In megabar shock waves, materials compress and undergo a phase transition to a dense charged-particle system that is dominated by strong correlations and quantum effects. This complex state, known as warm dense matter, exists in planetary interiors and many laboratory experiments (for example, during high-power laser interactions with solids or the compression phase of inertial confinement fusion implosions). Here, we apply record peak brightness X-rays at the Linac Coherent Light Source to resolve ionic interactions at atomic (ångström) scale lengths and to determine their physical properties. Our in situ measurements characterize the compressed lattice and resolve the transition to warm dense matter, demonstrating that short-range repulsion between ions must be accounted for to obtain accurate structure factor and equation of state data. In addition, the unique properties of the X-ray laser provide plasmon spectra that yield the temperature and density with unprecedented precision at micrometre-scale resolution in dynamic compression experiments. M aterials exposed to high pressures of 1 Mbar and above have recently been the subject of increased attention due to their importance for the physics of planetary formation 1-3 , for material science 4 and for inertial confinement fusion research 5 . The behaviour of shock-compressed aluminium is of particular interest because it has been proposed as a standard for shock-wave experiments 6 and is widely used for equation-of-state 7,8 and warm dense matter (WDM) 9,10 studies. At room temperature, aluminium has three delocalized electrons, so it provides a prototype for an ideal electron fluid. As temperatures and pressures increase, compressing and breaking ionic lattice bonds, strong ionic forces remain, resulting in significant deviations from a simple fluid.Simulations using density functional theory coupled to manyparticle molecular dynamics (DFT-MD) have evolved into an ab initio tool to explore this regime of high-pressure physics 11,12 . To date, these simulations have been used to predict physical properties derived from optical observations of particle and shock velocities. Studies of structural properties that are sensitive to many-particle electron-ion and ion-ion interaction physics 13 have been challenging 14 , although recent progress has been made using X-ray absorption spectroscopy 15,16 . Early experiments on fourth-generation light sources 17 have made use of X-ray diffraction and measured the structural evolution from elastic to plastic states 18 . However, pressures in the Mbar regime, as required for melting many solids, have only recently become available at the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS).Here we visualize, for the first time, the evolution of compressed matter across the melting line and the coexistence regime into a WDM state. The combination of high-power optical lasers and the X-ray beam at MEC provides high-resolution X-ray scattering at multi-Mbar pressures. Our data provide the io...

No abstract

Deep inside planets, extreme density, pressure, and temperature strongly modify the properties of the constituent materials. In particular, how much heat solids can sustain before melting under pressure is key to determining a planet's internal structure and evolution. We report laser-driven shock experiments on fused silica, a-quartz, and stishovite yielding equation-of-state and electronic conductivity data at unprecedented conditions and showing that the melting temperature of SiO 2 rises to 8300 K at a pressure of 500 gigapascals, comparable to the core-mantle boundary conditions for a 5-Earth mass super-Earth. We show that mantle silicates and core metal have comparable melting temperatures above 500 to 700 gigapascals, which could favor long-lived magma oceans for large terrestrial planets with implications for planetary magnetic-field generation in silicate magma layers deep inside such planets.U nderstanding the structure, formation, and evolution of giant planets and extrasolar terrestrial planets (super-Earths) discovered to date requires knowledge of the properties of basic constituents such as iron, magnesium oxide, and silica at the relevant extreme conditions, including pressures of 100s to 1000s of GPa. Melting is arguably the most important process determining the physical and chemical evolution of planetary interiors, as differentiation of a terrestrial planet into a dense metallic core surrounded by rocky mantle and atmosphere proceeds by gravitational separation of a liquid phase (1). Moreover, giant impacts during the terminal stages of planetary formation can cause large-scale melting and generate a magma ocean encompassing much of the planet's rocky constituents (2, 3). As mantle viscosity typically increases by more than 10 to 15 orders of magnitude upon solidification (4), the potential freezing of this magma ocean would greatly influence the planet's subsequent thermal evolution, geochemistry, and magnetic field.We used shock compression of fused silica, a-quartz, and stishovite to document the pressuredensity-temperature equation-of-state and optical properties (hence, electronic conductivity) of SiO 2 . Stishovite's high initial density allowed us to access unprecedented high densities, which extended the experimental melting line of SiO 2 to more than 500 GPa. In combination with melting data for other oxides and iron, the highpressure measurements provide constraints on the thermal structure and evolution of rocky planets and provide a benchmark for future theoretical (e.g., first-principles molecular dynamics), as well as experimental studies.We used a TW-power laser pulse to send a strong, but decaying, shock through a planar target assembly (Fig. 1, A and B) (5). Nanosecond streaked optical pyrometry (SOP) and Doppler velocity interferometry (VISAR) recorded the shock-front velocity, reflectivity, and thermal emission as a function of time (Fig. 1, C and D). We applied impedance matching to obtain pressuredensity data up to 2.5 TPa along the locus of shock (Hugoniot) states of sti...