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Study: Proximity to Sun’s Magnetic Field Influenced Mercury’s Hefty Iron Core

By Kimbra Cutlip

For decades, many scientists argued that hit-and-run collisions with other bodies during the formation of our solar system blew away much of Mercury’s rocky mantle and left the big, dense, metal core inside. But new research reveals that collisions don’t explain the planet’s composition—the sun’s magnetism does.

William McDonough, a professor of geology at the University of Maryland, and Takashi Yoshizaki of Tohoku University developed a model showing that the density, mass and iron content of a rocky planet’s core are influenced by its distance from the sun’s magnetic field. The paper describing the model was published today in the journal Progress in Earth and Planetary Science.

“The four inner planets of our solar system—Mercury, Venus, Earth and Mars—are made up of different proportions of metal and rock,” McDonough said. “There is a gradient in which the metal content in the core drops off as the planets get farther from the sun. Our paper explains how this happened by showing that the distribution of raw materials in the early-forming solar system was controlled by the sun’s magnetic field.”

McDonough previously developed a model for Earth’s composition that is commonly used by planetary scientists to determine the composition of exoplanets. (His seminal paper on this work has been cited more than 8,000 times.)

McDonough’s new model shows that during the early formation of our solar system, when the young sun was surrounded by a swirling cloud of dust and gas, grains of iron were drawn toward the center by the sun’s magnetic field. When the planets began to form from clumps of that dust and gas, planets closer to the sun incorporated more iron into their cores than those farther away.

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