Laser Research Provides Insight into Planet Formation

Phase changes in liquid magmas at pressures and temperatures that exist deep inside Earth-like planets could provide insight into the processes that govern planet formation.

Just as graphite, under high pressure, can be transformed into diamond, liquid magmas at high pressure may similarly be altered to become denser materials, researchers at the Lawrence Livermore National Laboratory said.

Using high-power lasers, LLNL scientists discovered that molten magnesium silicate subjected to increasing pressure undergoes a phase change, abruptly transforming into a denser liquid.

An artist’s conception of planet Kepler-22b, which orbits in a star’s habitable zone — the region around a star where liquid water, a requirement for life on Earth, could persist. The planet is 2.4 times the size of Earth. (Image: NASA)

“Phase changes between different types of melts have not been taken into account in planetary evolution models,” said lead scientist Dylan Spaulding, a University of California, Berkeley, graduate student who conducted most of his thesis work at the laboratory’s Jupiter Laser Facility. “But they could have played an important role during Earth’s formation and may indicate that extrasolar ‘Super Earth’ planets are structured differently from Earth.”

Melts play a significant role in planetary evolution and may influence the thermal transport and convective processes that govern the formation of a mantle and core early in a planet’s history, the scientists said. A liquid-liquid phase transition similar to what would be found in terrestrial planetary mantles could help explain the thermal-chemical evolution of exoplanet interiors, they said.

The team used LLNL’s Janus laser and the University of Rochester’s OMEGA during experiments to achieve the extreme temperatures and pressures that exist in the interiors of exoplanets. In each experiment, a powerful laser pulse generated a shock wave while it traveled through the sample. By looking for changes in the velocity of the shock and the temperature of the sample, the team identified discontinuities that signaled a phase change in the material.

“In this case, the decay in shock-velocity and thermal emission both reverse themselves during the same brief time interval,” Spaulding said.

The team concluded that a liquid-liquid phase transition in a silicate composition similar to what would be found in terrestrial planetary mantles could help explain the thermal-chemical evolution of exoplanet interiors.

The research, which was funded by the National Nuclear Security Administration, the National Science Foundation and the University of California, appears in the Feb. 10 issue of Physical Review Letters.

For more information, visit: www.llnl.gov