The pressure & temperature conditions at which iron melts are important for rocky planets because they determine the size of the liquid metal core, an important factor in understanding the potential for creating a radiation-shielding magnetic field. In new research, a team of scientists from Lawrence Livermore National Laboratory & elsewhere used high-energy lasers at the National Ignition Facility & X-ray Diffraction to determine iron melt curve-up to a pressure of 1,000 gigapascals (almost 10,000,000 atmospheres), 3 times the pressure of the Earth’s inner core & almost 4 times higher pressure than any previous experiment. They found that the core of liquid metal lasts longer for Earth-like exoplanets with masses 4-6 times larger that of Earth.
“The great wealth of iron in the rocky interiors of planets makes it necessary to understand the properties & reaction of iron at extreme conditions deep in the cores of more massive planets like Earth,” said Dr. Rick Kraus, physicist at Lawrence Livermore National Laboratory. .
“The iron melting curve is crucial to understanding the internal structure, thermal evolution, and potential for dynamo generated magnetospheres.
A magnetosphere is believed to be a major component of habitable terrestrial planets, like-it-is on Earth.
Our planet’s magnetodynamo is generated in the convecting liquid iron outer core surrounding the solid iron inner core & is driven by the latent heat released during iron solidification.
Given the importance of iron on terrestrial planets, accurate & precise physical properties at extreme pressures & temperatures are needed to predict what’s happening inside.
A first-order property of iron is melting point, which is still a matter of debate because of the conditions in the Earth’s interior.
The melting curve is the largest rheological transition that a material can go through, from a material with strength to a material without.
This is where a solid turns into a liquid & the temperature depends on the pressure of iron.
Through experiments, Dr. Kraus & colleagues studied length of dynamo action during core solidification to hexagonal closed packed structure within super-Earth exoplanets.
“We found that terrestrial exoplanets 4-6 times the mass of Earth have the longest dynamos, which provide an important shield against cosmic rays,” said Dr. Kraus.
“Beyond our interest in understanding the habitability of exoplanets, the technique we developed for iron will be applied to more programmatically relevant materials in forward.
The authors also obtained evidence that kinetics of solidification under such extreme conditions are rapid, taking only a few nanoseconds to-transition from a liquid to a solid, allowing them to observe the equilibrium phase boundary.
“This experimental finding improves our modeling of the time-dependent material response for all materials,” said Dr. Kraus.
The study was published in the journal Science.