Why does Mercury have such a big iron core? Magnetism!

Credit: University of MarylandNew research challenges the hypothesis that Mercury has a large core relative to its mantle, which is the layer between the planet's crust and core. Scientists have argued for decades that Mercury's rocky core was destroyed by hit-and-run collisions. This is what caused the formation of the solar system. New research shows that collisions are not to blame. The sun's magnetism is.William McDonough, a professor of geoology at the University of Maryland and Takashi Yoshizaki, from Tohoku University, developed a model that showed how the distance to the sun's magnetic fields affects the density, mass, and iron content of a rock planet's core. The model was described in a paper published in Progress in Earth and Planetary Science on July 2, 2021.McDonough stated that the four inner planets in our solar systemMercury Venus Earth Mars are made up of different amounts of metal and rock. McDonough explained that there is a gradient where the metal content of the core decreases as the planets move further from the sun. This paper explains how it happened. It shows that the sun's magnetic fields controlled the distribution of raw materials during the early formation of the solar system.McDonough had previously created a model of Earth's composition, which is used by planetary scientists to calculate the compositions of exoplanets. His seminal paper has been cited over 8,000 times.McDonough's model shows that iron grains were attracted to the center of the sun's magnetic field during the early formations of the solar system. The iron in the cores of the planets formed from clumps made up of this dust and gas.Researchers found that the magnetic field strength around the sun's magnetic field correlated with the density and iron content of a rocky planet’s core during its formation. The new study suggests that magnetic properties should be considered in future attempts to describe the compositions of rocky planets.For a planet to be able to sustain life, its core composition is crucial. For example, Earth's magnetosphere is formed by molten iron. This protects it from cosmic rays that can cause cancer. It also holds the bulk of the planet's Phosphorus, an essential nutrient to sustain carbon-based life.McDonough used existing models of planetary creation to determine the speed with which gas and dust were pulled into the center our solar system's formation. McDonough calculated the magnetic field generated by the sun's explosion and how it would draw iron through the gas cloud and dust.As the solar system cooled, gas and dust that weren't drawn to the sun started to clump together. Clumps closer to the sun will have been exposed to stronger magnetic fields and would therefore contain more iron. The iron was dragged into the core of the planets by gravitational forces as the clumps cooled and coalesced.McDonough used this model to calculate planetary formation. It revealed a gradient of metal content and density which corresponds exactly with what scientists know about planets in the solar system. Mercury's metallic core makes up around three quarters of its total mass. Earth's and Venus' cores are about one third of their total mass. Mars, the outermost rocky planet, has a smaller core of about one quarter of its mass.The new knowledge of the role of magnetism in planetary formation has created a problem in the study exoplanets. There is no way to determine the magnetic properties a star from Earth-based observations. Scientists determine the composition of an exoplanet by looking at the spectrum of light radiated by its sun. Different elements emit different wavelengths of radiation from stars, so measuring these wavelengths can reveal what the star and the planets surrounding it are made up.McDonough stated that it was no longer possible to simply say "Oh, the composition a star looks like...", and the planets surrounding it must also look like this. "Now, you need to say, "Each planet could contain more or less iron depending on the magnetic properties associated with the star during the early growth phase of the solar system."Scientists will next need to locate another planetary system similar to ours with rocky planets scattered over large distances from the central sun. Researchers could prove this theory by examining the density of the planets as they radiate from the sun in the same way that it does in the solar system.McDonough, W. F. and Yoshizaki T. published the research paper "Terrestrial Planet Compositions Controlled by Acretion Disk Magnetic Field," on July 2, 2021 in Progress in Earth and Planetary Science.Continue reading How was the carbon content of terrestrial and lunar mantles determined?More information: William F. McDonough and colleagues, Terrestrial Planet Compositions Controlled by the Acretion Disk Magnetic Field, Progress in Earth and Planetary Science (2021). William F. McDonough and colleagues, Terrestrial world compositions controlled with accretion disc magnetic field, (2021). DOI: 10.186/s40645-021-00429-4