Is the Earth's core lopsided?

Model of how Earth’s inner core froze into solid iron implies it may be only 500 million years old

For reasons unknown, Earth’s solid-iron inner core is growing faster on one side than the other, and it has been ever since it started to freeze out from molten iron more than half a billion years ago, according to a new study by seismologists at the University of California, Berkeley.

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The faster growth under Indonesia’s Banda Sea hasn’t left the core lopsided. Gravity evenly distributes the new growth — iron crystals that form as the molten iron cools — to maintain a spherical inner core that grows in radius by an average of 1 millimeter per year.

But the enhanced growth on one side suggests that something in Earth’s outer core or mantle under Indonesia is removing heat from the inner core at a faster rate than on the opposite side, under Brazil. Quicker cooling on one side would accelerate iron crystallization and inner core growth on that side. This has implications for Earth’s magnetic field and its history, because convection in the outer core driven by release of heat from the inner core is what today drives the dynamo that generates the magnetic field that protects us from dangerous particles from the sun.This has implications for Earth’s magnetic field and its history, because convection in the outer core driven by release of heat from the inner core is what today drives the dynamo that generates the magnetic field.

A cut-away of Earth’s interior shows the solid iron inner core (red) slowly growing by freezing of the liquid iron outer core (orange). Seismic waves travel through the Earth’s inner core faster between the north and south poles (blue arrows) than across the equator (green arrow). The researchers concluded that this difference in seismic wave speed with direction results from a preferred alignment of the crystals — hexagonally close packed iron-nickel alloys, which are themselves anisotropic — parallel with Earth’s rotation axis. (Graphic by Daniel Frost)

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 the direction of the north-south rotation axis than along the equator, an asymmetry that geologists attribute to iron crystals — which are asymmetric — having their long axes preferentially aligned along Earth’s axis.

If the core is solid crystalline iron, how do the iron crystals get oriented preferentially in one direction?

In an attempt to explain the observations, Frost and colleagues Marine Lasbleis of the Université de Nantes in France and Brian Chandler and Romanowicz of UC Berkeley created a computer model of crystal growth in the inner core that incorporates geodynamic growth models and the mineral physics of iron at high pressure and high temperature.

“The simplest model seemed a bit unusual — that the inner core is asymmetric,” Frost said. “The west side looks different from the east side all the way to the center, not just at the top of the inner core, as some have suggested. The only way we can explain that is by one side growing faster than the other.”

“The simplest model seemed a bit unusual — that the inner core is asymmetric,” Frost said. “The west side looks different from the east side all the way to the center, not just at the top of the inner core, as some have suggested. The only way we can explain that is by one side growing faster than the other.”

The model describes how asymmetric growth — about 60% higher in the east than the west — can preferentially orient iron crystals along the rotation axis, with more alignment in the west than in the east, and explain the difference in seismic wave velocity across the inner core.

“What we’re proposing in this paper is a model of lopsided solid convection in the inner core that reconciles seismic observations and plausible geodynamic boundary conditions,” Romanowicz said.

“What we’re proposing in this paper is a model of lopsided solid convection in the inner core that reconciles seismic observations and plausible geodynamic boundary conditions,” Romanowicz said.

Frost, Romanowicz and their colleagues will report their findings in this week’s issue of the journal Nature Geoscience.Probing Earth’s interior with seismic waves

Probing Earth’s interior with seismic waves

Earth’s interior is layered like an onion. The solid iron-nickel inner core — today 1,200 kilometers (745 miles) in radius, or about three-quarters the size of the moon — is surrounded by a fluid 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 overlain by a thin, cool, rocky crust at the surface.

This has implications for Earth’s magnetic field and its history, because convection in the outer core driven by release of heat from the inner core is what today drives the dynamo that generates the magnetic.

A cut-away of Earth’s interior shows the solid iron inner core (red) slowly growing by freezing of the liquid iron outer core (orange). Seismic waves travel through the Earth’s inner core faster between the north and south poles (blue arrows) than across the equator (green arrow). The researchers concluded that this difference in seismic wave speed with direction results from a preferred alignment of the crystals — hexagonally close packed iron-nickel alloys, which are themselves anisotropic — parallel with Earth’s rotation axis. (Graphic by Daniel Frost)

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 the direction of the north-south rotation axis than along the equator, an asymmetry that geologists attribute to iron crystals — which are asymmetric — having their long axes preferentially aligned along Earth’s axis.

If the core is solid crystalline iron, how do the iron crystals get oriented preferentially in one direction?

In an attempt to explain the observations, Frost and colleagues Marine Lasbleis of the Université de Nantes in France and Brian Chandler and Romanowicz of UC Berkeley created a computer model of crystal growth in the inner core that incorporates geodynamic growth models and the mineral physics of iron at high pressure and high temperature.

“The simplest model seemed a bit unusual — that the inner core is asymmetric,” Frost said. “The west side looks different from the east side all the way to the center, not just at the top of the inner core, as some have suggested. The only way we can explain that is by one side growing faster than the other.”

“The simplest model seemed a bit unusual — that the inner core is asymmetric,” Frost said. “The west side looks different from the east side all the way to the center, not just at the top of the inner core, as some have suggested. The only way we can explain that is by one side growing faster than the other.”

The model describes how asymmetric growth — about 60% higher in the east than the west — can preferentially orient iron crystals along the rotation axis, with more alignment in the west than in the east, and explain the difference in seismic wave velocity across the inner core.

“What we’re proposing in this paper is a model of lopsided solid convection in the inner core that reconciles seismic observations and plausible geodynamic boundary conditions,” Romanowicz said.

“What we’re proposing in this paper is a model of lopsided solid convection in the inner core that reconciles seismic observations and plausible geodynamic boundary conditions,” Romanowicz said.

There are no samples of Earth's core accessible for direct measurement, as there are for Earth's mantle. Information about Earth's core mostly comes from analysis of seismic waves and Earth's magnetic field.[3] The inner core is believed to be composed of an iron–nickel alloy with some other elements. The temperature at the inner core's surface is estimated to be approximately 5,700 K (5,430 °C; 9,800 °F), which is about the temperature at the surface of the Sun.[4]

Frost, Romanowicz and their colleagues will report their findings in this week’s issue of the journal Nature Geoscience.

Probing Earth’s interior with seismic waves

Earth’s interior is layered like an onion. The solid iron-nickel inner core — today 1,200 kilometers (745 miles) in radius, or about three-quarters the size of the moon — is surrounded by a fluid 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 overlain by a thin, cool, rocky crust at the surface.

In 2010 Bruce buffet determined that the average magnetic field in the liquid outer core is about 2.5 ml which is about 40 times to maximum strength at the service he started from the known fact that the Moon and Sun caused studs in the liquid outer core just as they do on the ocean of the service is emotion of the liquid through the local magnetic field creates electric current status by energy as he according to ohm's law. The core–mantle boundary (CMB) of Earth lies between the planet's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2891 km (1796 miles) depth beneath Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth due to the differences between the acoustic impedances of the solid mantle and the molten outer core. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not exist at all in the liquid portion of the core. Recent evidence suggests a distinct boundary layer directly above the CMB possibly made of a novel phase of the basic perovskite mineralogy of the deep mantle named post-perovskite. Seismic tomography studies have shown significant irregularities within the boundary zone and appear to be dominated by the African and Pacific Large Low-Shear-Velocity Provinces (LLSVP).[1]

Inner core super-rotation is a true eastward rotation of the inner core of Earth relative to its mantle, for a net rotation rate that is faster than Earth as a whole. A 1995 model of Earth's dynamo predicted super-rotations of up to 3 degrees per year; the following year, this prediction was supported by observed discrepancies in the time that p-waves take to travel through the inner and outer core.

Seismic observations have made use of a direction dependence (anisotropy) of the speed of seismic waves in the inner core, as well as spatial variations in the speed. Other estimates come from free oscillations of Earth. The results are inconsistent and the existence of a super-rotation is still controversial, but it is probably less than 0.1 degrees per year.

When geodynamo models take into account gravitational coupling between the inner core and mantle, it lowers the predicted super-rotation to as little as 1 degree per million years. For the inner core to rotate despite gravitational coupling, it must be able to change shape, which places constraints on its viscosity.

The average density of Earth is 5.515 g/cm3.[16] Because the average density of surface material is only around 3.0 g/cm3, we must conclude that denser materials exist within Earth's core. This result has been known since the Schiehallion experiment, performed in the 1770s. Charles Hutton in his 1778 report concluded that the mean density of the Earth must be about {\displaystyle {\tfrac {9}{5}}}{\tfrac {9}{5}} that of surface rock, concluding that the interior of the Earth must be metallic. Hutton estimated this metallic portion to occupy some 65% of the diameter of the Earth.[17] Hutton's estimate on the mean density of the Earth was still about 20% too low, at 4.5 g/cm3. Henry Cavendish in his torsion balance experiment of 1798 found a value of 5.45 g/cm3, within 1% of the modern value.[18] Seismic measurements show that the core is divided into two parts, a "solid" inner core with a radius of ≈1,220 km[19] and a liquid outer core extending beyond it to a radius of ≈3,400 km. The densities are between 9,900 and 12,200 kg/m3 in the outer core and 12,600–13,000 kg/m3 in the inner core.[20]

The inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed primarily of iron and some nickel. Since this layer is able to transmit shear waves (transverse seismic waves), it must be solid. Experimental evidence has at times been inconsistent with current crystal models of the core.[21] Other experimental studies show a discrepancy under high pressure: diamond anvil (static) studies at core pressures yield melting temperatures that are approximately 2000 K below those from shock laser (dynamic) studies.[22][23] The laser studies create plasma,[24] and the results are suggestive that constraining inner core conditions will depend on whether the inner core is a solid or is a plasma with the density of a solid. This is an area of active research.

In early stages of Earth's formation about 4.6 billion years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation (see also the iron catastrophe), while less-dense materials would have migrated to the crust. The core is thus believed to largely be composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials). Some have argued that the inner core may be in the form of a single iron crystal.[25][26]

Under laboratory conditions a sample of iron–nickel alloy was subjected to the corelike pressures by gripping it in a vise between 2 diamond tips (diamond anvil cell), and then heating to approximately 4000 K. The sample was observed with x-rays, and strongly supported the theory that Earth's inner core was made of giant crystals running north to south.[27][28]

The liquid outer core surrounds the inner core and is believed to be composed of iron mixed with nickel and trace amounts of lighter elements.

Some have speculated that the innermost part of the core is enriched in gold, platinum and other siderophile elements.[29]

The composition of the Earth bears strong similarities to that of certain chondrite meteorites, and even to some elements in the outer portion of the Sun.[30][31] Beginning as early as 1940, scientists, including Francis Birch, built geophysics upon the premise that Earth is like ordinary chondrites, the most common type of meteorite observed impacting Earth. This ignores the less abundant enstatite chondrites, which formed under extremely limited available oxygen, leading to certain normally oxyphile elements existing either partially or wholly in the alloy portion that corresponds to the core of Earth.

Dynamo theory suggests that convection in the outer core, combined with the Coriolis effect, gives rise to Earth's magnetic field. The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to stabilize the magnetic field generated by the liquid outer core. The average magnetic field in Earth's outer core is estimated to measure 25 Gauss (2.5 mT), 50 times stronger than the magnetic field at the surface.[32][33]

Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet; in 2005 a team of geophysicists estimated that Earth's inner core rotates approximately 0.3 to 0.5 degrees per year faster.;[34][35][36] However, more recent studies in 2011[which?] did not support this hypothesis. Other possible motions of the core be oscillatory or chaotic.[citation needed]

The current scientific explanation for Earth's temperature gradient is a combination of heat left over from the planet's initial formation, decay of radioactive elements, and freezing of the inner core.

The current scientific explanation for Earth's temperature gradient is a combination of heat left over from the planet's initial formation, decay of radioactive elements, and freezing of the inner core.