Shock compression of stishovite and melting of silica at planetary interior conditions

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...

High Pressure:One of the many Tools to Study Material Properties at Extreme Conditions

“…20 Recent shock melting studies on stishovite revealed that high pressure liquid SiO 2 is electrically conducting and could contribute to large magnetic fields in very large terrestrial exoplanets. These magnetic fields are similar to those contributed by iron cores of smaller planets like our earth 21 . With the help of variable composition evolutionary algorithms combined with ab initio calculations, several new binary and ternary stoichiometric compounds of the Mg-Si-O system like SiO 3 , SiO, MgSiO 3 , MgSi 3 O 12 have been predicted to form at ~TPa pressures.…”

On the Destruction of Cancer Cells Using Laser-Induced Shock-Waves: A Review on Experiments and Multiscale Computer Simulations

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Thermodynamically complete equation of state of MgO from true radiative shock temperature measurements on samples preheated to 1850 K

“…The predicted melting temperature for MgO at the Earth's core-mantle boundary pressure, ≈135 GPa, ranges from 6000 to 9000 K [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] (Fig. 1).…”