The pressure and temperature conditions at which iron melts are important for rocky planets because they determine the size of the liquid metal core, an important factor for understanding the potential for generating a radiation-shielding magnetic field. In new research, a team of scientists from the Lawrence Livermore National Laboratory and elsewhere used high-energy lasers at the National Ignition Facility and X-ray diffraction to determine the iron-melt curve up to a pressure of 1,000 gigapascals (nearly 10,000,000 atmospheres), three times the pressure of Earth’s inner core and nearly four times greater pressure than any previous experiments. They found that the liquid metal core lasted the longest for Earth-like exoplanets four to six times larger in mass than the Earth.

An artist’s conception of the cross section of a super-Earth with the National Ignition Facility target chamber superimposed over the mantle, looking into the core. Image credit: John Jett / Lawrence Livermore National Laboratory.

An artist’s conception of the cross section of a super-Earth with the National Ignition Facility target chamber superimposed over the mantle, looking into the core. Image credit: John Jett / Lawrence Livermore National Laboratory.

“The sheer wealth of iron within rocky planet interiors makes it necessary to understand the properties and response of iron at the extreme conditions deep within the cores of more massive Earth-like planets,” said Dr. Rick Kraus, a physicist at the Lawrence Livermore National Laboratory.

“The iron melting curve is critical to understanding the internal structure, thermal evolution, as well as the potential for dynamo-generated magnetospheres.”

A magnetosphere is believed to be an important component of habitable terrestrial planets, like it is on Earth.

The magnetodynamo of our planet is generated in the convecting liquid iron outer core surrounding the solid iron inner core and is powered by the latent heat released during solidification of the iron.

With the prominence of iron in terrestrial planets, accurate and precise physical properties at extreme pressure and temperatures are required to predict what is happening within their interiors.

A first-order property of iron is the melting point, which is still debated for the conditions of Earth’s interior.

The melt curve is the largest rheological transition a material can undergo, from a material with strength to one without.

It is where a solid turns to a liquid, and the temperature depends on the pressure of the iron.

Through the experiments, Dr. Kraus and colleagues determined the length of dynamo action during core solidification to the hexagonal close-packed structure within super-Earth exoplanets.

“We find that terrestrial exoplanets with four to six times Earth’s mass will have the longest dynamos, which provide important shielding against cosmic radiation,” Dr. Kraus said.

“Beyond our interest in understanding the habitability of exoplanets, the technique we’ve developed for iron will be applied to more programmatically relevant materials in the future.”

The authors also obtained evidence that the kinetics of solidification at such extreme conditions are fast, taking only nanoseconds to transition from a liquid to a solid, allowing them to observe the equilibrium phase boundary.

“This experimental insight is improving our modeling of the time-dependent material response for all materials,” Dr. Kraus said.

The study was published online today in the journal Science.

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Richard G. Kraus et al. 2022. Measuring the melting curve of iron at super-Earth core conditions. Science 375 (6577): 202-205; doi: 10.1126/science.abm1472




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