Earth’s core may have hardened just in time to save planet’s magnetic field | Science News for Students

Earth’s core may have hardened just in time to save planet’s magnetic field

Planet’s solid heart may be much younger than thought
Mar 1, 2019 — 6:45 am EST
a computer-generated mage showing the Earth's magnetic field

Earth’s magnetic field (illustrated) is powered by the circulation of an iron-rich fluid in the planet’s core. The solid inner core formed relatively “late,” saving a weakening magnetic field from collapse.

Tom Bridgman/NASA’s Scientific Visualization Studio, JPL NAIF

The inner core of planet Earth was once molten. But it solidified within the past 565 million years, a new study suggests. That would have been just in time to save the planet’s protective magnetic field from imminent collapse. It also would have kick-started that field into its current, powerful phase, the study says.

Scientists shared their new analysis in the February Nature Geoscience.

It supports something previously suggested by a computer model. That model had proposed that Earth’s inner core is relatively young.

Earth formed some 4.54 billion years ago. The new study offers clues to how — and how quickly — Earth has been losing heat since its formed. And that is key to learning how the planet’s magnetic shield formed. It also offers insight into heat-related motions within the mantle (between Earth’s crust and core), as well as into plate tectonics.

“We don’t have many real benchmarks for the thermal history of our planet,” says Peter Olson. He’s a geophysicist at Johns Hopkins University in Baltimore, Md. He was not involved in the new study. “We know [Earth’s] interior was hotter than today, because all planets lose heat. But we don’t know what the average temperature was a billion years ago, compared with today.” Pinning down when iron in the inner core began to crystallize could offer a window into how hot the interior of the planet was back then, he says.

The planet’s core is made of iron and nickel and has two layers. The inner core is solid. Around it sits a molten outer core. When that solid inner core formed has been a long-standing mystery. People have suggested it could have been anywhere from 500 million years ago to more than 2.5 billion years ago, says coauthor John Tarduno. He’s a geophysicist at the University of Rochester in New York.

The core’s two layers interact. And that drives Earth’s geodynamo — the circulation of an iron-rich fluid that powers the planet’s magnetic field. That field surrounds the planet. And that’s a good thing. It protects Earth from being battered by the solar wind, a constant flow of charged particles from the sun.

As the inner core cooled and crystallized, the chemical recipe of its surrounding fluid changed. The more buoyant liquid would have risen like a plume. The cooling crystals would have sunk. That density-driven circulation was self-sustaining. And it has driven a strong magnetic field with two opposing poles, north and south.

an illustration of how the Earth's core powers its magnetic field
Heat driving convection within Earth’s hot, molten core (orange) powered the planet’s magnetic field for billions of years. New evidence suggests that by about 565 million years ago, that field was weak and increasingly unstable (left). Later, the inner core began to solidify (red, at right). That strengthened the field, giving it relatively consistent north and south magnetic poles (right).
Roberto Molar Candanosa and Peter Driscoll/Nature Geoscience 2019

Ancient magnetism

Scientists have found traces of magnetism in ancient rocks. Those rocks suggest that Earth had a magnetic field as far back as 4.2 billion years ago. That earlier field was likely generated by heat within the planet. That heat would have driven circulation within the molten core.

But over time, computer models suggest, the heat-driven circulation wouldn’t have been strong enough alone go on powering a strong magnetic field. Instead, the field would have begun to shut down. And old rocks show evidence of that in weakening magnetic field intensities and rapid reversals in Earth’s North and Sound poles over millions of years.

Eventually, Earth’s inner core began to crystallize. That would have jump-started the geodynamo. It also would have generated a new, stronger magnetic field.

Now scientists think they’ve found evidence for when that magnetic-field breakdown took place.

Inclusions are iron-rich, needlelike grains in rocks. Those inclusions align themselves with the orientation of Earth’s magnetic field when the rocks formed. Analyses of those inclusions show that the planet’s magnetic field was extremely weak some 565 million years ago. Richard Bono led the research team that had been looking at a suite of rocks from Quebec, Canada. A geophysicist, Bono now works at the University of Liverpool in England. Those very weak magnetic fields, Tarduno says, suggested there was something happening in the core.

Earlier studies had found that the magnetic field was also rapidly reversing its North-to-South polarity at that time. And with the new data, this research indicates that Earth’s field may have been on the point of collapse some 565 million years ago. That would seem to mean that the inner core had not yet become solid. Fortunately for life on Earth, it eventually did.

The new finding is “potentially very important,” says Olson at Johns Hopkins. The rocks bearing the magnetic grains took a very long time to cool. That means the new data represent an average field intensity over a 100,000-year period. He says Bono’s team hasn’t just captured a snapshot of some fluctuating field, but instead a true, persistent signal. Computer modeling had suggested that the weak field might have lasted much longer — from about 900 million to 600 million years ago, or until the inner core solidified. More data from within that time span, as well as from other locations, would help to confirm that.  

Peter Driscoll is a geophysicist at the Carnegie Institution for Science in Washington, D.C. He was one of the scientists who estimated how long the weak phase might have lasted. Driscoll wrote a commentary that was published along with the new study. A young solid inner core also highlights lingering questions about how quickly Earth cooled, he notes. For example, “if the core is cooling quickly, that means it was very hot in the recent past, and that the lower mantle was very hot in the recent past.” Both could have still been molten just 1 billion to 2 billion years ago. “We absolutely do not see that in the rock record.”

Power Words

(more about Power Words)

align     (noun: alignment) To place or organize things in a patterned order, following an apparent line.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

buoyant     (n. buoyancy) An adjective for something that can float on or rise up within some liquid or gas.

circulation     (adj. circulatory) A term that refers to the pumping of some fluid repeatedly throughout a system of vessels. (in medicine) The pumping of blood through the arteries and smaller types of vessels (and from there into other organs and tissues).

coauthor     One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.

commentary     (in science) An opinion piece, often written to accompany — and add perspective to — a paper by others, which describes new research findings.

computer model     A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

core     Something — usually round-shaped — in the center of an object. (in geology) Earth’s innermost layer. Or, a long, tube-like sample drilled down into ice, soil or rock. Cores allow scientists to examine layers of sediment, dissolved chemicals, rock and fossils to see how the environment at one location changed through hundreds to thousands of years or more.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

density     The measure of how condensed some object is, found by dividing its mass by its volume.

field      (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

fundamental     Something that is basic or serves as the foundation for another thing or idea.

geodynamo     A proposed mechanism to explain why Earth’s magnetic field has not gone away. The idea is that convective movement of a largely iron-based fluid in the planet’s core generates the planet’s magnetic field. About the size of the moon but as hot as the surface of the sun, the solid inner core has been cooling and crystalizing. The heat it gives off causes the remaining core fluid to rise up and move due to Earth's rotation. Scientists suspect this fluid motion twists and breaks away part of the magnetic field, generating new magnetic field to replace that the diminishing old one.

geoscience     Any of a number of sciences, like geology or atmospheric science, concerned with better understanding Earth. People who work in this field are known as geoscientists.

inclusion     (in geology) Something trapped inside a mineral.

insight     The ability to gain an accurate and deep understanding of a situation just by thinking about it, instead of working out a solution through experimentation.

iron     A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.

liquid     A material that flows freely but keeps a constant volume, like water or oil.

magnetic field     An area of influence created by certain materials, called magnets, or by the movement of electric charges.

magnetism     The attractive influence, or force, created by certain materials, called magnets, or by the movement of electric charges.

mantle     (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.

molten     A word describing something that is melted, such as the liquid rock that makes up lava.

nickel     Number 28 on the periodic table of elements, this hard, silvery element resists oxidation and corrosion. That makes it a good coating for many other elements or for use in multi-metal alloys.

online     (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.

particle     A minute amount of something.

persistent     An adjective for something that is long-lasting.

plate tectonics     The processes governing the movements of massive pieces that make up Earth’s outer layer, which is called the lithosphere. Those processes cause the rock masses to rise from inside Earth, travel along its surface, and sink back down.

plume     (in geology) Fluids (air, water or magma typically) that move, largely intact, in a feather-like shape over long distances.

poles     (in Earth science and astronomy) The cold regions of the planet that exist farthest from the equator; the upper and lower ends of the virtual axis around which a celestial object rotates. (in physics and electrical engineering) The ends of a magnet.

solar wind     A flow of charged particles (including atomic nuclei) that have been ejected from the surface of the star, such as our sun. It can permeate the solar system. This is called a stellar wind, when from a star other than the sun.

thermal     Of or relating to heat. (in meteorology) A relatively small-scale, rising air current produced when Earth’s surface is heated. Thermals are a common source of low level turbulence for aircraft.

Citation

Journal:​ R.K. Bono et al. Young inner core inferred from Ediacaran ultra-low geomagnetic field intensity. Nature Geoscience. Vol. 12, February 2019, p. 143. doi:10.1038/s41561-018-0288-0.

Journal:​ P. Driscoll. Geodynamo recharged. Nature Geoscience. Vol. 12, February 2019, p. 83. doi:10.1038/s41561-019-0301-2.