The model of how the inner core of the Earth is frozen in solid iron suggests that it may be only 500 million years old.
For unknown reasons, the Earth’s inner core of solid iron is growing faster on one side of the other and has since begun to freeze from molten iron more than half a billion years ago, according to a new study by seismologists from University of California, Berkeley.
Faster growth under the sea Banda in Indonesia has not left the core aside. Gravity evenly distributes the new growth – iron crystals that form when the molten iron is cooled to maintain a spherical inner core that grows in a radius of an average of 1 millimeter per year.
But the increased growth on the one hand suggests that something in the outer core or mantle of the Earth below Indonesia removes heat from the inner core at a faster rate than on the opposite side, below Brazil. Faster cooling on one side would accelerate the crystallization of iron and the growth of the inner core on that side.
This has implications for the Earth’s magnetic field and its history, because convection in the outer core, driven by heat from the inner core, is what drives the dynamo today, which generates the magnetic field that protects us from dangerous particles from the sun.
“We provide a lot of free limits to the age of the inner core – between half a billion and 1.5 billion years – which can be helpful in the debate about how the magnetic field was generated before the solid inner core existed,” Barbara said. Romanovic, a professor at the University of Berkeley in the Department of Earth and Planetary Science and honorary director of the Berkeley Seismological Laboratory (BSL). “We know that the magnetic field existed 3 billion years ago, so at that time other processes must have conducted convection in the outer core.”
The young age of the inner core may mean that at the beginning of Earth’s history, the heat boiling in the liquid core came from light elements released from the iron, not from the crystallization of the iron we see today.
“The debate about the age of the inner core has been going on for a long time,” said Daniel Frost, an assistant scientist on the BSL project. “The complexity is: If the inner core has only been able to exist for 1.5 billion years, based on what we know about how it loses heat and how hot it is, then where does the older magnetic field come from?” Hence the idea of dissolved light elements that then freeze. “
The asymmetric growth of the inner core explains a mystery from three decades – that the crystallized iron in the core appears to be preferentially aligned along the axis of rotation of the earth, more to the west than to the east, while crystals can be expected to be randomly oriented.
Evidence for this alignment comes from measurements of the travel time of seismic waves from earthquakes through the inner core. Seismic waves travel faster in a north-south axis of rotation than along the equator, an asymmetry that geologists attribute to iron crystals – which are asymmetric – with their long axes preferentially aligned along the Earth’s axis.
If the core is solid crystalline iron, how do the iron crystals orient themselves preferably in one direction?
In an attempt to explain the observations, Frost and colleagues Marine Lasbleis of the University of Nantes in France and Brian Chandler and Romanowicz of UC Berkeley developed a computer model of crystal growth in the inner core that included geodynamic growth models and mineral physics of high pressure iron. and high temperature.
“The simplest model seemed a little unusual – that the inner core is asymmetrical,” said Frost. “The west side looks different from the east all the way to the center, not just at the top of the inner core, as some have suggested. The only way to explain this is that one country is growing faster than the other. “
The model describes how asymmetric growth – about 60% higher east to west – can preferentially orient iron crystals along the axis of rotation, with greater alignment to the west than to the east, and explains the difference in seismic wave velocities in the west. inner core.
“What we are proposing in this document is a model of one-way solid convection in the inner core, which combines seismic observations and plausible geodynamic boundary conditions,” Romanovic said.
Frost, Romanovic and their colleagues will report their findings in this week’s issue of the magazine Nature Geoscience is a monthly peer-reviewed scientific journal published by the Nature Publishing Group, which covers all aspects of Earth sciences, including theoretical research, modeling and fieldwork. Other related work is also published in fields that include atmospheric sciences, geology, geophysics, climatology, oceanography, paleontology, and space science. It was established in January 2008.
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Study of the Earth’s interior with seismic waves
The interior of the Earth is layered like an onion. The solid inner core of iron-nickel – today with a radius of 1,200 kilometers (745 miles) or about three-quarters of the size of the moon – is surrounded by a liquid outer core of molten iron and nickel about 2,400 kilometers (1,500 miles) thick. . The outer core is surrounded by a mantle of hot rock 2,900 kilometers (1,800 miles) thick and covered with a thin, cool, rocky crust on the surface.
Convection occurs both in the outer core, which boils slowly as the heat from the crystallizing iron exits the inner core, and in the mantle as the hotter rock moves upward to transfer that heat from the center of the planet to the surface. The vigorous boiling motion in the outer core of liquid iron creates the Earth’s magnetic field.
According to Frost’s computer model, which he created with the help of Lasbleis, as iron crystals grow, gravity redistributes excess growth east to west in the inner core. This movement of crystals in the rather soft solid of the inner core – which is close to the melting point of iron at these high pressures – aligns the crystal lattice along the axis of rotation of the Earth to a greater extent to the west than to the east.
The model correctly predicts new observations by researchers about the travel time of a seismic wave through the inner core: The anisotropy or difference in travel times, parallel and perpendicular to the axis of rotation, increases with depth and the strongest anisotropy shifts west of the earth’s axis at a turn of about 400 kilometers (250 miles).
The core core growth model also limits the proportion of nickel to iron in the center of the earth, Frost said. Its model does not accurately reproduce seismic observations unless nickel makes up between 4% and 8% of the inner core – which is close to the share of metal meteorites that were once probably the nuclei of dwarf planets in our solar system. The model also tells geologists how viscous or liquid the inner core is.
“We assume that the viscosity of the inner core is a relatively large, input parameter important for geodynamics studying the dynamo processes in the outer core,” Romanovic said.
Reference: “A Dynamic History of the Inner Core Bounded by Seismic Anisotropy” by Daniel A. Frost, Marin Lasblais, Brian Chandler, and Barbara Romanovich, June 3, 2021, Nature Geoscience.
DOI: 10.1038 / s41561-021-00761-w
Frost and Romanowicz were supported by grants from the National Science Foundation (EAR-1135452, EAR-1829283).