The discovery that more than 1000 planets orbit stars in our galaxy has revealed the broad diversity of planetary systems. These exoplanets, orbiting stars other than our own Sun, are diverse in size and physical properties, a fact that has changed our understanding of what might constitute a habitable world. It also shines new light on our own solar system and the quest to understand the formation of Earth and the origins of life. New dynamic-compression experiments reported in the 23 January 2015 edition of Science reveal the unusual properties of silica – the key constituent of rock – under the extreme pressures and temperatures relevant to planetary formation and interior evolution .
Using laser-driven shock compression and ultrafast diagnostics, DCO’s Marius Millot (Lawrence Livermore National Laboratories (LLNL), USA) and colleagues from Bayreuth University, Germany, LLNL, and the University of California Berkeley, USA were able to measure the melting temperature of silica at 500 GPa (5 million atmospheres), a pressure comparable to the core-mantle boundary pressure for a super-Earth planet (5 Earth masses), or gas giant planets comparable to Uranus and Neptune. This pressure is also representative of giant impacts that characterize the final stages of planet formation.
In combination with prior melting measurements of other oxides and iron, these new data indicate that mantle silicates and core metal have comparable melting temperatures above 300 to 500 GPa, suggesting that large rocky planets may commonly have long-lived oceans of magma – molten rock – at depth. Planetary magnetic fields can be formed in this liquid-rock layer.
In addition, the new study suggests that silica is likely solid inside the cores of Neptune, Uranus, Saturn, and Jupiter, which sets new constraints on improved models for the structure and evolution of these planets.
The newly published advances were made possible by a breakthrough in high-pressure crystal growth techniques at Bayreuth University in Germany. There, Natalia Dubrovinskaia and colleagues managed to synthesize millimeter-sized transparent polycrystals and beautiful single crystals of stishovite, a high-density form of silica (SiO2) usually found only in minute amounts near meteor-impact craters
These crystals enabled Millot and colleagues to conduct the first laser-driven shock compression study of stishovite using ultrafast optical pyrometry and velocimetry at the Omega laser at the University of Rochester, NY.
“Stishovite being much denser than quartz or fused-silica, it stays cooler under shock compression, and that allowed us to measure the melting temperature at a much higher pressure,” said Millot.
Reproducing the extreme conditions found deep inside giant planets, as well as during planet formation, in the laboratory is crucial; material properties that control the birth, evolution, and structure of large celestial bodies can now be directly measured.
Image: New laser-driven shock compression experiments on stishovite provide thermodynamic and electrical conductivity data at unprecedented conditions and reveal the unusual properties of rocks deep inside large exoplanets and giant planets. Credit: LLNL/NIF/NASA/LLE. Photo by E. Kowaluk (LLE)