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
|
Just
as graphite can transform into diamond under high pressure, liquid
magmas may similarly undergo major transformations at the pressures and
temperatures that exist deep inside Earth-like planets.
Using
high-powered lasers, scientists at Lawrence Livermore National
Laboratory and collaborators discovered that molten magnesium silicate
undergoes a phase change in the liquid state, abruptly transforming to a
more dense liquid with increasing pressure. The research provides
insight into planet formation.
“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 extra-solar ‘Super-Earth’ planets are
structured differently from Earth.”
Melts
play a key role in planetary evolution. The team said that
pressure-induced liquid-liquid phase separation in silicate magmas may
represent an important mechanism for global-scale chemical
differentiation and also may influence the thermal transport and
convective processes that govern the formation of a mantle and core
early in planetary history. Liquid-liquid phase separation is similar to
the difference between oil and vinegar—they want to separate because
they have different densities. In the new research, however, the
researchers noticed a sudden change between liquid states of silicate
magma that displayed different physical properties even though they both
have the same composition when high pressure and temperatures were
applied.
The
team used LLNL’s Janus laser and OMEGA at the University of Rochester
to conduct the experiments to achieve the extreme temperatures and
pressures that exist in the interiors of exoplanets—those objects
outside our solar system.
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 was able to
identify 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 appears in the Feb. 10 edition of the journal, Physical Review Letters.
Other
LLNL authors include Jon Eggert, Peter Celliers, Damien Hicks, Gilbert
Collins and Ray Smith. Other collaborators include UC Berkeley, the
Carnegie Institution of Washington and Howard University.
The
work was funded by the National Nuclear Security Administration, the
National Science Foundation and the University of California.
Scientists help define structure of exoplanets
