The discovery of more than 4,500 extra-solar planets has created a need for
modelling their interior structure and dynamics. As it turns out, iron plays
a key role.
Lawrence Livermore National Laboratory (LLNL) scientists and collaborators
have used lasers at the National Ignition Facility to experimentally
determine the high-pressure melting curve and structural properties of pure
iron up to 1,000 GPa (nearly 10,000,000 atmospheres), three times the
pressure of Earth's inner core and nearly four times greater pressure than
any previous experiments. The research appears in Science.
The team performed a series of experiments that emulate the conditions
observed by a parcel of iron descending toward the center of a super-Earth
core. The experiments were allocated as part of the NIF Discovery Science
program, which is open access and available to all researchers.
"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 Rick Kraus,
LLNL physicist and lead author of the paper. "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. Earth's magnetodynamo 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, the team 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," Kraus said.
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," including the Stockpile
Stewardship Program.
The melt curve is an incredibly sensitive constraint on an equation of state
model.
The team 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 the team to observe the equilibrium phase
boundary. "This experimental insight is improving our modeling of the
time-dependent material response for all materials," Kraus said.
Other Livermore team members include Suzanne Ali, Jon Belof, Lorin Benedict,
Joel Bernier, Dave Braun, Federica Coppari, Dayne Fratanduono, Sebastien
Hamel, Andy Krygier, Amy Lazicki, James McNaney, Marius Millot, Philip
Myint, Dane M. Sterbentz, Damian Swift, Chris Wehrenberg and Jon Eggert.
Researchers from the University of Illinois at Chicago, the Carnegie
Institution for Science, University of Rochester, Sandia National
Laboratory, California Institute of Technology, University of California
Davis and University of California Los Angeles also contributed to the
study.
Reference:
Richard G. Kraus, Measuring the Melting Curve of Iron at Super-Earth Core
Conditions, Science (2022).
DOI: 10.1126/science.abm1472.
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