Liquid silica at high pressure and temperature is shown to undergo significant structural modifications and profound changes in its electronic properties. Temperature measurements on shock waves in silica at 70-1,000 GPa indicate that the specific heat of liquid rises SiO(2) well above the Dulong-Petit limit, exhibiting a broad peak with temperature that is attributable to the growing structural disorder caused by bond breaking in the melt. The simultaneous sharp rise in optical reflectivity of liquid SiO(2) indicates that such dissociation causes the electrical and therefore thermal conductivities of silica to attain metalliclike values of 1-5 x 10(5) S/m and 24-600 W/m x K, respectively.

A high-intensity laser was used to shock-compress liquid deuterium to pressures from 22 to 340 gigapascals. In this regime deuterium is predicted to transform from an insulating molecular fluid to an atomic metallic fluid. Shock densities and pressures, determined by radiography, revealed an increase in compressibility near 100 gigapascals indicative of such a transition. Velocity interferometry measurements, obtained by reflecting a laser probe directly off the shock front in flight, demonstrated that deuterium shocked above 55 gigapascals has an electrical conductivity characteristic of a liquid metal and independently confirmed the radiography.

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...

Magnesium oxide (MgO) is representative of the rocky materials comprising the mantles of terrestrial planets, such that its properties at high temperatures and pressures reflect the nature of planetary interiors. Shock-compression experiments on MgO to pressures of 1.4 terapascals (TPa) reveal a sequence of two phase transformations: from B1 (sodium chloride) to B2 (cesium chloride) crystal structures above 0.36 TPa, and from electrically insulating solid to metallic liquid above 0.60 TPa. The transitions exhibit large latent heats that are likely to affect the structure and evolution of super-Earths. Together with data on other oxide liquids, we conclude that magmas deep inside terrestrial planets can be electrically conductive, enabling magnetic field-producing dynamo action within oxide-rich regions and blurring the distinction between planetary mantles and cores.

Simultaneous measurements of shock velocity and optical reflectance at 1064, 808, and 404 nm of a high pressure shock front propagating through liquid deuterium show a continuous increase in reflectance from below 10% and saturating at approximately (40-60)% in the range of shock velocities from 12 to 20 &mgr;m/ns (pressure range 17-50 GPa). The high optical reflectance is evidence that the shocked deuterium reaches a conducting state characteristic of a metallic fluid. Above 20 &mgr;m/ns shock velocity (50 GPa pressure) reflectance is constant indicating that the transformation is substantially complete.

A new compression technique, which enables the study of solids into the TPa regime, is described and used to ramp (or quasi-isentropically) compress diamond to a peak pressure of 1400 GPa. Diamond stress versus density data are reported to 800 GPa and suggest that the diamond phase is stable and has significant material strength up to at least this stress level. Data presented here are the highest ramp compression pressures by more than a factor of 5 and the highest-pressure solid equation-of-state data ever reported.

In situ x-ray diffraction studies of iron under shock conditions confirm unambiguously a phase change from the bcc (alpha) to hcp (epsilon) structure. Previous identification of this transition in shock-loaded iron has been inferred from the correlation between shock-wave-profile analyses and static high-pressure x-ray measurements. This correlation is intrinsically limited because dynamic loading can markedly affect the structural modifications of solids. The in situ measurements are consistent with a uniaxial collapse along the [001] direction and shuffling of alternate (110) planes of atoms, and are in good agreement with large-scale nonequilibrium molecular dynamics simulations.

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