New laser-driven compression experiments have reproduced the conditions deep inside exotic super-Earths and the cores of giant planets as well as the conditions that exist during the violent birth of Earth-like planets. The research documents the material properties that determine the processes governing the planets’ formation and evolution.
The experiments, reported in the Jan. 23 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, LLNL physicist Marius Millot and colleagues from LLNL, Bayreuth University in Germany, and the University of California, Berkeley, were able to measure the melting temperature of silica at 500 gigapascals (GPa) (five million atmospheres), a pressure comparable to the core-mantle boundary pressure for a super-Earth planet (five Earth masses), as well as for Uranus and Neptune. It also is the regime of giant impacts that characterize the final stages of planet formation.
“Deep inside planets, extreme density, pressure and temperature strongly modify the properties of the constituent materials,” Millot said. “How much heat solids can sustain before melting under pressure is key to determining a planet’s internal structure and evolution, and now we can measure it directly in the laboratory.”
In combination with prior melting measurements on other oxides and on iron, the new data indicate that mantle silicates and core metal have comparable melting temperatures above 300-500 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, our research suggests that silica is likely solid inside Neptune, Uranus, Saturn and Jupiter cores, which sets new constraints on future improved models for the structure and evolution of these planets,” Millot said.
The advances were made possible by a breakthrough in high-pressure crystal growth techniques at Bayreuth University. There, Natalia Dubrovinskaia and colleagues synthesized millimeter-sized transparent polycrystals and single crystals of stishovite, a high-density form of silica (SiO2) usually found only in minute amounts near meteor-impact craters.
The 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 Facility at the University of Rochester’s Laboratory for Laser Energetics.
“Stishovite, being much denser than quartz or fused-silica, stays cooler under shock compression, and that allowed us to measure the melting temperature at a much higher pressure,” Millot said. “Dynamic compression of planetary-relevant materials is a very exciting field right now. Deep inside planets hydrogen is a metallic fluid, helium rains, fluid silica is a metal and water may be superionic (exhibiting properties of both a liquid and a solid).”
In fact, the recent discovery of more than 1,000 exoplanets orbiting other stars in our galaxy reveals the broad diversity of planetary systems, planet sizes and properties. It also inspires a quest for habitable worlds hosting extraterrestrial life and shines new light on our own solar system.
Using the ability to reproduce in the laboratory the extreme conditions deep inside giant planets, as well as during planet formation, Millot and colleagues plan to study the exotic behavior of the main planetary constituents using dynamic compression to contribute to a better understanding of the formation of the Earth and the origin of life.
Joining Millot on the paper were LLNL colleagues David Braun, Peter Celliers, Rip Collins, and Jon Eggert; Natalia Dubrovinskaia, Ana Černok, Stephan Blaha, and Leonid Dubrovinsky of Bayreuth University; and Raymond Jeanloz of UC Berkeley. LLNL's Antonio Correa Barrios, Walter Unites, Carol Davis, Russell Wallace, and James Emig were acknowledged on the paper for their support of the work by preparing the targets.
The generation of cosmic magnetic fields has long intrigued astrophysicists. Since it was first described in 1959, the Weibel filamentation instability has generated much theoretical interest from astrophysicists and plasma physicists as a potential mechanism for seed magnetic field generation in the universe.
Direct observation of Weibel-generated magnetic fields, however, remained challenging for decades. In a Nature Physics paper published on Jan. 19, LLNL researchers report for the first time well-developed, oriented magnetic filaments generated by the Weibel mechanism in counter-streaming collisionless flows generated by high-power lasers.
“Comparison with 3D particle-in-cell simulations and a first-principles theoretical treatment proves that the magnetic field generation in such flows is real, and quite efficient,” said lead author Channing Huntington, a NIF&PS physicist.
The team’s findings demonstrate the power of the Weibel instability to produce small-scale seed magnetic fields throughout the cosmos, which can then be further amplified to larger scales, creating the ubiquitous magnetic fields that are seen to exist in astrophysical systems. In addition, Weibel-generated magnetic fields may trap plasma ions, creating localized shocks where cosmic ray particles could be accelerated.
The experiments were conducted at the OMEGA Laser Facility at the University of Rochester’s Laboratory for Laser Energetics. The researchers employed protons produced by the implosion of a D-3He (deuterium and helium) capsule at high flux, a technique pioneered on the OMEGA laser by members of the Plasma Science and Fusion Center at the Massachusetts Institute of Technology led by Rich Petrasso. This high-quality data unambiguously revealed the elusive Weibel filamentation instability, which is a fundamental result that, due to the scalability of this process, will have a strong impact on the thinking of astrophysicists. In addition, the 3D simulations performed to complement the data utilize cutting-edge advanced computation techniques, further extending the work’s applicability.
“It is well known that a range of magnetic field scales exist in the cosmos, but the origin of these fields has been an elusive question,” Huntington said. “Weibel instability has long been theorized as a mechanism to generate these fields, but this work offers the most compelling experimental evidence to date that this is indeed possible.”
Having developed a robust experimental platform and confirmed the generation of Weibel filamentation, the team envisions a broad range of follow-up experiments on OMEGA to test the magnetic field generation under conditions relevant to astrophysical systems (for example, in the presence of a pre-existing magnetic field, which may modify the instability growth). The researchers also have begun a set of experiments at NIF, where larger, faster plasma flows are believed to produce even higher fields and the Weibel-mediated shock formation will be fully mature. These experiments will reach conditions not previously achieved in a laboratory setting.
The research was conducted in collaboration with an international team from Japan, the United Kingdom, France, the Massachusetts Institute of Technology, Princeton University, the University of Rochester, and the University of Michigan.
Co-authors on this paper include Frederico Fiuza, J. Steve Ross, Matthew Levy, Chris Plechaty, Bruce Remington, Dmitri Ryutov and Hye-Sook Park of LLNL; Alex Zylstra, Chikang Li and Richard Petrasso of the Massachusetts Institute of Technology; R. Paul Drake and Carolyn Kuranz of the University of Michigan; Dustin Froula of the University of Rochester’s Laboratory for Laser Energetics; Gianluca Gregori and Jena Meinecke of the University of Oxford; Nathan Kugland of Lam Research Corporation; Taichi Morita, Youichi Sakawa and Hideaki Takabe of Osaka University’s Institute of Laser Engineering; and Anatoly Spitkovsky of Princeton University.
NIF&PS Chief Technology Officer Chris Barty presented an invited talk on “NIF and the Pursuit of Star Power with Lasers” on Jan. 23 during a meeting of the Danish Physical Society celebrating the opening of the International Year of Light and Light-based Technologies (IYL2015). The year-long celebration was initiated by the European Physical Society and endorsed by the United Nations Educational, Scientific and Cultural Organization (UNESCO) and the UN General Assembly.
Barty’s presentation reviewed the 50-plus-year pursuit of laser fusion at LLNL and introduced the advanced laser and optical technologies required for fusion as well as the potential applications and science possible with laser fusion systems.
IYL2015, which was officially opened on Jan. 19 at UNESCO headquarters in Paris, will highlight the importance of light and optical technologies in everyday life and for the development of society. This year marks a number of anniversaries related to light and the achievements of several eminent scientists, including the 50th anniversary of the discovery of the cosmic microwave background by Arno Penzias and Robert Wilson; the centenary of Einstein’s general theory of relativity; the 150th anniversary of James Clerk Maxwell’s equations uniting electricity and magnetism; and the 200th anniversary of Augustin-Jean Fresnel’s paper describing the wave nature of light.