Why does Mercury have such a big iron core? New research shows that proximity to the sun’s magnetic field determines a planet’s interior composition

A brand new examine disputes the prevailing speculation on why Mercury has an enormous core relative to its mantle (the layer between a planet’s core and crust). For many years, scientists argued that hit-and-run collisions with different our bodies in the course of the formation of our photo voltaic system blew away a lot of Mercury’s rocky mantle and left the massive, dense, steel core inside. However new analysis reveals that collisions are to not blame — the solar’s magnetism is.

William McDonough, a professor of geology on the College of Maryland, and Takashi Yoshizaki from Tohoku College developed a mannequin displaying that the density, mass and iron content material of a rocky planet’s core are influenced by its distance from the solar’s magnetic discipline. The paper describing the mannequin was printed on July 2, 2021, within the journal Progress in Earth and Planetary Science.

“The 4 inside planets of our photo voltaic system — Mercury, Venus, Earth and Mars — are made up of various proportions of steel and rock,” McDonough mentioned. “There’s a gradient wherein the steel content material within the core drops off because the planets get farther from the solar. Our paper explains how this occurred by displaying that the distribution of uncooked supplies within the early forming photo voltaic system was managed by the solar’s magnetic discipline.”

McDonough beforehand developed a mannequin for Earth’s composition that’s generally utilized by planetary scientists to find out the composition of exoplanets. (His seminal paper on this work has been cited greater than 8,000 instances.)

McDonough’s new mannequin reveals that in the course of the early formation of our photo voltaic system, when the younger solar was surrounded by a swirling cloud of mud and fuel, grains of iron had been drawn towards the middle by the solar’s magnetic discipline. When the planets started to type from clumps of that mud and fuel, planets nearer to the solar included extra iron into their cores than these farther away.

The researchers discovered that the density and proportion of iron in a rocky planet’s core correlates with the energy of the magnetic discipline across the solar throughout planetary formation. Their new examine means that magnetism needs to be factored into future makes an attempt to explain the composition of rocky planets, together with these exterior our photo voltaic system.


The composition of a planet’s core is necessary for its potential to help life. On Earth, as an illustration, a molten iron core creates a magnetosphere that protects the planet from cancer-causing cosmic rays. The core additionally incorporates nearly all of the planet’s phosphorus, which is a vital nutrient for sustaining carbon-based life.

Utilizing present fashions of planetary formation, McDonough decided the velocity at which fuel and mud was pulled into the middle of our photo voltaic system throughout its formation. He factored within the magnetic discipline that will have been generated by the solar because it burst into being and calculated how that magnetic discipline would draw iron by means of the mud and fuel cloud.

Because the early photo voltaic system started to chill, mud and fuel that weren’t drawn into the solar started to clump collectively. The clumps nearer to the solar would have been uncovered to a stronger magnetic discipline and thus would comprise extra iron than these farther away from the solar. Because the clumps coalesced and cooled into spinning planets, gravitational forces drew the iron into their core.

When McDonough included this mannequin into calculations of planetary formation, it revealed a gradient in steel content material and density that corresponds completely with what scientists know in regards to the planets in our photo voltaic system. Mercury has a metallic core that makes up about three-quarters of its mass. The cores of Earth and Venus are solely about one-third of their mass, and Mars, the outermost of the rocky planets, has a small core that’s solely about one-quarter of its mass.

This new understanding of the function magnetism performs in planetary formation creates a kink within the examine of exoplanets, as a result of there’s at present no technique to find out the magnetic properties of a star from Earth-based observations. Scientists infer the composition of an exoplanet primarily based on the spectrum of sunshine radiated from its solar. Totally different parts in a star emit radiation in several wavelengths, so measuring these wavelengths reveals what the star, and presumably the planets round it, are manufactured from.

“You’ll be able to not simply say, ‘Oh, the composition of a star appears to be like like this, so the planets round it should appear like this,'” McDonough mentioned. “Now it’s important to say, ‘Every planet may have kind of iron primarily based on the magnetic properties of the star within the early progress of the photo voltaic system.'”

The subsequent steps on this work shall be for scientists to seek out one other planetary system like ours — one with rocky planets unfold over vast distances from their central solar. If the density of the planets drops as they radiate out from the solar the best way it does in our photo voltaic system, researchers may affirm this new concept and infer {that a} magnetic discipline influenced planetary formation.