate–water–sulfur system at 5–10 GPa, up to approximately 1900°C, were used to elucidate sequential reactions: phase transformation and hydrogenation of iron, and formation of iron sulfide and silicates. The newly constructed X-ray imaging system achieved spatial resolution of approx. 10 μm for this study to show iron blob formation and motion. Sulfur distorted the iron blob shape and affected blob growth during heating by reducing the interfacial energy between molten iron and silicates. Light elements in the molten iron and the remaining silicate grains affected core–mantle segregation in the primitive Earth as temperatures increased. Carbon and silicon were incorporated into liquid Fe during later processes at higher temperatures. [ABSTRACT FROM AUTHOR]

A new hydrate form of potassium chloride, KCl·H2O, is identified for the first time by in situ single‐crystal X‐ray diffraction under high pressure. It has a monoclinic structure with lattice parameters of a = 5.687 (7), b = 6.3969 (3), c = 8.447 (3) Å and β = 107.08 (8)° at 2.23 (4) GPa and 295 K. The structure of this hydrate has K—Cl alignments similar to the B1 phase of anhydrous KCl, while water molecules intercalate among the ionic species. The coordination structures of the K and Cl atoms can be regarded as the intermediate states between the B1 and B2 phases of KCl. This finding provides a perspective on the structural interpretation of multicomponent materials and an additional candidate for bound water in salt–water systems under high pressure, such as inside of icy bodies. [ABSTRACT FROM AUTHOR]